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How to extend the javascript Array Class?

I am trying to extend the Array class to add a Sum method to it. This is my code below, what am I doing wrong?
Class tipArray extends Array{
sum(values) {
for(i in values) {
total +=i;
}
}
}
var testArray = new tipArray();
testArray = [1,2,3,4];
console.log(testArray.sum());
Expected output = 10

Start by imagining how you would sum an array (maybe something with reduce).
Turn that into a function.
Add that as a method on your class. You can use this to refer to the array.
(optional) ask yourself if you really need a subclass instead of a function that accepts an array.
class tipArray extends Array{
sum() {
// Consider making sure the array contains items where sum makes sense here.
return this.reduce((sum, current) => sum + current)
}
}
var testArray = new tipArray(1,2,3,4);
console.log(testArray.sum());
// add another element and take another sum
testArray.push(10)
console.log(testArray.sum());

class tipArray extends Array {
sum() {
let val = 0;
for (let i = 0; i < this.length; i++) {
val += this[i];
}
return val;
}
}
var testArray = new tipArray(1, 2, 3, 4);
console.log(testArray.sum());
console.log(testArray.length);
Inside of the sum method, you refer to the array via this.

This seems to me more like asking for the solution of a homework after doing half the research on how to code in JavaScript.
Keywords, like class in JavaScript needs to be lowercase.
You didn't declare any of the variables. For example in the for loop you keep adding i to total, but what was the initial value of total? Declare the total variable before the for loop as let total = 0
The loop index i also needs to be declared as for (let i in values) {
The sum function doesn't return anything as you don't have any return statements. In this case JavaScript returns undefined and that is what you see in your console log. Add return total after your for loop so that the function would return something.
No need to use reduce here or extending array prototype as the issue isn't because for loop not being suitable to calculate sums.
You create a new instance of your own array class only to override it with a simple array at the next line with testArray = [1,2,3,4], as that line is the same as testArray = new Array(1, 2, 3, 4);. Write either var testArray = new tipArray(1, 2, 3, 4) or testArray.push(1, 2, 3, 4).
In essence the solution should be what Geuis wrote.

How do I remove all the elements of an array using splice [duplicate]

This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
Is there a way to empty an array and if so possibly with .remove()?
For instance,
A = [1,2,3,4];
How can I empty that?

Ways to clear an existing array A:
Method 1
(this was my original answer to the question)
A = [];
This code will set the variable A to a new empty array. This is perfect if you don't have references to the original array A anywhere else because this actually creates a brand new (empty) array. You should be careful with this method because if you have referenced this array from another variable or property, the original array will remain unchanged. Only use this if you only reference the array by its original variable A.
This is also the fastest solution.
This code sample shows the issue you can encounter when using this method:
var arr1 = ['a','b','c','d','e','f'];
var arr2 = arr1; // Reference arr1 by another variable
arr1 = [];
console.log(arr2); // Output ['a','b','c','d','e','f']
Method 2 (as suggested by Matthew Crumley)
A.length = 0
This will clear the existing array by setting its length to 0. It also works when using "strict mode" in ECMAScript 5 because the length property of an array is a read/write property.
Method 3 (as suggested by Anthony)
A.splice(0,A.length)
Using .splice() will work perfectly, but since the .splice() function will return an array with all the removed items, it will actually return a copy of the original array. Benchmarks suggest that this has no effect on performance whatsoever.
Method 4 (as suggested by tanguy_k)
while(A.length > 0) {
A.pop();
}
This solution is not very succinct, and it is also the slowest solution, contrary to earlier benchmarks referenced in the original answer.
Performance
Of all the methods of clearing an existing array, methods 2 and 3 are very similar in performance and are a lot faster than method 4. See this benchmark.
As pointed out by Diadistis in their answer below, the original benchmarks that were used to determine the performance of the four methods described above were flawed. The original benchmark reused the cleared array so the second iteration was clearing an array that was already empty.
The following benchmark fixes this flaw: http://jsben.ch/#/hyj65. It clearly shows that methods #2 (length property) and #3 (splice) are the fastest (not counting method #1 which doesn't change the original array).
This has been a hot topic and the cause of a lot of controversy. There are actually many correct answers and because this answer has been marked as the accepted answer for a very long time, I will include all of the methods here.

If you need to keep the original array because you have other references to it that should be updated too, you can clear it without creating a new array by setting its length to zero:
A.length = 0;

Here the fastest working implementation while keeping the same array ("mutable"):
function clearArray(array) {
while (array.length > 0) {
array.pop();
}
}
FYI it cannot be simplified to while (array.pop()): the tests will fail.
FYI Map and Set define clear(), it would have seem logical to have clear() for Array too.
TypeScript version:
function clearArray<T>(array: T[]) {
while (array.length > 0) {
array.pop();
}
}
The corresponding tests:
describe('clearArray()', () => {
test('clear regular array', () => {
const array = [1, 2, 3, 4, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
test('clear array that contains undefined and null', () => {
const array = [1, undefined, 3, null, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
});
Here the updated jsPerf: http://jsperf.com/array-destroy/32 http://jsperf.com/array-destroy/152
jsPerf offline. Similar benchmark: https://jsben.ch/hyj65

