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adding values to TypeScript Object [duplicate]

How can I loop through all the entries in an array using JavaScript?

TL;DR
Your best bets are usually
a for-of loop (ES2015+ only; spec | MDN) - simple and async-friendly
for (const element of theArray) {
// ...use `element`...
}
forEach (ES5+ only; spec | MDN) (or its relatives some and such) - not async-friendly (but see details)
theArray.forEach(element => {
// ...use `element`...
});
a simple old-fashioned for loop - async-friendly
for (let index = 0; index < theArray.length; ++index) {
const element = theArray[index];
// ...use `element`...
}
(rarely) for-in with safeguards - async-friendly
for (const propertyName in theArray) {
if (/*...is an array element property (see below)...*/) {
const element = theArray[propertyName];
// ...use `element`...
}
}
Some quick "don't"s:
Don't use for-in unless you use it with safeguards or are at least aware of why it might bite you.
Don't use map if you're not using its return value.(There's sadly someone out there teaching map [spec / MDN] as though it were forEach — but as I write on my blog, that's not what it's for. If you aren't using the array it creates, don't use map.)
Don't use forEach if the callback does asynchronous work and you want the forEach to wait until that work is done (because it won't).
But there's lots more to explore, read on...
JavaScript has powerful semantics for looping through arrays and array-like objects. I've split the answer into two parts: Options for genuine arrays, and options for things that are just array-like, such as the arguments object, other iterable objects (ES2015+), DOM collections, and so on.
Okay, let's look at our options:
For Actual Arrays
You have five options (two supported basically forever, another added by ECMAScript 5 ["ES5"], and two more added in ECMAScript 2015 ("ES2015", aka "ES6"):
Use for-of (use an iterator implicitly) (ES2015+)
Use forEach and related (ES5+)
Use a simple for loop
Use for-in correctly
Use an iterator explicitly (ES2015+)
(You can see those old specs here: ES5, ES2015, but both have been superceded; the current editor's draft is always here.)
Details:
1. Use for-of (use an iterator implicitly) (ES2015+)
ES2015 added iterators and iterables to JavaScript. Arrays are iterable (so are strings, Maps, and Sets, as well as DOM collections and lists, as you'll see later). Iterable objects provide iterators for their values. The new for-of statement loops through the values returned by an iterator:
const a = ["a", "b", "c"];
for (const element of a) { // You can use `let` instead of `const` if you like
console.log(element);
}
// a
// b
// c
It doesn't get simpler than that! Under the covers, that gets an iterator from the array and loops through the values the iterator returns. The iterator provided by arrays provides the values of the array elements, in order beginning to end.
Notice how element is scoped to each loop iteration; trying to use element after the end of the loop would fail because it doesn't exist outside the loop body.
In theory, a for-of loop involves several function calls (one to get the iterator, then one to get each value from it). Even when that's true, it's nothing to worry about, function calls are very cheap in modern JavaScript engines (it bothered me for forEach [below] until I looked into it; details). But additionally, JavaScript engines optimize those calls away (in performance-critical code) when dealing with native iterators for things like arrays.
for-of is entirely async-friendly. If you need the work in a loop body to be done in series (not in parallel), an await in the loop body will wait for the promise to settle before continuing. Here's a silly example:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (const message of messages) {
await delay(400);
console.log(message);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
Note how the words appear with a delay before each one.
It's a matter of coding style, but for-of is the first thing I reach for when looping through anything iterable.
2. Use forEach and related
In any even vaguely-modern environment (so, not IE8) where you have access to the Array features added by ES5, you can use forEach (spec | MDN) if you're only dealing with synchronous code (or you don't need to wait for an asynchronous process to finish during the loop):
const a = ["a", "b", "c"];
a.forEach((element) => {
console.log(element);
});
forEach accepts a callback function and, optionally, a value to use as this when calling that callback (not used above). The callback is called for each element in the array, in order, skipping non-existent elements in sparse arrays. Although I only used one parameter above, the callback is called with three arguments: The element for that iteration, the index of that element, and a reference to the array you're iterating over (in case your function doesn't already have it handy).
Like for-of, forEach has the advantage that you don't have to declare indexing and value variables in the containing scope; in this case, they're supplied as arguments to the iteration function, and so nicely scoped to just that iteration.
Unlike for-of, forEach has the disadvantage that it doesn't understand async functions and await. If you use an async function as the callback, forEach does not wait for that function's promise to settle before continuing. Here's the async example from for-of using forEach instead — notice how there's an initial delay, but then all the text appears right away instead of waiting:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
// INCORRECT, doesn't wait before continuing,
// doesn't handle promise rejections
messages.forEach(async message => {
await delay(400);
console.log(message);
});
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
forEach is the "loop through them all" function, but ES5 defined several other useful "work your way through the array and do things" functions, including:
every (spec | MDN) - stops looping the first time the callback returns a falsy value
some (spec | MDN) - stops looping the first time the callback returns a truthy value
filter (spec | MDN) - creates a new array including elements where the callback returns a truthy value, omitting the ones where it doesn't
map (spec | MDN) - creates a new array from the values returned by the callback
reduce (spec | MDN) - builds up a value by repeatedly calling the callback, passing in previous values; see the spec for the details
reduceRight (spec | MDN) - like reduce, but works in descending rather than ascending order
As with forEach, if you use an async function as your callback, none of those waits for the function's promise to settle. That means:
Using an async function callback is never appropriate with every, some, and filter since they will treat the returned promise as though it were a truthy value; they don't wait for the promise to settle and then use the fulfillment value.
Using an async function callback is often appropriate with map, if the goal is to turn an array of something into an array of promises, perhaps for passing to one of the promise combinator functions (Promise.all, Promise.race, promise.allSettled, or Promise.any).
Using an async function callback is rarely appropriate with reduce or reduceRight, because (again) the callback will always return a promise. But there is an idiom of building a chain of promises from an array that uses reduce (const promise = array.reduce((p, element) => p.then(/*...something using `element`...*/));), but usually in those cases a for-of or for loop in an async function will be clearer and easier to debug.
3. Use a simple for loop
Sometimes the old ways are the best:
const a = ["a", "b", "c"];
for (let index = 0; index < a.length; ++index) {
const element = a[index];
console.log(element);
}
If the length of the array won't change during the loop, and it's in highly performance-sensitive code, a slightly more complicated version grabbing the length up front might be a tiny bit faster:
const a = ["a", "b", "c"];
for (let index = 0, len = a.length; index < len; ++index) {
const element = a[index];
console.log(element);
}
And/or counting backward:
const a = ["a", "b", "c"];
for (let index = a.length - 1; index >= 0; --index) {
const element = a[index];
console.log(element);
}
But with modern JavaScript engines, it's rare you need to eke out that last bit of juice.
Before ES2015, the loop variable had to exist in the containing scope, because var only has function-level scope, not block-level scope. But as you saw in the examples above, you can use let within the for to scope the variables to just the loop. And when you do that, the index variable is recreated for each loop iteration, meaning closures created in the loop body keep a reference to the index for that specific iteration, which solves the old "closures in loops" problem:
// (The `NodeList` from `querySelectorAll` is array-like)
const divs = document.querySelectorAll("div");
for (let index = 0; index < divs.length; ++index) {
divs[index].addEventListener('click', e => {
console.log("Index is: " + index);
});
}
<div>zero</div>
<div>one</div>
<div>two</div>
<div>three</div>
<div>four</div>
In the above, you get "Index is: 0" if you click the first and "Index is: 4" if you click the last. This does not work if you use var instead of let (you'd always see "Index is: 5").
Like for-of, for loops work well in async functions. Here's the earlier example using a for loop:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (let i = 0; i < messages.length; ++i) {
const message = messages[i];
await delay(400);
console.log(message);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
4. Use for-in correctly
for-in isn't for looping through arrays, it's for looping through the names of an object's properties. It does often seem to work for looping through arrays as a by-product of the fact that arrays are objects, but it doesn't just loop through the array indexes, it loops through all enumerable properties of the object (including inherited ones). (It also used to be that the order wasn't specified; it is now [details in this other answer], but even though the order is specified now, the rules are complex, there are exceptions, and relying on the order is not best practice.)
The only real use cases for for-in on an array are:
It's a sparse array with massive gaps in it, or
You're using non-element properties on the array object and you want to include them in the loop
Looking only at that first example: You can use for-in to visit those sparse array elements if you use appropriate safeguards:
// `a` is a sparse array
const a = [];
a[0] = "a";
a[10] = "b";
a[10000] = "c";
for (const name in a) {
if (Object.hasOwn(a, name) && // These checks are
/^0$|^[1-9]\d*$/.test(name) && // explained
name <= 4294967294 // below
) {
const element = a[name];
console.log(a[name]);
}
}
Note the three checks:
That the object has its own property by that name (not one it inherits from its prototype; this check is also often written as a.hasOwnProperty(name) but ES2022 adds Object.hasOwn which can be more reliable), and
That the name is all decimal digits (e.g., normal string form, not scientific notation), and
That the name's value when coerced to a number is <= 2^32 - 2 (which is 4,294,967,294). Where does that number come from? It's part of the definition of an array index in the specification. Other numbers (non-integers, negative numbers, numbers greater than 2^32 - 2) are not array indexes. The reason it's 2^32 - 2 is that that makes the greatest index value one lower than 2^32 - 1, which is the maximum value an array's length can have. (E.g., an array's length fits in a 32-bit unsigned integer.)
...although with that said, most code only does the hasOwnProperty check.
You wouldn't do that in inline code, of course. You'd write a utility function. Perhaps:
// Utility function for antiquated environments without `forEach`
const hasOwn = Object.prototype.hasOwnProperty.call.bind(Object.prototype.hasOwnProperty);
const rexNum = /^0$|^[1-9]\d*$/;
function sparseEach(array, callback, thisArg) {
for (const name in array) {
const index = +name;
if (hasOwn(a, name) &&
rexNum.test(name) &&
index <= 4294967294
) {
callback.call(thisArg, array[name], index, array);
}
}
}
const a = [];
a[5] = "five";
a[10] = "ten";
a[100000] = "one hundred thousand";
a.b = "bee";
sparseEach(a, (value, index) => {
console.log("Value at " + index + " is " + value);
});
Like for, for-in works well in asynchronous functions if the work within it needs to be done in series.
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (const name in messages) {
if (messages.hasOwnProperty(name)) { // Almost always this is the only check people do
const message = messages[name];
await delay(400);
console.log(message);
}
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
5. Use an iterator explicitly (ES2015+)
for-of uses an iterator implicitly, doing all the scut work for you. Sometimes, you might want to use an iterator explicitly. It looks like this:
const a = ["a", "b", "c"];
const it = a.values(); // Or `const it = a[Symbol.iterator]();` if you like
let entry;
while (!(entry = it.next()).done) {
const element = entry.value;
console.log(element);
}
An iterator is an object matching the Iterator definition in the specification. Its next method returns a new result object each time you call it. The result object has a property, done, telling us whether it's done, and a property value with the value for that iteration. (done is optional if it would be false, value is optional if it would be undefined.)
What you get for value varies depending on the iterator. On arrays, the default iterator provides the value of each array element ("a", "b", and "c" in the example earlier). Arrays also have three other methods that return iterators:
values(): This is an alias for the [Symbol.iterator] method that returns the default iterator.
keys(): Returns an iterator that provides each key (index) in the array. In the example above, it would provide "0", then "1", then "2" (yes, as strings).
entries(): Returns an iterator that provides [key, value] arrays.
Since iterator objects don't advance until you call next, they work well in async function loops. Here's the earlier for-of example using the iterator explicitly:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
const it = messages.values()
while (!(entry = it.next()).done) {
await delay(400);
const element = entry.value;
console.log(element);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
For Array-Like Objects
Aside from true arrays, there are also array-like objects that have a length property and properties with all-digits names: NodeList instances, HTMLCollection instances, the arguments object, etc. How do we loop through their contents?
Use most of the options above
At least some, and possibly most or even all, of the array approaches above apply equally well to array-like objects:
Use for-of (use an iterator implicitly) (ES2015+)
for-of uses the iterator provided by the object (if any). That includes host-provided objects (like DOM collections and lists). For instance, HTMLCollection instances from getElementsByXYZ methods and NodeLists instances from querySelectorAll both support iteration. (This is defined quite subtly by the HTML and DOM specifications. Basically, any object with length and indexed access is automatically iterable. It doesn't have to be marked iterable; that is used only for collections that, in addition to being iterable, support forEach, values, keys, and entries methods. NodeList does; HTMLCollection doesn't, but both are iterable.)
Here's an example of looping through div elements:
const divs = document.querySelectorAll("div");
for (const div of divs) {
div.textContent = Math.random();
}
<div>zero</div>
<div>one</div>
<div>two</div>
<div>three</div>
<div>four</div>
Use forEach and related (ES5+)
The various functions on Array.prototype are "intentionally generic" and can be used on array-like objects via Function#call (spec | MDN) or Function#apply (spec | MDN). (If you have to deal with IE8 or earlier [ouch], see the "Caveat for host-provided objects" at the end of this answer, but it's not an issue with vaguely-modern browsers.)
Suppose you wanted to use forEach on a Node's childNodes collection (which, being an HTMLCollection, doesn't have forEach natively). You'd do this:
Array.prototype.forEach.call(node.childNodes, (child) => {
// Do something with `child`
});
(Note, though, that you could just use for-of on node.childNodes.)
If you're going to do that a lot, you might want to grab a copy of the function reference into a variable for reuse, e.g.:
// (This is all presumably in a module or some scoping function)
const forEach = Array.prototype.forEach.call.bind(Array.prototype.forEach);
// Then later...
forEach(node.childNodes, (child) => {
// Do something with `child`
});
Use a simple for loop
Perhaps obviously, a simple for loop works for array-like objects.
Use an iterator explicitly (ES2015+)
See #1.
You may be able to get away with for-in (with safeguards), but with all of these more appropriate options, there's no reason to try.
Create a true array
Other times, you may want to convert an array-like object into a true array. Doing that is surprisingly easy:
Use Array.from
Array.from (spec) | (MDN) (ES2015+, but easily polyfilled) creates an array from an array-like object, optionally passing the entries through a mapping function first. So:
const divs = Array.from(document.querySelectorAll("div"));
...takes the NodeList from querySelectorAll and makes an array from it.
The mapping function is handy if you were going to map the contents in some way. For instance, if you wanted to get an array of the tag names of the elements with a given class:
// Typical use (with an arrow function):
const divs = Array.from(document.querySelectorAll(".some-class"), element => element.tagName);
// Traditional function (since `Array.from` can be polyfilled):
var divs = Array.from(document.querySelectorAll(".some-class"), function(element) {
return element.tagName;
});
Use spread syntax (...)
It's also possible to use ES2015's spread syntax. Like for-of, this uses the iterator provided by the object (see #1 in the previous section):
const trueArray = [...iterableObject];
So for instance, if we want to convert a NodeList into a true array, with spread syntax this becomes quite succinct:
const divs = [...document.querySelectorAll("div")];
Use the slice method of arrays
We can use the slice method of arrays, which like the other methods mentioned above is "intentionally generic" and so can be used with array-like objects, like this:
const trueArray = Array.prototype.slice.call(arrayLikeObject);
So for instance, if we want to convert a NodeList into a true array, we could do this:
const divs = Array.prototype.slice.call(document.querySelectorAll("div"));
(If you still have to handle IE8 [ouch], will fail; IE8 didn't let you use host-provided objects as this like that.)
Caveat for host-provided objects
If you use Array.prototype functions with host-provided array-like objects (for example, DOM collections and such provided by the browser rather than the JavaScript engine), obsolete browsers like IE8 didn't necessarily handle that way, so if you have to support them, be sure to test in your target environments. But it's not an issue with vaguely-modern browsers. (For non-browser environments, naturally it'll depend on the environment.)