A more cross-browser friendly and more optimal solution will be to use the splice method to empty the content of the array A as below:
A.splice(0, A.length);

The answers that have no less that 2739 upvotes by now are misleading and incorrect.
The question is: "How do you empty your existing array?" E.g. for A = [1,2,3,4].
Saying "A = [] is the answer" is ignorant and absolutely incorrect. [] == [] is false.
This is because these two arrays are two separate, individual objects, with their own two identities, taking up their own space in the digital world, each on its own.
Let's say your mother asks you to empty the trash can.
You don't bring in a new one as if you've done what you've been asked for.
Instead, you empty the trash can.
You don't replace the filled one with a new empty can, and you don't take the label "A" from the filled can and stick it to the new one as in A = [1,2,3,4]; A = [];
Emptying an array object is the easiest thing ever:
A.length = 0;
This way, the can under "A" is not only empty, but also as clean as new!
Furthermore, you are not required to remove the trash by hand until the can is empty! You were asked to empty the existing one, completely, in one turn, not to pick up the trash until the can gets empty, as in:
while(A.length > 0) {
A.pop();
}
Nor, to put your left hand at the bottom of the trash, holding it with your right at the top to be able to pull its content out as in:
A.splice(0, A.length);
No, you were asked to empty it:
A.length = 0;
This is the only code that correctly empties the contents of a given JavaScript array.

Performance test:
http://jsperf.com/array-clear-methods/3
a = []; // 37% slower
a.length = 0; // 89% slower
a.splice(0, a.length) // 97% slower
while (a.length > 0) {
a.pop();
} // Fastest

You can add this to your JavaScript file to allow your arrays to be "cleared":
Array.prototype.clear = function() {
this.splice(0, this.length);
};
Then you can use it like this:
var list = [1, 2, 3];
list.clear();
Or if you want to be sure you don't destroy something:
if (!Array.prototype.clear) {
Array.prototype.clear = function() {
this.splice(0, this.length);
};
}
Lots of people think you shouldn't modify native objects (like Array), and I'm inclined to agree. Please use caution in deciding how to handle this.

You can easily create a function to do that for you, change the length or even add it to native Array as remove() function for reuse.
Imagine you have this array:
var arr = [1, 2, 3, 4, 5]; //the array
OK, just simply run this:
arr.length = 0; //change the length
and the result is:
[] //result
easy way to empty an array...
Also using loop which is not necessary but just another way to do that:
/* could be arr.pop() or arr.splice(0)
don't need to return as main array get changed */
function remove(arr) {
while(arr.length) {
arr.shift();
}
}
There are also tricky way which you can think about, for example something like this:
arr.splice(0, arr.length); //[]
So if arr has 5 items, it will splice 5 items from 0, which means nothing will remain in the array.
Also other ways like simply reassign the array for example:
arr = []; //[]
If you look at the Array functions, there are many other ways to do this, but the most recommended one could be changing the length.
As I said in the first place, you can also prototype remove() as it's the answer to your question. you can simply choose one of the methods above and prototype it to Array object in JavaScript, something like:
Array.prototype.remove = Array.prototype.remove || function() {
this.splice(0, this.length);
};
and you can simply call it like this to empty any array in your javascript application:
arr.remove(); //[]

If you are using
a = [];
Then you are assigning new array reference to a, if reference in a is already assigned to any other variable, then it will not empty that array too and hence garbage collector will not collect that memory.
For ex.
var a=[1,2,3];
var b=a;
a=[];
console.log(b);// It will print [1,2,3];
or
a.length = 0;
When we specify a.length, we are just resetting boundaries of the array and memory for rest array elements will be connected by garbage collector.
Instead of these two solutions are better.
a.splice(0,a.length)
and
while(a.length > 0) {
a.pop();
}
As per previous answer by kenshou.html, second method is faster.

There is a lot of confusion and misinformation regarding the while;pop/shift performance both in answers and comments. The while/pop solution has (as expected) the worst performance. What's actually happening is that setup runs only once for each sample that runs the snippet in a loop. eg:
var arr = [];
for (var i = 0; i < 100; i++) {
arr.push(Math.random());
}
for (var j = 0; j < 1000; j++) {
while (arr.length > 0) {
arr.pop(); // this executes 100 times, not 100000
}
}
I have created a new test that works correctly :
http://jsperf.com/empty-javascript-array-redux
Warning: even in this version of the test you can't actually see the real difference because cloning the array takes up most of the test time. It still shows that splice is the fastest way to clear the array (not taking [] into consideration because while it is the fastest it's not actually clearing the existing array).

Array.prototype.clear = function() {
this.length = 0;
};
And call it: array.clear();

In case you are interested in the memory allocation, you may compare each approach using something like this jsfiddle in conjunction with chrome dev tools' timeline tab. You will want to use the trash bin icon at the bottom to force a garbage collection after 'clearing' the array. This should give you a more definite answer for the browser of your choice. A lot of answers here are old and I wouldn't rely on them but rather test as in #tanguy_k's answer above.
(for an intro to the aforementioned tab you can check out here)
Stackoverflow forces me to copy the jsfiddle so here it is:
<html>
<script>
var size = 1000*100
window.onload = function() {
document.getElementById("quantifier").value = size
}
function scaffold()
{
console.log("processing Scaffold...");
a = new Array
}
function start()
{
size = document.getElementById("quantifier").value
console.log("Starting... quantifier is " + size);
console.log("starting test")
for (i=0; i<size; i++){
a[i]="something"
}
console.log("done...")
}
function tearDown()
{
console.log("processing teardown");
a.length=0
}
</script>
<body>
<span style="color:green;">Quantifier:</span>
<input id="quantifier" style="color:green;" type="text"></input>
<button onclick="scaffold()">Scaffold</button>
<button onclick="start()">Start</button>
<button onclick="tearDown()">Clean</button>
<br/>
</body>
</html>
And you should take note that it may depend on the type of the array elements, as javascript manages strings differently than other primitive types, not to mention arrays of objects. The type may affect what happens.

Use a modified version of Jan's initial suggestion:
var originalLength = A.length;
for (var i = originalLength; i > 0; i--) {
A.pop();
}
Terser:
for (let i = A.length; i > 0;A.pop(),i--) {}
Or here's another take:
while(!A[Symbol.iterator]().next().done)A.shift()

A.splice(0);
I just did this on some code I am working on. It cleared the array.

If you use constants then you have no choice:
const numbers = [1, 2, 3]
You can not reasign:
numbers = []
You can only truncate:
numbers.length = 0

To Empty a Current memory location of an array use: 'myArray.length = 0' or 'myArray.pop() UN-till its length is 0'
length : You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases.
pop() : The pop method removes the last element from an array and returns that returns the removed value.
shift() : The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value.
Example:
var arr = ['77'];
arr.length = 20;
console.log("Increasing : ", arr); // (20) ["77", empty × 19]
arr.length = 12;
console.log("Truncating : ", arr); // (12) ["77", empty × 11]
var mainArr = new Array();
mainArr = ['1', '2', '3', '4'];
var refArr = mainArr;
console.log('Current', mainArr, 'Refered', refArr);
refArr.length = 3;
console.log('Length: ~ Current', mainArr, 'Refered', refArr);
mainArr.push('0');
console.log('Push to the End of Current Array Memory Location \n~ Current', mainArr, 'Refered', refArr);
mainArr.poptill_length(0);
console.log('Empty Array \n~ Current', mainArr, 'Refered', refArr);
Array.prototype.poptill_length = function (e) {
while (this.length) {
if( this.length == e ) break;
console.log('removed last element:', this.pop());
}
};
new Array() | [] Create an Array with new memory location by using Array constructor or array literal.
mainArr = []; // a new empty array is addressed to mainArr.
var arr = new Array('10'); // Array constructor
arr.unshift('1'); // add to the front
arr.push('15'); // add to the end
console.log("After Adding : ", arr); // ["1", "10", "15"]
arr.pop(); // remove from the end
arr.shift(); // remove from the front
console.log("After Removing : ", arr); // ["10"]
var arrLit = ['14', '17'];
console.log("array literal « ", indexedItem( arrLit ) ); // {0,14}{1,17}
function indexedItem( arr ) {
var indexedStr = "";
arr.forEach(function(item, index, array) {
indexedStr += "{"+index+","+item+"}";
console.log(item, index);
});
return indexedStr;
}
slice() : By using slice function we get an shallow copy of elements from the original array, with new memory address, So that any modification on cloneArr will not affect to an actual|original array.
var shallowCopy = mainArr.slice(); // this is how to make a copy
var cloneArr = mainArr.slice(0, 3);
console.log('Main', mainArr, '\tCloned', cloneArr);
cloneArr.length = 0; // Clears current memory location of an array.
console.log('Main', mainArr, '\tCloned', cloneArr);

I'm surprised no one has suggested this yet:
let xs = [1,2,3,4];
for (let i in xs)
delete xs[i];
This yields an array in quite a different state from the other solutions. In a sense, the array has been 'emptied':
xs
=> Array [ <4 empty slots> ]
[...xs]
=> Array [ undefined, undefined, undefined, undefined ]
xs.length
=> 4
xs[0]
=> ReferenceError: reference to undefined property xs[0]
You can produce an equivalent array with [,,,,] or Array(4)

How to simplify nested calls in nodejs/javascript for inquirer [duplicate]

In Java, you can use a for loop to traverse objects in an array as follows:
String[] myStringArray = {"Hello", "World"};
for (String s : myStringArray) {
// Do something
}
Can I do the same in JavaScript?