Note: This answer is hopelessly out-of-date. For a more modern approach, look at the methods available on an array. Methods of interest might be:
forEach
map
filter
zip
reduce
every
some
The standard way to iterate an array in JavaScript is a vanilla for-loop:
var length = arr.length,
element = null;
for (var i = 0; i < length; i++) {
element = arr[i];
// Do something with element
}
Note, however, that this approach is only good if you have a dense array, and each index is occupied by an element. If the array is sparse, then you can run into performance problems with this approach, since you will iterate over a lot of indices that do not really exist in the array. In this case, a for .. in-loop might be a better idea. However, you must use the appropriate safeguards to ensure that only the desired properties of the array (that is, the array elements) are acted upon, since the for..in-loop will also be enumerated in legacy browsers, or if the additional properties are defined as enumerable.
In ECMAScript 5 there will be a forEach method on the array prototype, but it is not supported in legacy browsers. So to be able to use it consistently you must either have an environment that supports it (for example, Node.js for server side JavaScript), or use a "Polyfill". The Polyfill for this functionality is, however, trivial and since it makes the code easier to read, it is a good polyfill to include.

If you’re using the jQuery library, you can use jQuery.each:
$.each(yourArray, function(index, value) {
// do your stuff here
});
EDIT :
As per question, user want code in javascript instead of jquery so the edit is
var length = yourArray.length;
for (var i = 0; i < length; i++) {
// Do something with yourArray[i].
}

Loop backwards
I think the reverse for loop deserves a mention here:
for (var i = array.length; i--; ) {
// process array[i]
}
Advantages:
You do not need to declare a temporary len variable, or compare against array.length on each iteration, either of which might be a minute optimisation.
Removing siblings from the DOM in reverse order is usually more efficient. (The browser needs to do less shifting of elements in its internal arrays.)
If you modify the array while looping, at or after index i (for example you remove or insert an item at array[i]), then a forward loop would skip the item that shifted left into position i, or re-process the ith item that was shifted right. In a traditional for loop, you could update i to point to the next item that needs processing - 1, but simply reversing the direction of iteration is often a simpler and more elegant solution.
Similarly, when modifying or removing nested DOM elements, processing in reverse can circumvent errors. For example, consider modifying the innerHTML of a parent node before handling its children. By the time the child node is reached it will be detached from the DOM, having been replaced by a newly created child when the parent's innerHTML was written.
It is shorter to type, and read, than some of the other options available. Although it loses to forEach() and to ES6's for ... of.
Disadvantages:
It processes the items in reverse order. If you were building a new array from the results, or printing things on screen, naturally the output will be reversed with respect to the original order.
Repeatedly inserting siblings into the DOM as a first child in order to retain their order is less efficient. (The browser would keep having to shift things right.) To create DOM nodes efficiently and in order, just loop forwards and append as normal (and also use a "document fragment").
The reverse loop is confusing to junior developers. (You may consider that an advantage, depending on your outlook.)
Should I always use it?
Some developers use the reverse for loop by default, unless there is a good reason to loop forwards.
Although the performance gains are usually insignificant, it sort of screams:
"Just do this to every item in the list, I don't care about the order!"
However in practice that is not actually a reliable indication of intent, since it is indistinguishable from those occasions when you do care about the order, and really do need to loop in reverse. So in fact another construct would be needed to accurately express the "don't care" intent, something currently unavailable in most languages, including ECMAScript, but which could be called, for example, forEachUnordered().
If order doesn't matter, and efficiency is a concern (in the innermost loop of a game or animation engine), then it may be acceptable to use the reverse for loop as your go-to pattern. Just remember that seeing a reverse for loop in existing code does not necessarily mean that the order irrelevant!
It was better to use forEach()
In general for higher level code where clarity and safety are greater concerns, I previously recommended using Array::forEach as your default pattern for looping (although these days I prefer to use for..of). Reasons to prefer forEach over a reverse loop are:
It is clearer to read.
It indicates that i is not going to be shifted within the block (which is always a possible surprise hiding in long for and while loops).
It gives you a free scope for closures.
It reduces leakage of local variables and accidental collision with (and mutation of) outer variables.
Then when you do see the reverse for loop in your code, that is a hint that it is reversed for a good reason (perhaps one of the reasons described above). And seeing a traditional forward for loop may indicate that shifting can take place.
(If the discussion of intent makes no sense to you, then you and your code may benefit from watching Crockford's lecture on Programming Style & Your Brain.)
It is now even better to use for..of!
There is a debate about whether for..of or forEach() are preferable:
For maximum browser support, for..of requires a polyfill for iterators, making your app slightly slower to execute and slightly larger to download.
For that reason (and to encourage use of map and filter), some front-end style guides ban for..of completely!
But the above concerns is not applicable to Node.js applications, where for..of is now well supported.
And furthermore await does not work inside forEach(). Using for..of is the clearest pattern in this case.
Personally, I tend to use whatever looks easiest to read, unless performance or minification has become a major concern. So these days I prefer to use for..of instead of forEach(), but I will always use map or filter or find or some when applicable.
(For the sake of my colleagues, I rarely use reduce.)
How does it work?
for (var i = 0; i < array.length; i++) { ... } // Forwards
for (var i = array.length; i--; ) { ... } // Reverse
You will notice that i-- is the middle clause (where we usually see a comparison) and the last clause is empty (where we usually see i++). That means that i-- is also used as the condition for continuation. Crucially, it is executed and checked before each iteration.
How can it start at array.length without exploding?
Because i-- runs before each iteration, on the first iteration we will actually be accessing the item at array.length - 1 which avoids any issues with Array-out-of-bounds undefined items.
Why doesn't it stop iterating before index 0?
The loop will stop iterating when the condition i-- evaluates to a falsey value (when it yields 0).
The trick is that unlike --i, the trailing i-- operator decrements i but yields the value before the decrement. Your console can demonstrate this:
> var i = 5; [i, i--, i];
[5, 5, 4]
So on the final iteration, i was previously 1 and the i-- expression changes it to 0 but actually yields 1 (truthy), and so the condition passes. On the next iteration i-- changes i to -1 but yields 0 (falsey), causing execution to immediately drop out of the bottom of the loop.
In the traditional forwards for loop, i++ and ++i are interchangeable (as Douglas Crockford points out). However in the reverse for loop, because our decrement is also our condition expression, we must stick with i-- if we want to process the item at index 0.
Trivia
Some people like to draw a little arrow in the reverse for loop, and end with a wink:
for (var i = array.length; i --> 0 ;) {
Credits go to WYL for showing me the benefits and horrors of the reverse for loop.

Some C-style languages use foreach to loop through enumerations. In JavaScript this is done with the for..in loop structure:
var index,
value;
for (index in obj) {
value = obj[index];
}
There is a catch. for..in will loop through each of the object's enumerable members, and the members on its prototype. To avoid reading values that are inherited through the object's prototype, simply check if the property belongs to the object:
for (i in obj) {
if (obj.hasOwnProperty(i)) {
//do stuff
}
}
Additionally, ECMAScript 5 has added a forEach method to Array.prototype which can be used to enumerate over an array using a calback (the polyfill is in the docs so you can still use it for older browsers):
arr.forEach(function (val, index, theArray) {
//do stuff
});
It's important to note that Array.prototype.forEach doesn't break when the callback returns false. jQuery and Underscore.js provide their own variations on each to provide loops that can be short-circuited.

for...of | forEach | map
Using modern JavaScript syntax to iterate through arrays
const fruits = ['🍎', '🍋', '🍌' ]
👉🏽   for...of
for (const fruit of fruits) {
console.log(fruit) // '🍎', '🍋', '🍌'
}
👉🏽   forEach
fruits.forEach(fruit => {
console.log(fruit) // '🍎', '🍋', '🍌'
})
👉🏽   map
*Different from the two above, map() creates a new array and expects you to return something after each iteration.
fruits.map(fruit => fruit) // ['🍎', '🍋', '🍌' ]
🛑  Important: As map() is meant to return a value at each iteration, it is an ideal method for transforming elements in arrays:
fruits.map(fruit => 'cool ' + fruit) // ['cool 🍎', 'cool 🍋', 'cool 🍌' ]
On the other hand, for...of and forEach( ) don't need to return anything and that's why we typically use them to perform logic tasks that manipulate stuff outside.
So to speak, you're going to find if () statements, side effects, and logging activities in these two.
👌🏾  TIP: you can also have the index (as well as the whole array) in each iteration in your .map() or .forEach() functions.
Just pass additional arguments to them:
fruits.map((fruit, i) => i + ' ' + fruit)
// ['0 🍎', '1 🍋', '2 🍌' ]
fruits.forEach((f, i, arr) => {
console.log( f + ' ' + i + ' ' + arr )
})
// 🍎 0 🍎, 🍋, 🍌,
// 🍋 1 🍎, 🍋, 🍌,
// 🍌 2 🍎, 🍋, 🍌,

If you want to loop over an array, use the standard three-part for loop.
for (var i = 0; i < myArray.length; i++) {
var arrayItem = myArray[i];
}
You can get some performance optimisations by caching myArray.length or iterating over it backwards.

If you don't mind emptying the array:
var x;
while(x = y.pop()){
alert(x); //do something
}
x will contain the last value of y and it will be removed from the array. You can also use shift() which will give and remove the first item from y.

A forEach implementation (see in jsFiddle):
function forEach(list,callback) {
var length = list.length;
for (var n = 0; n < length; n++) {
callback.call(list[n]);
}
}
var myArray = ['hello','world'];
forEach(
myArray,
function(){
alert(this); // do something
}
);

I know this is an old post, and there are so many great answers already. For a little more completeness I figured I'd throw in another one using AngularJS. Of course, this only applies if you're using Angular, obviously, nonetheless I'd like to put it anyway.
angular.forEach takes 2 arguments and an optional third argument. The first argument is the object (array) to iterate over, the second argument is the iterator function, and the optional third argument is the object context (basically referred to inside the loop as 'this'.
There are different ways to use the forEach loop of angular. The simplest and probably most used is
var temp = [1, 2, 3];
angular.forEach(temp, function(item) {
//item will be each element in the array
//do something
});
Another way that is useful for copying items from one array to another is
var temp = [1, 2, 3];
var temp2 = [];
angular.forEach(temp, function(item) {
this.push(item); //"this" refers to the array passed into the optional third parameter so, in this case, temp2.
}, temp2);
Though, you don't have to do that, you can simply do the following and it's equivalent to the previous example:
angular.forEach(temp, function(item) {
temp2.push(item);
});
Now there are pros and cons of using the angular.forEach function as opposed to the built in vanilla-flavored for loop.
Pros
Easy readability
Easy writability
If available, angular.forEach will use the ES5 forEach loop. Now, I will get to efficientcy in the cons section, as the forEach loops are much slower than the for loops. I mention this as a pro because it's nice to be consistent and standardized.
Consider the following 2 nested loops, which do exactly the same thing. Let's say that we have 2 arrays of objects and each object contains an array of results, each of which has a Value property that's a string (or whatever). And let's say we need to iterate over each of the results and if they're equal then perform some action:
angular.forEach(obj1.results, function(result1) {
angular.forEach(obj2.results, function(result2) {
if (result1.Value === result2.Value) {
//do something
}
});
});
//exact same with a for loop
for (var i = 0; i < obj1.results.length; i++) {
for (var j = 0; j < obj2.results.length; j++) {
if (obj1.results[i].Value === obj2.results[j].Value) {
//do something
}
}
}
Granted this is a very simple hypothetical example, but I've written triple embedded for loops using the second approach and it was very hard to read, and write for that matter.
Cons
Efficiency. angular.forEach, and the native forEach, for that matter, are both so much slower than the normal for loop....about 90% slower. So for large data sets, best to stick to the native for loop.
No break, continue, or return support. continue is actually supported by "accident", to continue in an angular.forEach you simple put a return; statement in the function like angular.forEach(array, function(item) { if (someConditionIsTrue) return; }); which will cause it to continue out of the function for that iteration. This is also due to the fact that the native forEach does not support break or continue either.
I'm sure there's various other pros and cons as well, and please feel free to add any that you see fit. I feel that, bottom line, if you need efficiency, stick with just the native for loop for your looping needs. But, if your datasets are smaller and a some efficiency is okay to give up in exchange for readability and writability, then by all means throw an angular.forEach in that bad boy.

As of ECMAScript 6:
list = [0, 1, 2, 3]
for (let obj of list) {
console.log(obj)
}
Where of avoids the oddities associated with in and makes it work like the for loop of any other language, and let binds i within the loop as opposed to within the function.
The braces ({}) can be omitted when there is only one command (e.g. in the example above).

Probably the for(i = 0; i < array.length; i++) loop is not the best choice. Why? If you have this:
var array = new Array();
array[1] = "Hello";
array[7] = "World";
array[11] = "!";
The method will call from array[0] to array[2]. First, this will first reference variables you don't even have, second you would not have the variables in the array, and third this will make the code bolder. Look here, it's what I use:
for(var i in array){
var el = array[i];
//If you want 'i' to be INT just put parseInt(i)
//Do something with el
}
And if you want it to be a function, you can do this:
function foreach(array, call){
for(var i in array){
call(array[i]);
}
}
If you want to break, a little more logic:
function foreach(array, call){
for(var i in array){
if(call(array[i]) == false){
break;
}
}
}
Example:
foreach(array, function(el){
if(el != "!"){
console.log(el);
} else {
console.log(el+"!!");
}
});
It returns:
//Hello
//World
//!!!

There are three implementations of foreach in jQuery as follows.
var a = [3,2];
$(a).each(function(){console.log(this.valueOf())}); //Method 1
$.each(a, function(){console.log(this.valueOf())}); //Method 2
$.each($(a), function(){console.log(this.valueOf())}); //Method 3

An easy solution now would be to use the underscore.js library. It's providing many useful tools, such as each and will automatically delegate the job to the native forEach if available.
A CodePen example of how it works is:
var arr = ["elemA", "elemB", "elemC"];
_.each(arr, function(elem, index, ar)
{
...
});
See also
Documentation for native Array.prototype.forEach().
In for_each...in (MDN) it is explained that for each (variable in object) is deprecated as the part of ECMA-357 (EAX) standard.
for...of (MDN) describes the next way of iterating using for (variable of object) as the part of the Harmony (ECMAScript 6) proposal.

There isn't any for each loop in native JavaScript. You can either use libraries to get this functionality (I recommend Underscore.js), use a simple for in loop.
for (var instance in objects) {
...
}
However, note that there may be reasons to use an even simpler for loop (see Stack Overflow question Why is using “for…in” with array iteration such a bad idea?)
var instance;
for (var i=0; i < objects.length; i++) {
var instance = objects[i];
...
}

ECMAScript 5 (the version on JavaScript) to work with Arrays:
forEach - Iterates through every item in the array and do whatever you need with each item.
['C', 'D', 'E'].forEach(function(element, index) {
console.log(element + " is #" + (index+1) + " in the musical scale");
});
// Output
// C is the #1 in musical scale
// D is the #2 in musical scale
// E is the #3 in musical scale
In case, more interested on operation on array using some inbuilt feature.
map - It creates a new array with the result of the callback function. This method is good to be used when you need to format the elements of your array.
// Let's upper case the items in the array
['bob', 'joe', 'jen'].map(function(elem) {
return elem.toUpperCase();
});
// Output: ['BOB', 'JOE', 'JEN']
reduce - As the name says, it reduces the array to a single value by calling the given function passing in the current element and the result of the previous execution.
[1,2,3,4].reduce(function(previous, current) {
return previous + current;
});
// Output: 10
// 1st iteration: previous=1, current=2 => result=3
// 2nd iteration: previous=3, current=3 => result=6
// 3rd iteration: previous=6, current=4 => result=10
every - Returns true or false if all the elements in the array pass the test in the callback function.
// Check if everybody has 18 years old of more.
var ages = [30, 43, 18, 5];
ages.every(function(elem) {
return elem >= 18;
});
// Output: false
filter - Very similar to every except that filter returns an array with the elements that return true to the given function.
// Finding the even numbers
[1,2,3,4,5,6].filter(function(elem){
return (elem % 2 == 0)
});
// Output: [2,4,6]

There are a few ways to loop through an array in JavaScript, as below:
for - it's the most common one. Full block of code for looping
var languages = ["Java", "JavaScript", "C#", "Python"];
var i, len, text;
for (i = 0, len = languages.length, text = ""; i < len; i++) {
text += languages[i] + "<br>";
}
document.getElementById("example").innerHTML = text;
<p id="example"></p>
while - loop while a condition is through. It seems to be the fastest loop
var text = "";
var i = 0;
while (i < 10) {
text += i + ") something<br>";
i++;
}
document.getElementById("example").innerHTML = text;
<p id="example"></p>
do/while - also loop through a block of code while the condition is true, will run at least one time
var text = ""
var i = 0;
do {
text += i + ") something <br>";
i++;
}
while (i < 10);
document.getElementById("example").innerHTML = text;
<p id="example"></p>
Functional loops - forEach, map, filter, also reduce (they loop through the function, but they are used if you need to do something with your array, etc.
// For example, in this case we loop through the number and double them up using the map function
var numbers = [65, 44, 12, 4];
document.getElementById("example").innerHTML = numbers.map(function(num){return num * 2});
<p id="example"></p>
For more information and examples about functional programming on arrays, look at the blog post Functional programming in JavaScript: map, filter and reduce.

This is an iterator for NON-sparse list where the index starts at 0, which is the typical scenario when dealing with document.getElementsByTagName or document.querySelectorAll)
function each( fn, data ) {
if(typeof fn == 'string')
eval('fn = function(data, i){' + fn + '}');
for(var i=0, L=this.length; i < L; i++)
fn.call( this[i], data, i );
return this;
}
Array.prototype.each = each;
Examples of usage:
Example #1
var arr = [];
[1, 2, 3].each( function(a){ a.push( this * this}, arr);
arr = [1, 4, 9]
Example #2
each.call(document.getElementsByTagName('p'), "this.className = data;",'blue');
Each p tag gets class="blue"
Example #3
each.call(document.getElementsByTagName('p'),
"if( i % 2 == 0) this.className = data;",
'red'
);
Every other p tag gets class="red">
Example #4
each.call(document.querySelectorAll('p.blue'),
function(newClass, i) {
if( i < 20 )
this.className = newClass;
}, 'green'
);
And finally the first 20 blue p tags are changed to green
Caution when using string as function: the function is created out-of-context and ought to be used only where you are certain of variable scoping. Otherwise, better to pass functions where scoping is more intuitive.

There's no inbuilt ability to break in forEach. To interrupt execution use the Array#some like below:
[1,2,3].some(function(number) {
return number === 1;
});
This works because some returns true as soon as any of the callbacks, executed in array order, returns true, short-circuiting the execution of the rest.
Original Answer
see Array prototype for some

I also would like to add this as a composition of a reverse loop and an answer above for someone that would like this syntax too.
var foo = [object,object,object];
for (var i = foo.length, item; item = foo[--i];) {
console.log(item);
}
Pros:
The benefit for this: You have the reference already in the first like that won't need to be declared later with another line. It is handy when looping trough the object array.
Cons:
This will break whenever the reference is false - falsey (undefined, etc.). It can be used as an advantage though. However, it would make it a little bit harder to read. And also depending on the browser it can be "not" optimized to work faster than the original one.

jQuery way using $.map:
var data = [1, 2, 3, 4, 5, 6, 7];
var newData = $.map(data, function(element) {
if (element % 2 == 0) {
return element;
}
});
// newData = [2, 4, 6];

Use for...of where possible
async/await support
Skips non-numeric props
Immutable index
for...of
✅
✅
✅
forEach()
❌
✅
✅
for...in
✅
❌
✅
Regular for
✅
✅
❌
As one can see in the table above, for...of should be used wherever it fits. Since it supports async functions, skips non-numeric properties and prevents messing up the loop by accidentally modifying the loop index.
Syntax
const nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const num of nums) {
/* Do something with num */
}
See for...of reference for more examples, link to specification and difference between for...of and for...in. Or maybe check this tutorial for some explanation on how they differ.

Using loops with ECMAScript 6 destructuring and the spread operator
Destructuring and using of the spread operator have proven quite useful for newcomers to ECMAScript 6 as being more human-readable/aesthetic, although some JavaScript veterans might consider it messy. Juniors or some other people might find it useful.
The following examples will use the for...of statement and the .forEach method.
Examples 6, 7, and 8 can be used with any functional loops like .map, .filter, .reduce, .sort, .every, .some. For more information about these methods, check out the Array Object.
Example 1: Normal for...of loop - no tricks here.
let arrSimple = ['a', 'b', 'c'];
for (let letter of arrSimple) {
console.log(letter);
}
Example 2: Split words to characters
let arrFruits = ['apple', 'orange', 'banana'];
for (let [firstLetter, ...restOfTheWord] of arrFruits) {
// Create a shallow copy using the spread operator
let [lastLetter] = [...restOfTheWord].reverse();
console.log(firstLetter, lastLetter, restOfTheWord);
}
Example 3: Looping with a key and value
// let arrSimple = ['a', 'b', 'c'];
// Instead of keeping an index in `i` as per example `for(let i = 0 ; i<arrSimple.length;i++)`
// this example will use a multi-dimensional array of the following format type:
// `arrWithIndex: [number, string][]`
let arrWithIndex = [
[0, 'a'],
[1, 'b'],
[2, 'c'],
];
// Same thing can be achieved using `.map` method
// let arrWithIndex = arrSimple.map((i, idx) => [idx, i]);
// Same thing can be achieved using `Object.entries`
// NOTE: `Object.entries` method doesn't work on Internet Explorer unless it's polyfilled
// let arrWithIndex = Object.entries(arrSimple);
for (let [key, value] of arrWithIndex) {
console.log(key, value);
}
Example 4: Get object properties inline
let arrWithObjects = [{
name: 'Jon',
age: 32
},
{
name: 'Elise',
age: 33
}
];
for (let { name, age: aliasForAge } of arrWithObjects) {
console.log(name, aliasForAge);
}
Example 5: Get deep object properties of what you need
let arrWithObjectsWithArr = [{
name: 'Jon',
age: 32,
tags: ['driver', 'chef', 'jogger']
},
{
name: 'Elise',
age: 33,
tags: ['best chef', 'singer', 'dancer']
}
];
for (let { name, tags: [firstItemFromTags, ...restOfTags] } of arrWithObjectsWithArr) {
console.log(name, firstItemFromTags, restOfTags);
}
Example 6: Is Example 3 used with .forEach
let arrWithIndex = [
[0, 'a'],
[1, 'b'],
[2, 'c'],
];
// Not to be confused here, `forEachIndex` is the real index
// `mappedIndex` was created by "another user", so you can't really trust it
arrWithIndex.forEach(([mappedIndex, item], forEachIndex) => {
console.log(forEachIndex, mappedIndex, item);
});
Example 7: Is Example 4 used with .forEach
let arrWithObjects = [{
name: 'Jon',
age: 32
},
{
name: 'Elise',
age: 33
}
];
// NOTE: Destructuring objects while using shorthand functions
// are required to be surrounded by parentheses
arrWithObjects.forEach( ({ name, age: aliasForAge }) => {
console.log(name, aliasForAge)
});
Example 8: Is Example 5 used with .forEach
let arrWithObjectsWithArr = [{
name: 'Jon',
age: 32,
tags: ['driver', 'chef', 'jogger']
},
{
name: 'Elise',
age: 33,
tags: ['best chef', 'singer', 'dancer']
}
];
arrWithObjectsWithArr.forEach(({
name,
tags: [firstItemFromTags, ...restOfTags]
}) => {
console.log(name, firstItemFromTags, restOfTags);
});

Summary:
When iterating over an array, we often want to accomplish one of the following goals:
We want to iterate over the array and create a new array:
Array.prototype.map
We want to iterate over the array and don't create a new array:
Array.prototype.forEach
for..of loop
In JavaScript, there are many ways of accomplishing both of these goals. However, some are more convenient than others. Below you can find some commonly used methods (the most convenient IMO) to accomplish array iteration in JavaScript.
Creating new array: Map
map() is a function located on Array.prototype which can transform every element of an array and then returns a new array. map() takes as an argument a callback function and works in the following manner:
let arr = [1, 2, 3, 4, 5];
let newArr = arr.map((element, index, array) => {
return element * 2;
})
console.log(arr);
console.log(newArr);
The callback which we have passed into map() as an argument gets executed for every element. Then an array gets returned which has the same length as the original array. In this new array element is transformed by the callback function passed in as an argument to map().
The distinct difference between map and another loop mechanism like forEach and a for..of loop is that map returns a new array and leaves the old array intact (except if you explicitly manipulate it with thinks like splice).
Also, note that the map function's callback provides the index number of the current iteration as a second argument. Furthermore, does the third argument provide the array on which map was called? Sometimes these properties can be very useful.
Loop using forEach
forEach is a function which is located on Array.prototype which takes a callback function as an argument. It then executes this callback function for every element in the array. In contrast to the map() function, the forEach function returns nothing (undefined). For example:
let arr = [1, 2, 3, 4, 5];
arr.forEach((element, index, array) => {
console.log(element * 2);
if (index === 4) {
console.log(array)
}
// index, and oldArray are provided as 2nd and 3th argument by the callback
})
console.log(arr);
Just like the map function, the forEach callback provides the index number of the current iteration as a second argument. Also, does the third argument provide the array on which forEach was called?
Loop through elements using for..of
The for..of loop loops through every element of an array (or any other iterable object). It works in the following manner:
let arr = [1, 2, 3, 4, 5];
for(let element of arr) {
console.log(element * 2);
}
In the above example, element stands for an array element and arr is the array which we want to loop. Note that the name element is arbitrary, and we could have picked any other name like 'el' or something more declarative when this is applicable.
Don't confuse the for..in loop with the for..of loop. for..in will loop through all enumerable properties of the array whereas the for..of loop will only loop through the array elements. For example:
let arr = [1, 2, 3, 4, 5];
arr.foo = 'foo';
for(let element of arr) {
console.log(element);
}
for(let element in arr) {
console.log(element);
}