Three main options:
for (var i = 0; i < xs.length; i++) { console.log(xs[i]); }
xs.forEach((x, i) => console.log(x));
for (const x of xs) { console.log(x); }
Detailed examples are below.
1. Sequential for loop:
var myStringArray = ["Hello","World"];
var arrayLength = myStringArray.length;
for (var i = 0; i < arrayLength; i++) {
console.log(myStringArray[i]);
//Do something
}
Pros
Works on every environment
You can use break and continue flow control statements
Cons
Too verbose
Imperative
Easy to have off-by-one errors (sometimes also called a fence post error)
2. Array.prototype.forEach:
The ES5 specification introduced a lot of beneficial array methods. One of them, the Array.prototype.forEach, gave us a concise way to iterate over an array:
const array = ["one", "two", "three"]
array.forEach(function (item, index) {
console.log(item, index);
});
Being almost ten years as the time of writing that the ES5 specification was released (Dec. 2009), it has been implemented by nearly all modern engines in the desktop, server, and mobile environments, so it's safe to use them.
And with the ES6 arrow function syntax, it's even more succinct:
array.forEach(item => console.log(item));
Arrow functions are also widely implemented unless you plan to support ancient platforms (e.g., Internet Explorer 11); you are also safe to go.
Pros
Very short and succinct.
Declarative
Cons
Cannot use break / continue
Normally, you can replace the need to break out of imperative loops by filtering the array elements before iterating them, for example:
array.filter(item => item.condition < 10)
.forEach(item => console.log(item))
Keep in mind if you are iterating an array to build another array from it, you should use map. I've seen this anti-pattern so many times.
Anti-pattern:
const numbers = [1,2,3,4,5], doubled = [];
numbers.forEach((n, i) => { doubled[i] = n * 2 });
Proper use case of map:
const numbers = [1,2,3,4,5];
const doubled = numbers.map(n => n * 2);
console.log(doubled);
Also, if you are trying to reduce the array to a value, for example, you want to sum an array of numbers, you should use the reduce method.
Anti-pattern:
const numbers = [1,2,3,4,5];
const sum = 0;
numbers.forEach(num => { sum += num });
Proper use of reduce:
const numbers = [1,2,3,4,5];
const sum = numbers.reduce((total, n) => total + n, 0);
console.log(sum);
3. ES6 for-of statement:
The ES6 standard introduces the concept of iterable objects and defines a new construct for traversing data, the for...of statement.
This statement works for any kind of iterable object and also for generators (any object that has a \[Symbol.iterator\] property).
Array objects are by definition built-in iterables in ES6, so you can use this statement on them:
let colors = ['red', 'green', 'blue'];
for (const color of colors){
console.log(color);
}
Pros
It can iterate over a large variety of objects.
Can use normal flow control statements (break / continue).
Useful to iterate serially asynchronous values.
Cons
If you are targeting older browsers, the transpiled output might surprise you.
Do not use for...in
#zipcodeman suggests the use of the for...in statement, but for iterating arrays for-in should be avoided, that statement is meant to enumerate object properties.
It shouldn't be used for array-like objects because:
The order of iteration is not guaranteed; the array indexes may not be visited in numeric order.
Inherited properties are also enumerated.
The second point is that it can give you a lot of problems, for example, if you extend the Array.prototype object to include a method there, that property will also be enumerated.
For example:
Array.prototype.foo = "foo!";
var array = ['a', 'b', 'c'];
for (var i in array) {
console.log(array[i]);
}
The above code will console log "a", "b", "c", and "foo!".
That can be particularly a problem if you use some library that relies heavily on native prototypes augmentation (such as MooTools).
The for-in statement, as I said before, is there to enumerate object properties, for example:
var obj = {
"a": 1,
"b": 2,
"c": 3
};
for (var prop in obj) {
if (obj.hasOwnProperty(prop)) {
// or if (Object.prototype.hasOwnProperty.call(obj,prop)) for safety...
console.log("prop: " + prop + " value: " + obj[prop])
}
}
In the above example, the hasOwnProperty method allows you to enumerate only own properties. That's it, only the properties that the object physically has, no inherited properties.
I would recommend you to read the following article:
Enumeration VS Iteration