Performance
Today (2019-12-18) I perform test on my macOS v10.13.6 (High Sierra), on Chrome v 79.0, Safari v13.0.4 and Firefox v71.0 (64 bit) - conclusions about optimisation (and micro-optimisation which usually is not worth to introduce it to code because the benefit is small, but code complexity grows).
It looks like the traditional for i (Aa) is a good choice to write fast code on all browsers.
The other solutions, like for-of (Ad), all in group C.... are usually 2 - 10 (and more) times slower than Aa, but for small arrays it is ok to use it - for the sake of increase code clarity.
The loops with array length cached in n (Ab, Bb, Be) are sometimes faster, sometimes not. Probably compilers automatically detect this situation and introduce caching. The speed differences between the cached and no-cached versions (Aa, Ba, Bd) are about ~1%, so it looks like introduce n is a micro-optimisation.
The i-- like solutions where the loop starts from the last array element (Ac, Bc) are usually ~30% slower than forward solutions - probably the reason is the way of CPU memory cache working - forward memory reading is more optimal for CPU caching). Is recommended to NOT USE such solutions.
Details
In tests we calculate the sum of array elements. I perform a test for small arrays (10 elements) and big arrays (1M elements) and divide them into three groups:
A - for tests
B - while tests
C - other/alternative methods
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
//let arr = Array.from(Array(1000000), (x, i) => i%10);
function Aa(a, s=0) {
for(let i=0; i<a.length; i++) {
s += a[i];
}
console.log('Aa=', s);
}
function Ab(a, s=0) {
let n = a.length;
for(let i=0; i<n; i++) {
s += a[i];
}
console.log('Ab=', s);
}
function Ac(a, s=0) {
for(let i=a.length; i--;) {
s += a[i];
}
console.log('Ac=', s);
}
function Ad(a, s=0) {
for(let x of a) {
s += x;
}
console.log('Ad=', s);
}
function Ae(a, s=0) {
for(let i in a) if (a.hasOwnProperty(i)) {
s += a[i];
}
console.log('Ae=', s);
}
function Ba(a, s=0) {
let i = -1;
while(++i < a.length) {
s+= a[i];
}
console.log('Ba=', s);
}
function Bb(a, s=0) {
let i = -1;
let n = a.length;
while(++i < n) {
s+= a[i];
}
console.log('Bb=', s);
}
function Bc(a, s=0) {
let i = a.length;
while(i--) {
s += a[i];
}
console.log('Bc=', s);
}
function Bd(a, s=0) {
let i = 0;
do {
s+= a[i]
} while (++i < a.length);
console.log('Bd=', s);
}
function Be(a, s=0) {
let i = 0;
let n = a.length;
do {
s += a[i]
} while (++i < n);
console.log('Be=', s);
}
function Bf(a, s=0) {
const it = a.values();
let e;
while (!(e = it.next()).done) {
s+= e.value;
}
console.log('Bf=', s);
}
function Ca(a, s=0) {
a.map(x => { s+=x });
console.log('Ca=', s);
}
function Cb(a, s=0) {
a.forEach(x => { s+=x });
console.log('Cb=', s);
}
function Cc(a, s=0) {
a.every(x => (s += x, 1));
console.log('Cc=', s);
}
function Cd(a, s=0) {
a.filter(x => { s+=x });
console.log('Cd=',s);
}
function Ce(a, s=0) {
a.reduce((z, c) => { s+=c }, 0);
console.log('Ce=', s);
}
function Cf(a, s=0) {
a.reduceRight((z, c) => { s += c }, 0);
console.log('Cf=', s);
}
function Cg(a, s=0) {
a.some(x => { s += x } );
console.log('Cg=', s);
}
function Ch(a, s=0) {
Array.from(a, x=> s += x);
console.log('Cc=', s);
}
Aa(arr);
Ab(arr);
Ac(arr);
Ad(arr);
Ae(arr);
Ba(arr);
Bb(arr);
Bc(arr);
Bd(arr);
Be(arr);
Bf(arr);
Ca(arr);
Cb(arr);
Cc(arr);
Cd(arr);
Ce(arr);
Cf(arr);
Cg(arr);
Ch(arr);
<p style="color: red">This snippets only PRESENTS code used for benchmark - it not perform test itself</p>
Cross browser results
Results for all tested browsers
browsers**
Array with 10 elements
Results for Chrome. You can perform the test on your machine here.
Array with 1,000,000 elements
Results for Chrome. You can perform the test on your machine here

A way closest to your idea would be to use Array.forEach() which accepts a closure function which will be executed for each element of the array.
myArray.forEach(
(item) => {
// Do something
console.log(item);
}
);
Another viable way would be to use Array.map() which works in the same way, but it also takes all values that you return and returns them in a new array (essentially mapping each element to a new one), like this:
var myArray = [1, 2, 3];
myArray = myArray.map(
(item) => {
return item + 1;
}
);
console.log(myArray); // [2, 3, 4]

As per the new updated feature ECMAScript 6 (ES6) and ECMAScript 2015, you can use the following options with loops:
for loops
for(var i = 0; i < 5; i++){
console.log(i);
}
// Output: 0,1,2,3,4
for...in loops
let obj = {"a":1, "b":2}
for(let k in obj){
console.log(k)
}
// Output: a,b
Array.forEach()
let array = [1,2,3,4]
array.forEach((x) => {
console.log(x);
})
// Output: 1,2,3,4
for...of loops
let array = [1,2,3,4]
for(let x of array){
console.log(x);
}
// Output: 1,2,3,4
while loops
let x = 0
while(x < 5){
console.log(x)
x++
}
// Output: 1,2,3,4
do...while loops
let x = 0
do{
console.log(x)
x++
}while(x < 5)
// Output: 1,2,3,4

The lambda syntax doesn't usually work in Internet Explorer 10 or below.
I usually use the
[].forEach.call(arrayName,function(value,index){
console.log("value of the looped element" + value);
console.log("index of the looped element" + index);
});
If you are a jQuery fan and already have a jQuery file running, you should reverse the positions of the index and value parameters
$("#ul>li").each(function(**index, value**){
console.log("value of the looped element" + value);
console.log("index of the looped element" + index);
});

You can call forEach like this:
forEach will iterate over the array you provide and for each iteration it will have element which holds the value of that iteration. If you need index you can get the current index by passing the i as the second parameter in the callback function for forEach.
Foreach is basically a High Order Function, Which takes another function as its parameter.
let theArray= [1,3,2];
theArray.forEach((element) => {
// Use the element of the array
console.log(element)
}
Output:
1
3
2
You can also iterate over an array like this:
for (let i=0; i<theArray.length; i++) {
console.log(i); // i will have the value of each index
}

If you want to use forEach(), it will look like -
theArray.forEach ( element => {
console.log(element);
});
If you want to use for(), it will look like -
for(let idx = 0; idx < theArray.length; idx++){
let element = theArray[idx];
console.log(element);
}

How can we understand and create this parameter inside function in javascript?

I am a newbie to JS language. I have a lot of confusion in variable declaration that is created with functions. It is usual when you create and call as follows:
function sum(a, b) {
return a + b
}
We know a, b can be called later by values, no doubt in that
console.log(sum(1, 2))
But I have a doubt in parameters passed in in-built functions and in general. They are:
var words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
const result = words.filter(function (word) { return word.length > 6} );
console.log(result);
How this word parameter inside anonymous function grasps each element in array? What is the concept behind that?
Not only in this in-built function, this is my Javascript doubt in general. Every time it confuses me. Kindly someone explain me how to create and use this kind of parameters efficiently.

From my understanding, you're confused on how the anonymous / lambda function operates on the indefinite number of parameters.
Lets write our own filter function to see how it works. First, we'll need two things, an array of objects / values, and some way to map these values to either true (should be in the results array) or false (should not be in the results array). Then all we need to do is select the values that evaluated to try and return them.
// Define array
var words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
// Define filter
function hasAnE(word) {
return word.includes('e');
}
// Examples
hasAnE(words[0]); // false
hasAnE(words[1]); // false
hasAnE(words[2]); // true
// Create an array for our results
filteredWords = [];
// Go through our words array and check for matches
for (var index = 0; index < words.length; index ++) {
word = words[index];
// Map each element to true / false
filterResult = hasAnE(word);
// If it matches the filter, add it to the results
if (filterResult) {
filteredWords.push(word);
}
}
console.log(filteredWords); // [ 'elite', 'exuberant', 'destruction', 'present' ]
Ok, so we can build a this filter functionality ourselves, but how do we get to words.filter(hasAnE)? Well the first thing you need to understand is that in JavaScript, you can pass around functions themselves like variables. This means that we can write a function that takes another function as a parameter (the passed function is called a 'callback').
function filterWords(wordsArray, filter) {
filteredWords = [];
for (var index = 0; index < wordsArray.length; index ++) {
word = wordsArray[index];
// Map each element to true / false
filterResult = filter(word);
// If it matches the filter, add it to the results
if (filterResult) {
filteredWords.push(word);
}
}
return filteredWords;
}
console.log(filterWords(words, hasAnE)); // [ 'elite', 'exuberant', 'destruction', 'present' ]
Notice we get the same results as before. JavaScript allows us to not only pass the filter as an input parameter, but also call it seamlessly. But how is it that we can apply the filter directly to the words array itself? This is where prototypes come in handy as it allows me to change the 'base' code for an existing class. For instance, Array.prototype gives me access to all the default fields and methods of the Array class (which our words array is a one of). So using our knowledge that functions can be stored as variables, we can do this:
function filterWordsPrototype(filter) {
filteredWords = [];
for (var index = 0; index < this.length; index ++) {
word = this[index];
// Map each element to true / false
filterResult = filter(word);
// If it matches the filter, add it to the results
if (filterResult) {
filteredWords.push(word);
}
}
return filteredWords;
}
Array.prototype.filterWords = filterWordsPrototype;
console.log(words.filterWords(hasAnE)); // [ 'elite', 'exuberant', 'destruction', 'present' ]
No magic here. and our final words.filterWords(hasAnE) looks very close the original words.filter(someFunction) Notice how I got rid of the array parameter and changed all the references to the array to this instead. That is because I set the function as a method of the array itself (and of all arrays), so this now refers to the array I'm calling the function on.
Now of course the actual implementation is far more efficient, safe, and verbose than this, but I hope I've answered your question about the what's roughly going on under the hood.

As Daniel has pointed out in the comment your function is a callback function (as in don't call use we'll call you later).
Under the hood the array filter function is implemented something like this:
Array.prototype.filter = function (callback) {
const goodValues = []
for (let i = 0; i < this.length; i++) {
if (callback(this[i])) { // your callback is used here
goodValues.push(this[i])
}
}
return goodValues
}
The gist of it is that you supply a function (functions are objects too) as a parameter. Then the filter implementation of filter will use your function to determine if it should keep a value or throw it away.
The Array.prototype just means that every time you create a new array [] it will have the filter function. Although strongly discoursed you can add your own functions to array as well. Sometimes I had a dump helper to understand the intermediate values if you chain many maps and filters.
Here is a demo

Use reduce function to compare symbol in the two string

I try to compare two strings in array on equal symbols or char,this code works, but how to implement it in ES6 with reduce method, if I have more than two strings an array. I need to return true if the string in the first element of the array contains all of the letters of the string in the second element of the array. But how to create the more flexible function if I have more than 2 elments in the array.
function mutation(arr) {
var arr2 = arr.map(item => item.toLowerCase().split(''));
for (i=0;i<arr2[1].length;i++) {
if (arr2[0].indexOf(arr2[1][i]) < 0)
return false;
}
return true;
}
mutation(["hello", "hey"]);

#Palaniichuk I thought your original algorithm was pretty solid. To handle your request I was able to create a solution that uses reduce.
I do have one question for you. Should the array increase in size, how would the strings be evaluated?
The reason I ask is because using a helper function like this might help you scale this algorithm. Of course it all depends on how the input changes. How the inputs are evaluated.
function makeStr(string) {
const reducer = string.split('').reduce((a, b) => {
a[b] = a[b] + 1 || 1;
return a;
}, {});
return Object.keys(reducer).sort().join('');
}
function secondMutation(arr) {
const array = [...arr].map(makeStr);
return array[0].includes(array[1]);
};
console.log(secondMutation(["hello", "hell"]));

How to loop through a json array in Flask by javascript? [duplicate]

How can I loop through all the entries in an array using JavaScript?