Yes, assuming your implementation includes the for...of feature introduced in ECMAScript 2015 (the "Harmony" release)... which is a pretty safe assumption these days.
It works like this:
// REQUIRES ECMASCRIPT 2015+
var s, myStringArray = ["Hello", "World"];
for (s of myStringArray) {
// ... do something with s ...
}
Or better yet, since ECMAScript 2015 also provides block-scoped variables:
// REQUIRES ECMASCRIPT 2015+
const myStringArray = ["Hello", "World"];
for (const s of myStringArray) {
// ... do something with s ...
}
// s is no longer defined here
(The variable s is different on each iteration, but can still be declared const inside the loop body as long as it isn't modified there.)
A note on sparse arrays: an array in JavaScript may not actually store as many items as reported by its length; that number is simply one greater than the highest index at which a value is stored. If the array holds fewer elements than indicated by its length, its said to be sparse. For example, it's perfectly legitimate to have an array with items only at indexes 3, 12, and 247; the length of such an array is 248, though it is only actually storing 3 values. If you try to access an item at any other index, the array will appear to have the undefined value there, but the array is nonetheless is distinct from one that actually has undefined values stored. You can see this difference in a number of ways, for example in the way the Node REPL displays arrays:
> a // array with only one item, at index 12
[ <12 empty items>, 1 ]
> a[0] // appears to have undefined at index 0
undefined
> a[0]=undefined // but if we put an actual undefined there
undefined
> a // it now looks like this
[ undefined, <11 empty items>, 1 ]
So when you want to "loop through" an array, you have a question to answer: do you want to loop over the full range indicated by its length and process undefineds for any missing elements, or do you only want to process the elements actually present? There are plenty of applications for both approaches; it just depends on what you're using the array for.
If you iterate over an array with for..of, the body of the loop is executed length times, and the loop control variable is set to undefined for any items not actually present in the array. Depending on the details of your "do something with" code, that behavior may be what you want, but if not, you should use a different approach.
Of course, some developers have no choice but to use a different approach anyway, because for whatever reason they're targeting a version of JavaScript that doesn't yet support for...of.
As long as your JavaScript implementation is compliant with the previous edition of the ECMAScript specification (which rules out, for example, versions of Internet Explorer before 9), then you can use the Array#forEach iterator method instead of a loop. In that case, you pass a function to be called on each item in the array:
var myStringArray = [ "Hello", "World" ];
myStringArray.forEach( function(s) {
// ... do something with s ...
} );
You can of course use an arrow function if your implementation supports ES6+:
myStringArray.forEach( s => {
// ... do something with s ...
} );
Unlike for...of, .forEach only calls the function for elements that are actually present in the array. If passed our hypothetical array with three elements and a length of 248, it will only call the function three times, not 248 times. If this is how you want to handle sparse arrays, .forEach may be the way to go even if your interpreter supports for...of.
The final option, which works in all versions of JavaScript, is an explicit counting loop. You simply count from 0 up to one less than the length and use the counter as an index. The basic loop looks like this:
var i, s, myStringArray = [ "Hello", "World" ], len = myStringArray.length;
for (i=0; i<len; ++i) {
s = myStringArray[i];
// ... do something with s ...
}
One advantage of this approach is that you can choose how to handle sparse arrays. The above code will run the body of the loop the full length times, with s set to undefined for any missing elements, just like for..of; if you instead want to handle only the actually-present elements of a sparse array, like .forEach, you can add a simple in test on the index:
var i, s, myStringArray = [ "Hello", "World" ], len = myStringArray.length;
for (i=0; i<len; ++i) {
if (i in myStringArray) {
s = myStringArray[i];
// ... do something with s ...
}
}
Depending on your implementation's optimizations, assigning the length value to the local variable (as opposed to including the full myStringArray.length expression in the loop condition) can make a significant difference in performance since it skips a property lookup each time through. You may see the length caching done in the loop initialization clause, like this:
var i, len, myStringArray = [ "Hello", "World" ];
for (len = myStringArray.length, i=0; i<len; ++i) {
The explicit counting loop also means you have access to the index of each value, should you want it. The index is also passed as an extra parameter to the function you pass to forEach, so you can access it that way as well:
myStringArray.forEach( (s,i) => {
// ... do something with s and i ...
});
for...of doesn't give you the index associated with each object, but as long as the object you're iterating over is actually an instance of Array (and not one of the other iterable types for..of works on), you can use the Array#entries method to change it to an array of [index, item] pairs, and then iterate over that:
for (const [i, s] of myStringArray.entries()) {
// ... do something with s and i ...
}
The for...in syntax mentioned by others is for looping over an object's properties; since an Array in JavaScript is just an object with numeric property names (and an automatically-updated length property), you can theoretically loop over an Array with it. But the problem is that it doesn't restrict itself to the numeric property values (remember that even methods are actually just properties whose value is a closure), nor is it guaranteed to iterate over those in numeric order. Therefore, the for...in syntax should not be used for looping through Arrays.

You can use map, which is a functional programming technique that's also available in other languages like Python and Haskell.
[1,2,3,4].map( function(item) {
alert(item);
})
The general syntax is:
array.map(func)
In general func would take one parameter, which is an item of the array. But in the case of JavaScript, it can take a second parameter which is the item's index, and a third parameter which is the array itself.
The return value of array.map is another array, so you can use it like this:
var x = [1,2,3,4].map( function(item) {return item * 10;});
And now x is [10,20,30,40].
You don't have to write the function inline. It could be a separate function.
var item_processor = function(item) {
// Do something complicated to an item
}
new_list = my_list.map(item_processor);
which would be sort-of equivalent to:
for (item in my_list) {item_processor(item);}
Except you don't get the new_list.

for (const s of myStringArray) {
(Directly answering your question: now you can!)
Most other answers are right, but they do not mention (as of this writing) that ECMAScript  6  2015 is bringing a new mechanism for doing iteration, the for..of loop.
This new syntax is the most elegant way to iterate an array in JavaScript (as long you don't need the iteration index).
It currently works with Firefox 13+, Chrome 37+ and it does not natively work with other browsers (see browser compatibility below). Luckily we have JavaScript compilers (such as Babel) that allow us to use next-generation features today.
It also works on Node.js (I tested it on version 0.12.0).
Iterating an array
// You could also use "let" or "const" instead of "var" for block scope.
for (var letter of ["a", "b", "c"]) {
console.log(letter);
}
Iterating an array of objects
const band = [
{firstName : 'John', lastName: 'Lennon'},
{firstName : 'Paul', lastName: 'McCartney'}
];
for(const member of band){
console.log(member.firstName + ' ' + member.lastName);
}
Iterating a generator:
(example extracted from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of)
function* fibonacci() { // A generator function
let [prev, curr] = [1, 1];
while (true) {
[prev, curr] = [curr, prev + curr];
yield curr;
}
}
for (const n of fibonacci()) {
console.log(n);
// Truncate the sequence at 1000
if (n >= 1000) {
break;
}
}
Compatibility table:
http://kangax.github.io/compat-table/es6/#test-for..of_loops
Specification: http://wiki.ecmascript.org/doku.php?id=harmony:iterators
}

In JavaScript it's not advisable to loop through an Array with a for-in loop, but it's better to use a for loop such as:
for(var i=0, len=myArray.length; i < len; i++){}
It's optimized as well ("caching" the array length). If you'd like to learn more, read my post on the subject.

6 different methods to loop through the array
You can loop through an array by many different methods. I have sorted my 6 favorite methods from top to bottom.
1. Using for loop
When it's to simply loop through an array, the for loop is my first choice.
let array = [1, 2, 3, 4, 5];
for (let i = 0; i < array.length; i++) {
console.log(array[i]);
}
2. Using forEach loop
forEach loop is a modern way to loop through the array. Also, it gives more flexibility and control over the array and elements.
let array = [1, 2, 3, 4, 5];
array.forEach((element) => {
console.log(element);
});
3. Using for...of
for...of loop gives you direct access to the array elements.
let array = [1, 2, 3, 4, 5];
for (let element of array) {
console.log(element);
}
4. Using for...in loop
for...in gives you a key using which you can access array elements.
let array = [1, 2, 3, 4, 5];
for(let index in array){
console.log(array[index]);
}
5. Using while loop
while loop is can be used to loop through the array as well.
let array = [1, 2, 3, 4, 5];
let length = array.length;
while(length > 0){
console.log(array[array.length - length]);
length--;
}
6. Using do...while loop
Likewise, I use do...while loop
let array = [1, 2, 3, 4, 5];
let length = array.length;
do {
console.log(array[array.length - length]);
length--;
}
while (length > 0)

Opera, Safari, Firefox and Chrome now all share a set of enhanced Array methods for optimizing many common loops.
You may not need all of them, but they can be very useful, or would be if every browser supported them.
Mozilla Labs published the algorithms they and WebKit both use, so that you can add them yourself.
filter returns an array of items that satisfy some condition or test.
every returns true if every array member passes the test.
some returns true if any pass the test.
forEach runs a function on each array member and doesn't return anything.
map is like forEach, but it returns an array of the results of the operation for each element.
These methods all take a function for their first argument and have an optional second argument, which is an object whose scope you want to impose on the array members as they loop through the function.
Ignore it until you need it.
indexOf and lastIndexOf find the appropriate position of the first or last element that matches its argument exactly.
(function(){
var p, ap= Array.prototype, p2={
filter: function(fun, scope){
var L= this.length, A= [], i= 0, val;
if(typeof fun== 'function'){
while(i< L){
if(i in this){
val= this[i];
if(fun.call(scope, val, i, this)){
A[A.length]= val;
}
}
++i;
}
}
return A;
},
every: function(fun, scope){
var L= this.length, i= 0;
if(typeof fun== 'function'){
while(i<L){
if(i in this && !fun.call(scope, this[i], i, this))
return false;
++i;
}
return true;
}
return null;
},
forEach: function(fun, scope){
var L= this.length, i= 0;
if(typeof fun== 'function'){
while(i< L){
if(i in this){
fun.call(scope, this[i], i, this);
}
++i;
}
}
return this;
},
indexOf: function(what, i){
i= i || 0;
var L= this.length;
while(i< L){
if(this[i]=== what)
return i;
++i;
}
return -1;
},
lastIndexOf: function(what, i){
var L= this.length;
i= i || L-1;
if(isNaN(i) || i>= L)
i= L-1;
else
if(i< 0) i += L;
while(i> -1){
if(this[i]=== what)
return i;
--i;
}
return -1;
},
map: function(fun, scope){
var L= this.length, A= Array(this.length), i= 0, val;
if(typeof fun== 'function'){
while(i< L){
if(i in this){
A[i]= fun.call(scope, this[i], i, this);
}
++i;
}
return A;
}
},
some: function(fun, scope){
var i= 0, L= this.length;
if(typeof fun== 'function'){
while(i<L){
if(i in this && fun.call(scope, this[i], i, this))
return true;
++i;
}
return false;
}
}
}
for(p in p2){
if(!ap[p])
ap[p]= p2[p];
}
return true;
})();