TL;DR
Your best bets are usually
a for-of loop (ES2015+ only; spec | MDN) - simple and async-friendly
for (const element of theArray) {
// ...use `element`...
}
forEach (ES5+ only; spec | MDN) (or its relatives some and such) - not async-friendly (but see details)
theArray.forEach(element => {
// ...use `element`...
});
a simple old-fashioned for loop - async-friendly
for (let index = 0; index < theArray.length; ++index) {
const element = theArray[index];
// ...use `element`...
}
(rarely) for-in with safeguards - async-friendly
for (const propertyName in theArray) {
if (/*...is an array element property (see below)...*/) {
const element = theArray[propertyName];
// ...use `element`...
}
}
Some quick "don't"s:
Don't use for-in unless you use it with safeguards or are at least aware of why it might bite you.
Don't use map if you're not using its return value.(There's sadly someone out there teaching map [spec / MDN] as though it were forEach — but as I write on my blog, that's not what it's for. If you aren't using the array it creates, don't use map.)
Don't use forEach if the callback does asynchronous work and you want the forEach to wait until that work is done (because it won't).
But there's lots more to explore, read on...
JavaScript has powerful semantics for looping through arrays and array-like objects. I've split the answer into two parts: Options for genuine arrays, and options for things that are just array-like, such as the arguments object, other iterable objects (ES2015+), DOM collections, and so on.
Okay, let's look at our options:
For Actual Arrays
You have five options (two supported basically forever, another added by ECMAScript 5 ["ES5"], and two more added in ECMAScript 2015 ("ES2015", aka "ES6"):
Use for-of (use an iterator implicitly) (ES2015+)
Use forEach and related (ES5+)
Use a simple for loop
Use for-in correctly
Use an iterator explicitly (ES2015+)
(You can see those old specs here: ES5, ES2015, but both have been superceded; the current editor's draft is always here.)
Details:
1. Use for-of (use an iterator implicitly) (ES2015+)
ES2015 added iterators and iterables to JavaScript. Arrays are iterable (so are strings, Maps, and Sets, as well as DOM collections and lists, as you'll see later). Iterable objects provide iterators for their values. The new for-of statement loops through the values returned by an iterator:
const a = ["a", "b", "c"];
for (const element of a) { // You can use `let` instead of `const` if you like
console.log(element);
}
// a
// b
// c
It doesn't get simpler than that! Under the covers, that gets an iterator from the array and loops through the values the iterator returns. The iterator provided by arrays provides the values of the array elements, in order beginning to end.
Notice how element is scoped to each loop iteration; trying to use element after the end of the loop would fail because it doesn't exist outside the loop body.
In theory, a for-of loop involves several function calls (one to get the iterator, then one to get each value from it). Even when that's true, it's nothing to worry about, function calls are very cheap in modern JavaScript engines (it bothered me for forEach [below] until I looked into it; details). But additionally, JavaScript engines optimize those calls away (in performance-critical code) when dealing with native iterators for things like arrays.
for-of is entirely async-friendly. If you need the work in a loop body to be done in series (not in parallel), an await in the loop body will wait for the promise to settle before continuing. Here's a silly example:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (const message of messages) {
await delay(400);
console.log(message);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
Note how the words appear with a delay before each one.
It's a matter of coding style, but for-of is the first thing I reach for when looping through anything iterable.
2. Use forEach and related
In any even vaguely-modern environment (so, not IE8) where you have access to the Array features added by ES5, you can use forEach (spec | MDN) if you're only dealing with synchronous code (or you don't need to wait for an asynchronous process to finish during the loop):
const a = ["a", "b", "c"];
a.forEach((element) => {
console.log(element);
});
forEach accepts a callback function and, optionally, a value to use as this when calling that callback (not used above). The callback is called for each element in the array, in order, skipping non-existent elements in sparse arrays. Although I only used one parameter above, the callback is called with three arguments: The element for that iteration, the index of that element, and a reference to the array you're iterating over (in case your function doesn't already have it handy).
Like for-of, forEach has the advantage that you don't have to declare indexing and value variables in the containing scope; in this case, they're supplied as arguments to the iteration function, and so nicely scoped to just that iteration.
Unlike for-of, forEach has the disadvantage that it doesn't understand async functions and await. If you use an async function as the callback, forEach does not wait for that function's promise to settle before continuing. Here's the async example from for-of using forEach instead — notice how there's an initial delay, but then all the text appears right away instead of waiting:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
// INCORRECT, doesn't wait before continuing,
// doesn't handle promise rejections
messages.forEach(async message => {
await delay(400);
console.log(message);
});
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
forEach is the "loop through them all" function, but ES5 defined several other useful "work your way through the array and do things" functions, including:
every (spec | MDN) - stops looping the first time the callback returns a falsy value
some (spec | MDN) - stops looping the first time the callback returns a truthy value
filter (spec | MDN) - creates a new array including elements where the callback returns a truthy value, omitting the ones where it doesn't
map (spec | MDN) - creates a new array from the values returned by the callback
reduce (spec | MDN) - builds up a value by repeatedly calling the callback, passing in previous values; see the spec for the details
reduceRight (spec | MDN) - like reduce, but works in descending rather than ascending order
As with forEach, if you use an async function as your callback, none of those waits for the function's promise to settle. That means:
Using an async function callback is never appropriate with every, some, and filter since they will treat the returned promise as though it were a truthy value; they don't wait for the promise to settle and then use the fulfillment value.
Using an async function callback is often appropriate with map, if the goal is to turn an array of something into an array of promises, perhaps for passing to one of the promise combinator functions (Promise.all, Promise.race, promise.allSettled, or Promise.any).
Using an async function callback is rarely appropriate with reduce or reduceRight, because (again) the callback will always return a promise. But there is an idiom of building a chain of promises from an array that uses reduce (const promise = array.reduce((p, element) => p.then(/*...something using `element`...*/));), but usually in those cases a for-of or for loop in an async function will be clearer and easier to debug.
3. Use a simple for loop
Sometimes the old ways are the best:
const a = ["a", "b", "c"];
for (let index = 0; index < a.length; ++index) {
const element = a[index];
console.log(element);
}
If the length of the array won't change during the loop, and it's in highly performance-sensitive code, a slightly more complicated version grabbing the length up front might be a tiny bit faster:
const a = ["a", "b", "c"];
for (let index = 0, len = a.length; index < len; ++index) {
const element = a[index];
console.log(element);
}
And/or counting backward:
const a = ["a", "b", "c"];
for (let index = a.length - 1; index >= 0; --index) {
const element = a[index];
console.log(element);
}
But with modern JavaScript engines, it's rare you need to eke out that last bit of juice.
Before ES2015, the loop variable had to exist in the containing scope, because var only has function-level scope, not block-level scope. But as you saw in the examples above, you can use let within the for to scope the variables to just the loop. And when you do that, the index variable is recreated for each loop iteration, meaning closures created in the loop body keep a reference to the index for that specific iteration, which solves the old "closures in loops" problem:
// (The `NodeList` from `querySelectorAll` is array-like)
const divs = document.querySelectorAll("div");
for (let index = 0; index < divs.length; ++index) {
divs[index].addEventListener('click', e => {
console.log("Index is: " + index);
});
}
<div>zero</div>
<div>one</div>
<div>two</div>
<div>three</div>
<div>four</div>
In the above, you get "Index is: 0" if you click the first and "Index is: 4" if you click the last. This does not work if you use var instead of let (you'd always see "Index is: 5").
Like for-of, for loops work well in async functions. Here's the earlier example using a for loop:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (let i = 0; i < messages.length; ++i) {
const message = messages[i];
await delay(400);
console.log(message);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
4. Use for-in correctly
for-in isn't for looping through arrays, it's for looping through the names of an object's properties. It does often seem to work for looping through arrays as a by-product of the fact that arrays are objects, but it doesn't just loop through the array indexes, it loops through all enumerable properties of the object (including inherited ones). (It also used to be that the order wasn't specified; it is now [details in this other answer], but even though the order is specified now, the rules are complex, there are exceptions, and relying on the order is not best practice.)
The only real use cases for for-in on an array are:
It's a sparse array with massive gaps in it, or
You're using non-element properties on the array object and you want to include them in the loop
Looking only at that first example: You can use for-in to visit those sparse array elements if you use appropriate safeguards:
// `a` is a sparse array
const a = [];
a[0] = "a";
a[10] = "b";
a[10000] = "c";
for (const name in a) {
if (Object.hasOwn(a, name) && // These checks are
/^0$|^[1-9]\d*$/.test(name) && // explained
name <= 4294967294 // below
) {
const element = a[name];
console.log(a[name]);
}
}
Note the three checks:
That the object has its own property by that name (not one it inherits from its prototype; this check is also often written as a.hasOwnProperty(name) but ES2022 adds Object.hasOwn which can be more reliable), and
That the name is all decimal digits (e.g., normal string form, not scientific notation), and
That the name's value when coerced to a number is <= 2^32 - 2 (which is 4,294,967,294). Where does that number come from? It's part of the definition of an array index in the specification. Other numbers (non-integers, negative numbers, numbers greater than 2^32 - 2) are not array indexes. The reason it's 2^32 - 2 is that that makes the greatest index value one lower than 2^32 - 1, which is the maximum value an array's length can have. (E.g., an array's length fits in a 32-bit unsigned integer.)
...although with that said, most code only does the hasOwnProperty check.
You wouldn't do that in inline code, of course. You'd write a utility function. Perhaps:
// Utility function for antiquated environments without `forEach`
const hasOwn = Object.prototype.hasOwnProperty.call.bind(Object.prototype.hasOwnProperty);
const rexNum = /^0$|^[1-9]\d*$/;
function sparseEach(array, callback, thisArg) {
for (const name in array) {
const index = +name;
if (hasOwn(a, name) &&
rexNum.test(name) &&
index <= 4294967294
) {
callback.call(thisArg, array[name], index, array);
}
}
}
const a = [];
a[5] = "five";
a[10] = "ten";
a[100000] = "one hundred thousand";
a.b = "bee";
sparseEach(a, (value, index) => {
console.log("Value at " + index + " is " + value);
});
Like for, for-in works well in asynchronous functions if the work within it needs to be done in series.
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
for (const name in messages) {
if (messages.hasOwnProperty(name)) { // Almost always this is the only check people do
const message = messages[name];
await delay(400);
console.log(message);
}
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
5. Use an iterator explicitly (ES2015+)
for-of uses an iterator implicitly, doing all the scut work for you. Sometimes, you might want to use an iterator explicitly. It looks like this:
const a = ["a", "b", "c"];
const it = a.values(); // Or `const it = a[Symbol.iterator]();` if you like
let entry;
while (!(entry = it.next()).done) {
const element = entry.value;
console.log(element);
}
An iterator is an object matching the Iterator definition in the specification. Its next method returns a new result object each time you call it. The result object has a property, done, telling us whether it's done, and a property value with the value for that iteration. (done is optional if it would be false, value is optional if it would be undefined.)
What you get for value varies depending on the iterator. On arrays, the default iterator provides the value of each array element ("a", "b", and "c" in the example earlier). Arrays also have three other methods that return iterators:
values(): This is an alias for the [Symbol.iterator] method that returns the default iterator.
keys(): Returns an iterator that provides each key (index) in the array. In the example above, it would provide "0", then "1", then "2" (yes, as strings).
entries(): Returns an iterator that provides [key, value] arrays.
Since iterator objects don't advance until you call next, they work well in async function loops. Here's the earlier for-of example using the iterator explicitly:
function delay(ms) {
return new Promise(resolve => {
setTimeout(resolve, ms);
});
}
async function showSlowly(messages) {
const it = messages.values()
while (!(entry = it.next()).done) {
await delay(400);
const element = entry.value;
console.log(element);
}
}
showSlowly([
"So", "long", "and", "thanks", "for", "all", "the", "fish!"
]);
// `.catch` omitted because we know it never rejects
For Array-Like Objects
Aside from true arrays, there are also array-like objects that have a length property and properties with all-digits names: NodeList instances, HTMLCollection instances, the arguments object, etc. How do we loop through their contents?
Use most of the options above
At least some, and possibly most or even all, of the array approaches above apply equally well to array-like objects:
Use for-of (use an iterator implicitly) (ES2015+)
for-of uses the iterator provided by the object (if any). That includes host-provided objects (like DOM collections and lists). For instance, HTMLCollection instances from getElementsByXYZ methods and NodeLists instances from querySelectorAll both support iteration. (This is defined quite subtly by the HTML and DOM specifications. Basically, any object with length and indexed access is automatically iterable. It doesn't have to be marked iterable; that is used only for collections that, in addition to being iterable, support forEach, values, keys, and entries methods. NodeList does; HTMLCollection doesn't, but both are iterable.)
Here's an example of looping through div elements:
const divs = document.querySelectorAll("div");
for (const div of divs) {
div.textContent = Math.random();
}
<div>zero</div>
<div>one</div>
<div>two</div>
<div>three</div>
<div>four</div>
Use forEach and related (ES5+)
The various functions on Array.prototype are "intentionally generic" and can be used on array-like objects via Function#call (spec | MDN) or Function#apply (spec | MDN). (If you have to deal with IE8 or earlier [ouch], see the "Caveat for host-provided objects" at the end of this answer, but it's not an issue with vaguely-modern browsers.)
Suppose you wanted to use forEach on a Node's childNodes collection (which, being an HTMLCollection, doesn't have forEach natively). You'd do this:
Array.prototype.forEach.call(node.childNodes, (child) => {
// Do something with `child`
});
(Note, though, that you could just use for-of on node.childNodes.)
If you're going to do that a lot, you might want to grab a copy of the function reference into a variable for reuse, e.g.:
// (This is all presumably in a module or some scoping function)
const forEach = Array.prototype.forEach.call.bind(Array.prototype.forEach);
// Then later...
forEach(node.childNodes, (child) => {
// Do something with `child`
});
Use a simple for loop
Perhaps obviously, a simple for loop works for array-like objects.
Use an iterator explicitly (ES2015+)
See #1.
You may be able to get away with for-in (with safeguards), but with all of these more appropriate options, there's no reason to try.
Create a true array
Other times, you may want to convert an array-like object into a true array. Doing that is surprisingly easy:
Use Array.from
Array.from (spec) | (MDN) (ES2015+, but easily polyfilled) creates an array from an array-like object, optionally passing the entries through a mapping function first. So:
const divs = Array.from(document.querySelectorAll("div"));
...takes the NodeList from querySelectorAll and makes an array from it.
The mapping function is handy if you were going to map the contents in some way. For instance, if you wanted to get an array of the tag names of the elements with a given class:
// Typical use (with an arrow function):
const divs = Array.from(document.querySelectorAll(".some-class"), element => element.tagName);
// Traditional function (since `Array.from` can be polyfilled):
var divs = Array.from(document.querySelectorAll(".some-class"), function(element) {
return element.tagName;
});
Use spread syntax (...)
It's also possible to use ES2015's spread syntax. Like for-of, this uses the iterator provided by the object (see #1 in the previous section):
const trueArray = [...iterableObject];
So for instance, if we want to convert a NodeList into a true array, with spread syntax this becomes quite succinct:
const divs = [...document.querySelectorAll("div")];
Use the slice method of arrays
We can use the slice method of arrays, which like the other methods mentioned above is "intentionally generic" and so can be used with array-like objects, like this:
const trueArray = Array.prototype.slice.call(arrayLikeObject);
So for instance, if we want to convert a NodeList into a true array, we could do this:
const divs = Array.prototype.slice.call(document.querySelectorAll("div"));
(If you still have to handle IE8 [ouch], will fail; IE8 didn't let you use host-provided objects as this like that.)
Caveat for host-provided objects
If you use Array.prototype functions with host-provided array-like objects (for example, DOM collections and such provided by the browser rather than the JavaScript engine), obsolete browsers like IE8 didn't necessarily handle that way, so if you have to support them, be sure to test in your target environments. But it's not an issue with vaguely-modern browsers. (For non-browser environments, naturally it'll depend on the environment.)

Note: This answer is hopelessly out-of-date. For a more modern approach, look at the methods available on an array. Methods of interest might be:
forEach
map
filter
zip
reduce
every
some
The standard way to iterate an array in JavaScript is a vanilla for-loop:
var length = arr.length,
element = null;
for (var i = 0; i < length; i++) {
element = arr[i];
// Do something with element
}
Note, however, that this approach is only good if you have a dense array, and each index is occupied by an element. If the array is sparse, then you can run into performance problems with this approach, since you will iterate over a lot of indices that do not really exist in the array. In this case, a for .. in-loop might be a better idea. However, you must use the appropriate safeguards to ensure that only the desired properties of the array (that is, the array elements) are acted upon, since the for..in-loop will also be enumerated in legacy browsers, or if the additional properties are defined as enumerable.
In ECMAScript 5 there will be a forEach method on the array prototype, but it is not supported in legacy browsers. So to be able to use it consistently you must either have an environment that supports it (for example, Node.js for server side JavaScript), or use a "Polyfill". The Polyfill for this functionality is, however, trivial and since it makes the code easier to read, it is a good polyfill to include.

If you’re using the jQuery library, you can use jQuery.each:
$.each(yourArray, function(index, value) {
// do your stuff here
});
EDIT :
As per question, user want code in javascript instead of jquery so the edit is
var length = yourArray.length;
for (var i = 0; i < length; i++) {
// Do something with yourArray[i].
}

Loop backwards
I think the reverse for loop deserves a mention here:
for (var i = array.length; i--; ) {
// process array[i]
}
Advantages:
You do not need to declare a temporary len variable, or compare against array.length on each iteration, either of which might be a minute optimisation.
Removing siblings from the DOM in reverse order is usually more efficient. (The browser needs to do less shifting of elements in its internal arrays.)
If you modify the array while looping, at or after index i (for example you remove or insert an item at array[i]), then a forward loop would skip the item that shifted left into position i, or re-process the ith item that was shifted right. In a traditional for loop, you could update i to point to the next item that needs processing - 1, but simply reversing the direction of iteration is often a simpler and more elegant solution.
Similarly, when modifying or removing nested DOM elements, processing in reverse can circumvent errors. For example, consider modifying the innerHTML of a parent node before handling its children. By the time the child node is reached it will be detached from the DOM, having been replaced by a newly created child when the parent's innerHTML was written.
It is shorter to type, and read, than some of the other options available. Although it loses to forEach() and to ES6's for ... of.
Disadvantages:
It processes the items in reverse order. If you were building a new array from the results, or printing things on screen, naturally the output will be reversed with respect to the original order.
Repeatedly inserting siblings into the DOM as a first child in order to retain their order is less efficient. (The browser would keep having to shift things right.) To create DOM nodes efficiently and in order, just loop forwards and append as normal (and also use a "document fragment").
The reverse loop is confusing to junior developers. (You may consider that an advantage, depending on your outlook.)
Should I always use it?
Some developers use the reverse for loop by default, unless there is a good reason to loop forwards.
Although the performance gains are usually insignificant, it sort of screams:
"Just do this to every item in the list, I don't care about the order!"
However in practice that is not actually a reliable indication of intent, since it is indistinguishable from those occasions when you do care about the order, and really do need to loop in reverse. So in fact another construct would be needed to accurately express the "don't care" intent, something currently unavailable in most languages, including ECMAScript, but which could be called, for example, forEachUnordered().
If order doesn't matter, and efficiency is a concern (in the innermost loop of a game or animation engine), then it may be acceptable to use the reverse for loop as your go-to pattern. Just remember that seeing a reverse for loop in existing code does not necessarily mean that the order irrelevant!
It was better to use forEach()
In general for higher level code where clarity and safety are greater concerns, I previously recommended using Array::forEach as your default pattern for looping (although these days I prefer to use for..of). Reasons to prefer forEach over a reverse loop are:
It is clearer to read.
It indicates that i is not going to be shifted within the block (which is always a possible surprise hiding in long for and while loops).
It gives you a free scope for closures.
It reduces leakage of local variables and accidental collision with (and mutation of) outer variables.
Then when you do see the reverse for loop in your code, that is a hint that it is reversed for a good reason (perhaps one of the reasons described above). And seeing a traditional forward for loop may indicate that shifting can take place.
(If the discussion of intent makes no sense to you, then you and your code may benefit from watching Crockford's lecture on Programming Style & Your Brain.)
It is now even better to use for..of!
There is a debate about whether for..of or forEach() are preferable:
For maximum browser support, for..of requires a polyfill for iterators, making your app slightly slower to execute and slightly larger to download.
For that reason (and to encourage use of map and filter), some front-end style guides ban for..of completely!
But the above concerns is not applicable to Node.js applications, where for..of is now well supported.
And furthermore await does not work inside forEach(). Using for..of is the clearest pattern in this case.
Personally, I tend to use whatever looks easiest to read, unless performance or minification has become a major concern. So these days I prefer to use for..of instead of forEach(), but I will always use map or filter or find or some when applicable.
(For the sake of my colleagues, I rarely use reduce.)
How does it work?
for (var i = 0; i < array.length; i++) { ... } // Forwards
for (var i = array.length; i--; ) { ... } // Reverse
You will notice that i-- is the middle clause (where we usually see a comparison) and the last clause is empty (where we usually see i++). That means that i-- is also used as the condition for continuation. Crucially, it is executed and checked before each iteration.
How can it start at array.length without exploding?
Because i-- runs before each iteration, on the first iteration we will actually be accessing the item at array.length - 1 which avoids any issues with Array-out-of-bounds undefined items.
Why doesn't it stop iterating before index 0?
The loop will stop iterating when the condition i-- evaluates to a falsey value (when it yields 0).
The trick is that unlike --i, the trailing i-- operator decrements i but yields the value before the decrement. Your console can demonstrate this:
> var i = 5; [i, i--, i];
[5, 5, 4]
So on the final iteration, i was previously 1 and the i-- expression changes it to 0 but actually yields 1 (truthy), and so the condition passes. On the next iteration i-- changes i to -1 but yields 0 (falsey), causing execution to immediately drop out of the bottom of the loop.
In the traditional forwards for loop, i++ and ++i are interchangeable (as Douglas Crockford points out). However in the reverse for loop, because our decrement is also our condition expression, we must stick with i-- if we want to process the item at index 0.
Trivia
Some people like to draw a little arrow in the reverse for loop, and end with a wink:
for (var i = array.length; i --> 0 ;) {
Credits go to WYL for showing me the benefits and horrors of the reverse for loop.

Some C-style languages use foreach to loop through enumerations. In JavaScript this is done with the for..in loop structure:
var index,
value;
for (index in obj) {
value = obj[index];
}
There is a catch. for..in will loop through each of the object's enumerable members, and the members on its prototype. To avoid reading values that are inherited through the object's prototype, simply check if the property belongs to the object:
for (i in obj) {
if (obj.hasOwnProperty(i)) {
//do stuff
}
}
Additionally, ECMAScript 5 has added a forEach method to Array.prototype which can be used to enumerate over an array using a calback (the polyfill is in the docs so you can still use it for older browsers):
arr.forEach(function (val, index, theArray) {
//do stuff
});
It's important to note that Array.prototype.forEach doesn't break when the callback returns false. jQuery and Underscore.js provide their own variations on each to provide loops that can be short-circuited.

for...of | forEach | map
Using modern JavaScript syntax to iterate through arrays
const fruits = ['🍎', '🍋', '🍌' ]
👉🏽   for...of
for (const fruit of fruits) {
console.log(fruit) // '🍎', '🍋', '🍌'
}
👉🏽   forEach
fruits.forEach(fruit => {
console.log(fruit) // '🍎', '🍋', '🍌'
})
👉🏽   map
*Different from the two above, map() creates a new array and expects you to return something after each iteration.
fruits.map(fruit => fruit) // ['🍎', '🍋', '🍌' ]
🛑  Important: As map() is meant to return a value at each iteration, it is an ideal method for transforming elements in arrays:
fruits.map(fruit => 'cool ' + fruit) // ['cool 🍎', 'cool 🍋', 'cool 🍌' ]
On the other hand, for...of and forEach( ) don't need to return anything and that's why we typically use them to perform logic tasks that manipulate stuff outside.
So to speak, you're going to find if () statements, side effects, and logging activities in these two.
👌🏾  TIP: you can also have the index (as well as the whole array) in each iteration in your .map() or .forEach() functions.
Just pass additional arguments to them:
fruits.map((fruit, i) => i + ' ' + fruit)
// ['0 🍎', '1 🍋', '2 🍌' ]
fruits.forEach((f, i, arr) => {
console.log( f + ' ' + i + ' ' + arr )
})
// 🍎 0 🍎, 🍋, 🍌,
// 🍋 1 🍎, 🍋, 🍌,
// 🍌 2 🍎, 🍋, 🍌,

If you want to loop over an array, use the standard three-part for loop.
for (var i = 0; i < myArray.length; i++) {
var arrayItem = myArray[i];
}
You can get some performance optimisations by caching myArray.length or iterating over it backwards.

If you don't mind emptying the array:
var x;
while(x = y.pop()){
alert(x); //do something
}
x will contain the last value of y and it will be removed from the array. You can also use shift() which will give and remove the first item from y.

A forEach implementation (see in jsFiddle):
function forEach(list,callback) {
var length = list.length;
for (var n = 0; n < length; n++) {
callback.call(list[n]);
}
}
var myArray = ['hello','world'];
forEach(
myArray,
function(){
alert(this); // do something
}
);

I know this is an old post, and there are so many great answers already. For a little more completeness I figured I'd throw in another one using AngularJS. Of course, this only applies if you're using Angular, obviously, nonetheless I'd like to put it anyway.
angular.forEach takes 2 arguments and an optional third argument. The first argument is the object (array) to iterate over, the second argument is the iterator function, and the optional third argument is the object context (basically referred to inside the loop as 'this'.
There are different ways to use the forEach loop of angular. The simplest and probably most used is
var temp = [1, 2, 3];
angular.forEach(temp, function(item) {
//item will be each element in the array
//do something
});
Another way that is useful for copying items from one array to another is
var temp = [1, 2, 3];
var temp2 = [];
angular.forEach(temp, function(item) {
this.push(item); //"this" refers to the array passed into the optional third parameter so, in this case, temp2.
}, temp2);
Though, you don't have to do that, you can simply do the following and it's equivalent to the previous example:
angular.forEach(temp, function(item) {
temp2.push(item);
});
Now there are pros and cons of using the angular.forEach function as opposed to the built in vanilla-flavored for loop.
Pros
Easy readability
Easy writability
If available, angular.forEach will use the ES5 forEach loop. Now, I will get to efficientcy in the cons section, as the forEach loops are much slower than the for loops. I mention this as a pro because it's nice to be consistent and standardized.
Consider the following 2 nested loops, which do exactly the same thing. Let's say that we have 2 arrays of objects and each object contains an array of results, each of which has a Value property that's a string (or whatever). And let's say we need to iterate over each of the results and if they're equal then perform some action:
angular.forEach(obj1.results, function(result1) {
angular.forEach(obj2.results, function(result2) {
if (result1.Value === result2.Value) {
//do something
}
});
});
//exact same with a for loop
for (var i = 0; i < obj1.results.length; i++) {
for (var j = 0; j < obj2.results.length; j++) {
if (obj1.results[i].Value === obj2.results[j].Value) {
//do something
}
}
}
Granted this is a very simple hypothetical example, but I've written triple embedded for loops using the second approach and it was very hard to read, and write for that matter.
Cons
Efficiency. angular.forEach, and the native forEach, for that matter, are both so much slower than the normal for loop....about 90% slower. So for large data sets, best to stick to the native for loop.
No break, continue, or return support. continue is actually supported by "accident", to continue in an angular.forEach you simple put a return; statement in the function like angular.forEach(array, function(item) { if (someConditionIsTrue) return; }); which will cause it to continue out of the function for that iteration. This is also due to the fact that the native forEach does not support break or continue either.
I'm sure there's various other pros and cons as well, and please feel free to add any that you see fit. I feel that, bottom line, if you need efficiency, stick with just the native for loop for your looping needs. But, if your datasets are smaller and a some efficiency is okay to give up in exchange for readability and writability, then by all means throw an angular.forEach in that bad boy.

As of ECMAScript 6:
list = [0, 1, 2, 3]
for (let obj of list) {
console.log(obj)
}
Where of avoids the oddities associated with in and makes it work like the for loop of any other language, and let binds i within the loop as opposed to within the function.
The braces ({}) can be omitted when there is only one command (e.g. in the example above).

Probably the for(i = 0; i < array.length; i++) loop is not the best choice. Why? If you have this:
var array = new Array();
array[1] = "Hello";
array[7] = "World";
array[11] = "!";
The method will call from array[0] to array[2]. First, this will first reference variables you don't even have, second you would not have the variables in the array, and third this will make the code bolder. Look here, it's what I use:
for(var i in array){
var el = array[i];
//If you want 'i' to be INT just put parseInt(i)
//Do something with el
}
And if you want it to be a function, you can do this:
function foreach(array, call){
for(var i in array){
call(array[i]);
}
}
If you want to break, a little more logic:
function foreach(array, call){
for(var i in array){
if(call(array[i]) == false){
break;
}
}
}
Example:
foreach(array, function(el){
if(el != "!"){
console.log(el);
} else {
console.log(el+"!!");
}
});
It returns:
//Hello
//World
//!!!