Introduction
Since my time in college, I've programmed in Java, JavaScript, Pascal, ABAP, PHP, Progress 4GL, C/C++ and possibly a few other languages I can't think of right now.
While they all have their own linguistic idiosyncrasies, each of these languages share many of the same basic concepts. Such concepts include procedures / functions, IF-statements, FOR-loops, and WHILE-loops.
A traditional for-loop
A traditional for loop has three components:
The initialization: executed before the look block is executed the first time
The condition: checks a condition every time before the loop block is executed, and quits the loop if false
The afterthought: performed every time after the loop block is executed
These three components are separated from each other by a ; symbol. Content for each of these three components is optional, which means that the following is the most minimal for loop possible:
for (;;) {
// Do stuff
}
Of course, you will need to include an if(condition === true) { break; } or an if(condition === true) { return; } somewhere inside that for-loop to get it to stop running.
Usually, though, the initialization is used to declare an index, the condition is used to compare that index with a minimum or maximum value, and the afterthought is used to increment the index:
for (var i = 0, length = 10; i < length; i++) {
console.log(i);
}
Using a traditional for loop to loop through an array
The traditional way to loop through an array, is this:
for (var i = 0, length = myArray.length; i < length; i++) {
console.log(myArray[i]);
}
Or, if you prefer to loop backwards, you do this:
for (var i = myArray.length - 1; i > -1; i--) {
console.log(myArray[i]);
}
There are, however, many variations possible, like for example this one:
for (var key = 0, value = myArray[key], length = myArray.length; key < length; value = myArray[++key]) {
console.log(value);
}
...or this one...
var i = 0, length = myArray.length;
for (; i < length;) {
console.log(myArray[i]);
i++;
}
...or this one:
var key = 0, value;
for (; value = myArray[key++];){
console.log(value);
}
Whichever works best is largely a matter of both personal taste and the specific use case you're implementing.
Note that each of these variations is supported by all browsers, including very very old ones!
A while loop
One alternative to a for loop is a while loop. To loop through an array, you could do this:
var key = 0;
while(value = myArray[key++]){
console.log(value);
}
Like traditional for loops, while loops are supported by even the oldest of browsers.
Also, note that every while loop can be rewritten as a for loop. For example, the while loop hereabove behaves the exact same way as this for-loop:
for(var key = 0; value = myArray[key++];){
console.log(value);
}
For...in and for...of
In JavaScript, you can also do this:
for (i in myArray) {
console.log(myArray[i]);
}
This should be used with care, however, as it doesn't behave the same as a traditional for loop in all cases, and there are potential side-effects that need to be considered. See Why is using "for...in" for array iteration a bad idea? for more details.
As an alternative to for...in, there's now also for for...of. The following example shows the difference between a for...of loop and a for...in loop:
var myArray = [3, 5, 7];
myArray.foo = "hello";
for (var i in myArray) {
console.log(i); // logs 0, 1, 2, "foo"
}
for (var i of myArray) {
console.log(i); // logs 3, 5, 7
}
Additionally, you need to consider that no version of Internet Explorer supports for...of (Edge 12+ does) and that for...in requires at least Internet Explorer 10.
Array.prototype.forEach()
An alternative to for-loops is Array.prototype.forEach(), which uses the following syntax:
myArray.forEach(function(value, key, myArray) {
console.log(value);
});
Array.prototype.forEach() is supported by all modern browsers, as well as Internet Explorer 9 and later.
Libraries
Finally, many utility libraries also have their own foreach variation. AFAIK, the three most popular ones are these:
jQuery.each(), in jQuery:
$.each(myArray, function(key, value) {
console.log(value);
});
_.each(), in Underscore.js:
_.each(myArray, function(value, key, myArray) {
console.log(value);
});
_.forEach(), in Lodash:
_.forEach(myArray, function(value, key) {
console.log(value);
});

Use the while loop...
var i = 0, item, items = ['one', 'two', 'three'];
while(item = items[i++]){
console.log(item);
}
It logs: 'one', 'two', and 'three'
And for the reverse order, an even more efficient loop:
var items = ['one', 'two', 'three'], i = items.length;
while(i--){
console.log(items[i]);
}
It logs: 'three', 'two', and 'one'
Or the classical for loop:
var items = ['one', 'two', 'three']
for(var i=0, l = items.length; i < l; i++){
console.log(items[i]);
}
It logs: 'one','two','three'
Reference: Google Closure: How not to write JavaScript

If you want a terse way to write a fast loop and you can iterate in reverse:
for (var i=myArray.length;i--;){
var item=myArray[i];
}
This has the benefit of caching the length (similar to for (var i=0, len=myArray.length; i<len; ++i) and unlike for (var i=0; i<myArray.length; ++i)) while being fewer characters to type.
There are even some times when you ought to iterate in reverse, such as when iterating over a live NodeList where you plan on removing items from the DOM during iteration.