There are three implementations of foreach in jQuery as follows.
var a = [3,2];
$(a).each(function(){console.log(this.valueOf())}); //Method 1
$.each(a, function(){console.log(this.valueOf())}); //Method 2
$.each($(a), function(){console.log(this.valueOf())}); //Method 3

An easy solution now would be to use the underscore.js library. It's providing many useful tools, such as each and will automatically delegate the job to the native forEach if available.
A CodePen example of how it works is:
var arr = ["elemA", "elemB", "elemC"];
_.each(arr, function(elem, index, ar)
{
...
});
See also
Documentation for native Array.prototype.forEach().
In for_each...in (MDN) it is explained that for each (variable in object) is deprecated as the part of ECMA-357 (EAX) standard.
for...of (MDN) describes the next way of iterating using for (variable of object) as the part of the Harmony (ECMAScript 6) proposal.

There isn't any for each loop in native JavaScript. You can either use libraries to get this functionality (I recommend Underscore.js), use a simple for in loop.
for (var instance in objects) {
...
}
However, note that there may be reasons to use an even simpler for loop (see Stack Overflow question Why is using “for…in” with array iteration such a bad idea?)
var instance;
for (var i=0; i < objects.length; i++) {
var instance = objects[i];
...
}

ECMAScript 5 (the version on JavaScript) to work with Arrays:
forEach - Iterates through every item in the array and do whatever you need with each item.
['C', 'D', 'E'].forEach(function(element, index) {
console.log(element + " is #" + (index+1) + " in the musical scale");
});
// Output
// C is the #1 in musical scale
// D is the #2 in musical scale
// E is the #3 in musical scale
In case, more interested on operation on array using some inbuilt feature.
map - It creates a new array with the result of the callback function. This method is good to be used when you need to format the elements of your array.
// Let's upper case the items in the array
['bob', 'joe', 'jen'].map(function(elem) {
return elem.toUpperCase();
});
// Output: ['BOB', 'JOE', 'JEN']
reduce - As the name says, it reduces the array to a single value by calling the given function passing in the current element and the result of the previous execution.
[1,2,3,4].reduce(function(previous, current) {
return previous + current;
});
// Output: 10
// 1st iteration: previous=1, current=2 => result=3
// 2nd iteration: previous=3, current=3 => result=6
// 3rd iteration: previous=6, current=4 => result=10
every - Returns true or false if all the elements in the array pass the test in the callback function.
// Check if everybody has 18 years old of more.
var ages = [30, 43, 18, 5];
ages.every(function(elem) {
return elem >= 18;
});
// Output: false
filter - Very similar to every except that filter returns an array with the elements that return true to the given function.
// Finding the even numbers
[1,2,3,4,5,6].filter(function(elem){
return (elem % 2 == 0)
});
// Output: [2,4,6]

There are a few ways to loop through an array in JavaScript, as below:
for - it's the most common one. Full block of code for looping
var languages = ["Java", "JavaScript", "C#", "Python"];
var i, len, text;
for (i = 0, len = languages.length, text = ""; i < len; i++) {
text += languages[i] + "<br>";
}
document.getElementById("example").innerHTML = text;
<p id="example"></p>
while - loop while a condition is through. It seems to be the fastest loop
var text = "";
var i = 0;
while (i < 10) {
text += i + ") something<br>";
i++;
}
document.getElementById("example").innerHTML = text;
<p id="example"></p>
do/while - also loop through a block of code while the condition is true, will run at least one time
var text = ""
var i = 0;
do {
text += i + ") something <br>";
i++;
}
while (i < 10);
document.getElementById("example").innerHTML = text;
<p id="example"></p>
Functional loops - forEach, map, filter, also reduce (they loop through the function, but they are used if you need to do something with your array, etc.
// For example, in this case we loop through the number and double them up using the map function
var numbers = [65, 44, 12, 4];
document.getElementById("example").innerHTML = numbers.map(function(num){return num * 2});
<p id="example"></p>
For more information and examples about functional programming on arrays, look at the blog post Functional programming in JavaScript: map, filter and reduce.

This is an iterator for NON-sparse list where the index starts at 0, which is the typical scenario when dealing with document.getElementsByTagName or document.querySelectorAll)
function each( fn, data ) {
if(typeof fn == 'string')
eval('fn = function(data, i){' + fn + '}');
for(var i=0, L=this.length; i < L; i++)
fn.call( this[i], data, i );
return this;
}
Array.prototype.each = each;
Examples of usage:
Example #1
var arr = [];
[1, 2, 3].each( function(a){ a.push( this * this}, arr);
arr = [1, 4, 9]
Example #2
each.call(document.getElementsByTagName('p'), "this.className = data;",'blue');
Each p tag gets class="blue"
Example #3
each.call(document.getElementsByTagName('p'),
"if( i % 2 == 0) this.className = data;",
'red'
);
Every other p tag gets class="red">
Example #4
each.call(document.querySelectorAll('p.blue'),
function(newClass, i) {
if( i < 20 )
this.className = newClass;
}, 'green'
);
And finally the first 20 blue p tags are changed to green
Caution when using string as function: the function is created out-of-context and ought to be used only where you are certain of variable scoping. Otherwise, better to pass functions where scoping is more intuitive.

There's no inbuilt ability to break in forEach. To interrupt execution use the Array#some like below:
[1,2,3].some(function(number) {
return number === 1;
});
This works because some returns true as soon as any of the callbacks, executed in array order, returns true, short-circuiting the execution of the rest.
Original Answer
see Array prototype for some

I also would like to add this as a composition of a reverse loop and an answer above for someone that would like this syntax too.
var foo = [object,object,object];
for (var i = foo.length, item; item = foo[--i];) {
console.log(item);
}
Pros:
The benefit for this: You have the reference already in the first like that won't need to be declared later with another line. It is handy when looping trough the object array.
Cons:
This will break whenever the reference is false - falsey (undefined, etc.). It can be used as an advantage though. However, it would make it a little bit harder to read. And also depending on the browser it can be "not" optimized to work faster than the original one.

jQuery way using $.map:
var data = [1, 2, 3, 4, 5, 6, 7];
var newData = $.map(data, function(element) {
if (element % 2 == 0) {
return element;
}
});
// newData = [2, 4, 6];

Use for...of where possible
async/await support
Skips non-numeric props
Immutable index
for...of
✅
✅
✅
forEach()
❌
✅
✅
for...in
✅
❌
✅
Regular for
✅
✅
❌
As one can see in the table above, for...of should be used wherever it fits. Since it supports async functions, skips non-numeric properties and prevents messing up the loop by accidentally modifying the loop index.
Syntax
const nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const num of nums) {
/* Do something with num */
}
See for...of reference for more examples, link to specification and difference between for...of and for...in. Or maybe check this tutorial for some explanation on how they differ.

Using loops with ECMAScript 6 destructuring and the spread operator
Destructuring and using of the spread operator have proven quite useful for newcomers to ECMAScript 6 as being more human-readable/aesthetic, although some JavaScript veterans might consider it messy. Juniors or some other people might find it useful.
The following examples will use the for...of statement and the .forEach method.
Examples 6, 7, and 8 can be used with any functional loops like .map, .filter, .reduce, .sort, .every, .some. For more information about these methods, check out the Array Object.
Example 1: Normal for...of loop - no tricks here.
let arrSimple = ['a', 'b', 'c'];
for (let letter of arrSimple) {
console.log(letter);
}
Example 2: Split words to characters
let arrFruits = ['apple', 'orange', 'banana'];
for (let [firstLetter, ...restOfTheWord] of arrFruits) {
// Create a shallow copy using the spread operator
let [lastLetter] = [...restOfTheWord].reverse();
console.log(firstLetter, lastLetter, restOfTheWord);
}
Example 3: Looping with a key and value
// let arrSimple = ['a', 'b', 'c'];
// Instead of keeping an index in `i` as per example `for(let i = 0 ; i<arrSimple.length;i++)`
// this example will use a multi-dimensional array of the following format type:
// `arrWithIndex: [number, string][]`
let arrWithIndex = [
[0, 'a'],
[1, 'b'],
[2, 'c'],
];
// Same thing can be achieved using `.map` method
// let arrWithIndex = arrSimple.map((i, idx) => [idx, i]);
// Same thing can be achieved using `Object.entries`
// NOTE: `Object.entries` method doesn't work on Internet Explorer unless it's polyfilled
// let arrWithIndex = Object.entries(arrSimple);
for (let [key, value] of arrWithIndex) {
console.log(key, value);
}
Example 4: Get object properties inline
let arrWithObjects = [{
name: 'Jon',
age: 32
},
{
name: 'Elise',
age: 33
}
];
for (let { name, age: aliasForAge } of arrWithObjects) {
console.log(name, aliasForAge);
}
Example 5: Get deep object properties of what you need
let arrWithObjectsWithArr = [{
name: 'Jon',
age: 32,
tags: ['driver', 'chef', 'jogger']
},
{
name: 'Elise',
age: 33,
tags: ['best chef', 'singer', 'dancer']
}
];
for (let { name, tags: [firstItemFromTags, ...restOfTags] } of arrWithObjectsWithArr) {
console.log(name, firstItemFromTags, restOfTags);
}
Example 6: Is Example 3 used with .forEach
let arrWithIndex = [
[0, 'a'],
[1, 'b'],
[2, 'c'],
];
// Not to be confused here, `forEachIndex` is the real index
// `mappedIndex` was created by "another user", so you can't really trust it
arrWithIndex.forEach(([mappedIndex, item], forEachIndex) => {
console.log(forEachIndex, mappedIndex, item);
});
Example 7: Is Example 4 used with .forEach
let arrWithObjects = [{
name: 'Jon',
age: 32
},
{
name: 'Elise',
age: 33
}
];
// NOTE: Destructuring objects while using shorthand functions
// are required to be surrounded by parentheses
arrWithObjects.forEach( ({ name, age: aliasForAge }) => {
console.log(name, aliasForAge)
});
Example 8: Is Example 5 used with .forEach
let arrWithObjectsWithArr = [{
name: 'Jon',
age: 32,
tags: ['driver', 'chef', 'jogger']
},
{
name: 'Elise',
age: 33,
tags: ['best chef', 'singer', 'dancer']
}
];
arrWithObjectsWithArr.forEach(({
name,
tags: [firstItemFromTags, ...restOfTags]
}) => {
console.log(name, firstItemFromTags, restOfTags);
});

Summary:
When iterating over an array, we often want to accomplish one of the following goals:
We want to iterate over the array and create a new array:
Array.prototype.map
We want to iterate over the array and don't create a new array:
Array.prototype.forEach
for..of loop
In JavaScript, there are many ways of accomplishing both of these goals. However, some are more convenient than others. Below you can find some commonly used methods (the most convenient IMO) to accomplish array iteration in JavaScript.
Creating new array: Map
map() is a function located on Array.prototype which can transform every element of an array and then returns a new array. map() takes as an argument a callback function and works in the following manner:
let arr = [1, 2, 3, 4, 5];
let newArr = arr.map((element, index, array) => {
return element * 2;
})
console.log(arr);
console.log(newArr);
The callback which we have passed into map() as an argument gets executed for every element. Then an array gets returned which has the same length as the original array. In this new array element is transformed by the callback function passed in as an argument to map().
The distinct difference between map and another loop mechanism like forEach and a for..of loop is that map returns a new array and leaves the old array intact (except if you explicitly manipulate it with thinks like splice).
Also, note that the map function's callback provides the index number of the current iteration as a second argument. Furthermore, does the third argument provide the array on which map was called? Sometimes these properties can be very useful.
Loop using forEach
forEach is a function which is located on Array.prototype which takes a callback function as an argument. It then executes this callback function for every element in the array. In contrast to the map() function, the forEach function returns nothing (undefined). For example:
let arr = [1, 2, 3, 4, 5];
arr.forEach((element, index, array) => {
console.log(element * 2);
if (index === 4) {
console.log(array)
}
// index, and oldArray are provided as 2nd and 3th argument by the callback
})
console.log(arr);
Just like the map function, the forEach callback provides the index number of the current iteration as a second argument. Also, does the third argument provide the array on which forEach was called?
Loop through elements using for..of
The for..of loop loops through every element of an array (or any other iterable object). It works in the following manner:
let arr = [1, 2, 3, 4, 5];
for(let element of arr) {
console.log(element * 2);
}
In the above example, element stands for an array element and arr is the array which we want to loop. Note that the name element is arbitrary, and we could have picked any other name like 'el' or something more declarative when this is applicable.
Don't confuse the for..in loop with the for..of loop. for..in will loop through all enumerable properties of the array whereas the for..of loop will only loop through the array elements. For example:
let arr = [1, 2, 3, 4, 5];
arr.foo = 'foo';
for(let element of arr) {
console.log(element);
}
for(let element in arr) {
console.log(element);
}

Performance
Today (2019-12-18) I perform test on my macOS v10.13.6 (High Sierra), on Chrome v 79.0, Safari v13.0.4 and Firefox v71.0 (64 bit) - conclusions about optimisation (and micro-optimisation which usually is not worth to introduce it to code because the benefit is small, but code complexity grows).
It looks like the traditional for i (Aa) is a good choice to write fast code on all browsers.
The other solutions, like for-of (Ad), all in group C.... are usually 2 - 10 (and more) times slower than Aa, but for small arrays it is ok to use it - for the sake of increase code clarity.
The loops with array length cached in n (Ab, Bb, Be) are sometimes faster, sometimes not. Probably compilers automatically detect this situation and introduce caching. The speed differences between the cached and no-cached versions (Aa, Ba, Bd) are about ~1%, so it looks like introduce n is a micro-optimisation.
The i-- like solutions where the loop starts from the last array element (Ac, Bc) are usually ~30% slower than forward solutions - probably the reason is the way of CPU memory cache working - forward memory reading is more optimal for CPU caching). Is recommended to NOT USE such solutions.
Details
In tests we calculate the sum of array elements. I perform a test for small arrays (10 elements) and big arrays (1M elements) and divide them into three groups:
A - for tests
B - while tests
C - other/alternative methods
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
//let arr = Array.from(Array(1000000), (x, i) => i%10);
function Aa(a, s=0) {
for(let i=0; i<a.length; i++) {
s += a[i];
}
console.log('Aa=', s);
}
function Ab(a, s=0) {
let n = a.length;
for(let i=0; i<n; i++) {
s += a[i];
}
console.log('Ab=', s);
}
function Ac(a, s=0) {
for(let i=a.length; i--;) {
s += a[i];
}
console.log('Ac=', s);
}
function Ad(a, s=0) {
for(let x of a) {
s += x;
}
console.log('Ad=', s);
}
function Ae(a, s=0) {
for(let i in a) if (a.hasOwnProperty(i)) {
s += a[i];
}
console.log('Ae=', s);
}
function Ba(a, s=0) {
let i = -1;
while(++i < a.length) {
s+= a[i];
}
console.log('Ba=', s);
}
function Bb(a, s=0) {
let i = -1;
let n = a.length;
while(++i < n) {
s+= a[i];
}
console.log('Bb=', s);
}
function Bc(a, s=0) {
let i = a.length;
while(i--) {
s += a[i];
}
console.log('Bc=', s);
}
function Bd(a, s=0) {
let i = 0;
do {
s+= a[i]
} while (++i < a.length);
console.log('Bd=', s);
}
function Be(a, s=0) {
let i = 0;
let n = a.length;
do {
s += a[i]
} while (++i < n);
console.log('Be=', s);
}
function Bf(a, s=0) {
const it = a.values();
let e;
while (!(e = it.next()).done) {
s+= e.value;
}
console.log('Bf=', s);
}
function Ca(a, s=0) {
a.map(x => { s+=x });
console.log('Ca=', s);
}
function Cb(a, s=0) {
a.forEach(x => { s+=x });
console.log('Cb=', s);
}
function Cc(a, s=0) {
a.every(x => (s += x, 1));
console.log('Cc=', s);
}
function Cd(a, s=0) {
a.filter(x => { s+=x });
console.log('Cd=',s);
}
function Ce(a, s=0) {
a.reduce((z, c) => { s+=c }, 0);
console.log('Ce=', s);
}
function Cf(a, s=0) {
a.reduceRight((z, c) => { s += c }, 0);
console.log('Cf=', s);
}
function Cg(a, s=0) {
a.some(x => { s += x } );
console.log('Cg=', s);
}
function Ch(a, s=0) {
Array.from(a, x=> s += x);
console.log('Cc=', s);
}
Aa(arr);
Ab(arr);
Ac(arr);
Ad(arr);
Ae(arr);
Ba(arr);
Bb(arr);
Bc(arr);
Bd(arr);
Be(arr);
Bf(arr);
Ca(arr);
Cb(arr);
Cc(arr);
Cd(arr);
Ce(arr);
Cf(arr);
Cg(arr);
Ch(arr);
<p style="color: red">This snippets only PRESENTS code used for benchmark - it not perform test itself</p>
Cross browser results
Results for all tested browsers
browsers**
Array with 10 elements
Results for Chrome. You can perform the test on your machine here.
Array with 1,000,000 elements
Results for Chrome. You can perform the test on your machine here

A way closest to your idea would be to use Array.forEach() which accepts a closure function which will be executed for each element of the array.
myArray.forEach(
(item) => {
// Do something
console.log(item);
}
);
Another viable way would be to use Array.map() which works in the same way, but it also takes all values that you return and returns them in a new array (essentially mapping each element to a new one), like this:
var myArray = [1, 2, 3];
myArray = myArray.map(
(item) => {
return item + 1;
}
);
console.log(myArray); // [2, 3, 4]

As per the new updated feature ECMAScript 6 (ES6) and ECMAScript 2015, you can use the following options with loops:
for loops
for(var i = 0; i < 5; i++){
console.log(i);
}
// Output: 0,1,2,3,4
for...in loops
let obj = {"a":1, "b":2}
for(let k in obj){
console.log(k)
}
// Output: a,b
Array.forEach()
let array = [1,2,3,4]
array.forEach((x) => {
console.log(x);
})
// Output: 1,2,3,4
for...of loops
let array = [1,2,3,4]
for(let x of array){
console.log(x);
}
// Output: 1,2,3,4
while loops
let x = 0
while(x < 5){
console.log(x)
x++
}
// Output: 1,2,3,4
do...while loops
let x = 0
do{
console.log(x)
x++
}while(x < 5)
// Output: 1,2,3,4

The lambda syntax doesn't usually work in Internet Explorer 10 or below.
I usually use the
[].forEach.call(arrayName,function(value,index){
console.log("value of the looped element" + value);
console.log("index of the looped element" + index);
});
If you are a jQuery fan and already have a jQuery file running, you should reverse the positions of the index and value parameters
$("#ul>li").each(function(**index, value**){
console.log("value of the looped element" + value);
console.log("index of the looped element" + index);
});

You can call forEach like this:
forEach will iterate over the array you provide and for each iteration it will have element which holds the value of that iteration. If you need index you can get the current index by passing the i as the second parameter in the callback function for forEach.
Foreach is basically a High Order Function, Which takes another function as its parameter.
let theArray= [1,3,2];
theArray.forEach((element) => {
// Use the element of the array
console.log(element)
}
Output:
1
3
2
You can also iterate over an array like this:
for (let i=0; i<theArray.length; i++) {
console.log(i); // i will have the value of each index
}

If you want to use forEach(), it will look like -
theArray.forEach ( element => {
console.log(element);
});
If you want to use for(), it will look like -
for(let idx = 0; idx < theArray.length; idx++){
let element = theArray[idx];
console.log(element);
}

Removing elements with Array.map in JavaScript

I would like to filter an array of items by using the map() function. Here is a code snippet:
var filteredItems = items.map(function(item)
{
if( ...some condition... )
{
return item;
}
});
The problem is that filtered out items still uses space in the array and I would like to completely wipe them out.
Any idea?
EDIT: Thanks, I forgot about filter(), what I wanted is actually a filter() then a map().
EDIT2: Thanks for pointing that map() and filter() are not implemented in all browsers, although my specific code was not intended to run in a browser.

You should use the filter method rather than map unless you want to mutate the items in the array, in addition to filtering.
eg.
var filteredItems = items.filter(function(item)
{
return ...some condition...;
});
[Edit: Of course you could always do sourceArray.filter(...).map(...) to both filter and mutate]

Inspired by writing this answer, I ended up later expanding and writing a blog post going over this in careful detail. I recommend checking that out if you want to develop a deeper understanding of how to think about this problem--I try to explain it piece by piece, and also give a JSperf comparison at the end, going over speed considerations.
That said, **The tl;dr is this:
To accomplish what you're asking for (filtering and mapping within one function call), you would use Array.reduce()**.
However, the more readable and (less importantly) usually significantly faster2 approach is to just use filter and map chained together:
[1,2,3].filter(num => num > 2).map(num => num * 2)
What follows is a description of how Array.reduce() works, and how it can be used to accomplish filter and map in one iteration. Again, if this is too condensed, I highly recommend seeing the blog post linked above, which is a much more friendly intro with clear examples and progression.
You give reduce an argument that is a (usually anonymous) function.
That anonymous function takes two parameters--one (like the anonymous functions passed in to map/filter/forEach) is the iteratee to be operated on. There is another argument for the anonymous function passed to reduce, however, that those functions do not accept, and that is the value that will be passed along between function calls, often referred to as the memo.
Note that while Array.filter() takes only one argument (a function), Array.reduce() also takes an important (though optional) second argument: an initial value for 'memo' that will be passed into that anonymous function as its first argument, and subsequently can be mutated and passed along between function calls. (If it is not supplied, then 'memo' in the first anonymous function call will by default be the first iteratee, and the 'iteratee' argument will actually be the second value in the array)
In our case, we'll pass in an empty array to start, and then choose whether to inject our iteratee into our array or not based on our function--this is the filtering process.
Finally, we'll return our 'array in progress' on each anonymous function call, and reduce will take that return value and pass it as an argument (called memo) to its next function call.
This allows filter and map to happen in one iteration, cutting down our number of required iterations in half--just doing twice as much work each iteration, though, so nothing is really saved other than function calls, which are not so expensive in javascript.
For a more complete explanation, refer to MDN docs (or to my post referenced at the beginning of this answer).
Basic example of a Reduce call:
let array = [1,2,3];
const initialMemo = [];
array = array.reduce((memo, iteratee) => {
// if condition is our filter
if (iteratee > 1) {
// what happens inside the filter is the map
memo.push(iteratee * 2);
}
// this return value will be passed in as the 'memo' argument
// to the next call of this function, and this function will have
// every element passed into it at some point.
return memo;
}, initialMemo)
console.log(array) // [4,6], equivalent to [(2 * 2), (3 * 2)]
more succinct version:
[1,2,3].reduce((memo, value) => value > 1 ? memo.concat(value * 2) : memo, [])
Notice that the first iteratee was not greater than one, and so was filtered. Also note the initialMemo, named just to make its existence clear and draw attention to it. Once again, it is passed in as 'memo' to the first anonymous function call, and then the returned value of the anonymous function is passed in as the 'memo' argument to the next function.
Another example of the classic use case for memo would be returning the smallest or largest number in an array. Example:
[7,4,1,99,57,2,1,100].reduce((memo, val) => memo > val ? memo : val)
// ^this would return the largest number in the list.
An example of how to write your own reduce function (this often helps understanding functions like these, I find):
test_arr = [];
// we accept an anonymous function, and an optional 'initial memo' value.
test_arr.my_reducer = function(reduceFunc, initialMemo) {
// if we did not pass in a second argument, then our first memo value
// will be whatever is in index zero. (Otherwise, it will
// be that second argument.)
const initialMemoIsIndexZero = arguments.length < 2;
// here we use that logic to set the memo value accordingly.
let memo = initialMemoIsIndexZero ? this[0] : initialMemo;
// here we use that same boolean to decide whether the first
// value we pass in as iteratee is either the first or second
// element
const initialIteratee = initialMemoIsIndexZero ? 1 : 0;
for (var i = initialIteratee; i < this.length; i++) {
// memo is either the argument passed in above, or the
// first item in the list. initialIteratee is either the
// first item in the list, or the second item in the list.
memo = reduceFunc(memo, this[i]);
// or, more technically complete, give access to base array
// and index to the reducer as well:
// memo = reduceFunc(memo, this[i], i, this);
}
// after we've compressed the array into a single value,
// we return it.
return memo;
}
The real implementation allows access to things like the index, for example, but I hope this helps you get an uncomplicated feel for the gist of it.

That's not what map does. You really want Array.filter. Or if you really want to remove the elements from the original list, you're going to need to do it imperatively with a for loop.

Array Filter method
var arr = [1, 2, 3]
// ES5 syntax
arr = arr.filter(function(item){ return item != 3 })
// ES2015 syntax
arr = arr.filter(item => item != 3)
console.log( arr )

You must note however that the Array.filter is not supported in all browser so, you must to prototyped:
//This prototype is provided by the Mozilla foundation and
//is distributed under the MIT license.
//http://www.ibiblio.org/pub/Linux/LICENSES/mit.license
if (!Array.prototype.filter)
{
Array.prototype.filter = function(fun /*, thisp*/)
{
var len = this.length;
if (typeof fun != "function")
throw new TypeError();
var res = new Array();
var thisp = arguments[1];
for (var i = 0; i < len; i++)
{
if (i in this)
{
var val = this[i]; // in case fun mutates this
if (fun.call(thisp, val, i, this))
res.push(val);
}
}
return res;
};
}
And doing so, you can prototype any method you may need.

TLDR: Use map (returning undefined when needed) and then filter.
First, I believe that a map + filter function is useful since you don't want to repeat a computation in both. Swift originally called this function flatMap but then renamed it to compactMap.
For example, if we don't have a compactMap function, we might end up with computation defined twice:
let array = [1, 2, 3, 4, 5, 6, 7, 8];
let mapped = array
.filter(x => {
let computation = x / 2 + 1;
let isIncluded = computation % 2 === 0;
return isIncluded;
})
.map(x => {
let computation = x / 2 + 1;
return `${x} is included because ${computation} is even`
})
// Output: [2 is included because 2 is even, 6 is included because 4 is even]
Thus compactMap would be useful to reduce duplicate code.
A really simple way to do something similar to compactMap is to:
Map on real values or undefined.
Filter out all the undefined values.
This of course relies on you never needing to return undefined values as part of your original map function.
Example:
let array = [1, 2, 3, 4, 5, 6, 7, 8];
let mapped = array
.map(x => {
let computation = x / 2 + 1;
let isIncluded = computation % 2 === 0;
if (isIncluded) {
return `${x} is included because ${computation} is even`
} else {
return undefined
}
})
.filter(x => typeof x !== "undefined")

I just wrote array intersection that correctly handles also duplicates
https://gist.github.com/gkucmierz/8ee04544fa842411f7553ef66ac2fcf0
// array intersection that correctly handles also duplicates
const intersection = (a1, a2) => {
const cnt = new Map();
a2.map(el => cnt[el] = el in cnt ? cnt[el] + 1 : 1);
return a1.filter(el => el in cnt && 0 < cnt[el]--);
};
const l = console.log;
l(intersection('1234'.split``, '3456'.split``)); // [ '3', '4' ]
l(intersection('12344'.split``, '3456'.split``)); // [ '3', '4' ]
l(intersection('1234'.split``, '33456'.split``)); // [ '3', '4' ]
l(intersection('12334'.split``, '33456'.split``)); // [ '3', '3', '4' ]

First you can use map and with chaining you can use filter
state.map(item => {
if(item.id === action.item.id){
return {
id : action.item.id,
name : item.name,
price: item.price,
quantity : item.quantity-1
}
}else{
return item;
}
}).filter(item => {
if(item.quantity <= 0){
return false;
}else{
return true;
}
});

following statement cleans object using map function.
var arraytoclean = [{v:65, toberemoved:"gronf"}, {v:12, toberemoved:null}, {v:4}];
arraytoclean.map((x,i)=>x.toberemoved=undefined);
console.dir(arraytoclean);