Some use cases of looping through an array in the functional programming way in JavaScript:
1. Just loop through an array
const myArray = [{x:100}, {x:200}, {x:300}];
myArray.forEach((element, index, array) => {
console.log(element.x); // 100, 200, 300
console.log(index); // 0, 1, 2
console.log(array); // same myArray object 3 times
});
Note: Array.prototype.forEach() is not a functional way strictly speaking, as the function it takes as the input parameter is not supposed to return a value, which thus cannot be regarded as a pure function.
2. Check if any of the elements in an array pass a test
const people = [
{name: 'John', age: 23},
{name: 'Andrew', age: 3},
{name: 'Peter', age: 8},
{name: 'Hanna', age: 14},
{name: 'Adam', age: 37}];
const anyAdult = people.some(person => person.age >= 18);
console.log(anyAdult); // true
3. Transform to a new array
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => element.x);
console.log(newArray); // [100, 200, 300]
Note: The map() method creates a new array with the results of calling a provided function on every element in the calling array.
4. Sum up a particular property, and calculate its average
const myArray = [{x:100}, {x:200}, {x:300}];
const sum = myArray.map(element => element.x).reduce((a, b) => a + b, 0);
console.log(sum); // 600 = 0 + 100 + 200 + 300
const average = sum / myArray.length;
console.log(average); // 200
5. Create a new array based on the original but without modifying it
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray= myArray.map(element => {
return {
...element,
x: element.x * 2
};
});
console.log(myArray); // [100, 200, 300]
console.log(newArray); // [200, 400, 600]
6. Count the number of each category
const people = [
{name: 'John', group: 'A'},
{name: 'Andrew', group: 'C'},
{name: 'Peter', group: 'A'},
{name: 'James', group: 'B'},
{name: 'Hanna', group: 'A'},
{name: 'Adam', group: 'B'}];
const groupInfo = people.reduce((groups, person) => {
const {A = 0, B = 0, C = 0} = groups;
if (person.group === 'A') {
return {...groups, A: A + 1};
} else if (person.group === 'B') {
return {...groups, B: B + 1};
} else {
return {...groups, C: C + 1};
}
}, {});
console.log(groupInfo); // {A: 3, C: 1, B: 2}
7. Retrieve a subset of an array based on particular criteria
const myArray = [{x:100}, {x:200}, {x:300}];
const newArray = myArray.filter(element => element.x > 250);
console.log(newArray); // [{x:300}]
Note: The filter() method creates a new array with all elements that pass the test implemented by the provided function.
8. Sort an array
const people = [
{ name: "John", age: 21 },
{ name: "Peter", age: 31 },
{ name: "Andrew", age: 29 },
{ name: "Thomas", age: 25 }
];
let sortByAge = people.sort(function (p1, p2) {
return p1.age - p2.age;
});
console.log(sortByAge);
9. Find an element in an array
const people = [ {name: "john", age:23},
{name: "john", age:43},
{name: "jim", age:101},
{name: "bob", age:67} ];
const john = people.find(person => person.name === 'john');
console.log(john);
The Array.prototype.find() method returns the value of the first element in the array that satisfies the provided testing function.
References
Array.prototype.some()
Array.prototype.forEach()
Array.prototype.map()
Array.prototype.filter()
Array.prototype.sort()
Spread syntax
Array.prototype.find()

Yes, you can do the same in JavaScript using a loop, but not limited to that. There are many ways to do a loop over arrays in JavaScript. Imagine you have this array below, and you'd like to do a loop over it:
var arr = [1, 2, 3, 4, 5];
These are the solutions:
1) For loop
A for loop is a common way looping through arrays in JavaScript, but it is no considered as the fastest solutions for large arrays:
for (var i=0, l=arr.length; i<l; i++) {
console.log(arr[i]);
}
2) While loop
A while loop is considered as the fastest way to loop through long arrays, but it is usually less used in the JavaScript code:
let i=0;
while (arr.length>i) {
console.log(arr[i]);
i++;
}
3) Do while
A do while is doing the same thing as while with some syntax difference as below:
let i=0;
do {
console.log(arr[i]);
i++;
}
while (arr.length>i);
These are the main ways to do JavaScript loops, but there are a few more ways to do that.
Also we use a for in loop for looping over objects in JavaScript.
Also look at the map(), filter(), reduce(), etc. functions on an Array in JavaScript. They may do things much faster and better than using while and for.
This is a good article if you like to learn more about the asynchronous functions over arrays in JavaScript.
Functional programming has been making quite a splash in the
development world these days. And for good reason: Functional
techniques can help you write more declarative code that is easier to
understand at a glance, refactor, and test.
One of the cornerstones of functional programming is its special use
of lists and list operations. And those things are exactly what the
sound like they are: arrays of things, and the stuff you do to them.
But the functional mindset treats them a bit differently than you
might expect.
This article will take a close look at what I like to call the "big
three" list operations: map, filter, and reduce. Wrapping your head
around these three functions is an important step towards being able
to write clean functional code, and opens the doors to the vastly
powerful techniques of functional and reactive programming.
It also means you'll never have to write a for loop again.
Read more>> here:

There is a way to do it where you have very little implicit scope in your loop and do away with extra variables.
var i = 0,
item;
// Note this is weak to sparse arrays or falsey values
for ( ; item = myStringArray[i++] ; ){
item; // This is the string at the index.
}
Or if you really want to get the id and have a really classical for loop:
var i = 0,
len = myStringArray.length; // Cache the length
for ( ; i < len ; i++ ){
myStringArray[i]; // Don't use this if you plan on changing the length of the array
}
Modern browsers all support iterator methods forEach, map, reduce, filter and a host of other methods on the Array prototype.

There are various way to loop through array in JavaScript.
Generic loop:
var i;
for (i = 0; i < substr.length; ++i) {
// Do something with `substr[i]`
}
ES5's forEach:
substr.forEach(function(item) {
// Do something with `item`
});
jQuery.each:
jQuery.each(substr, function(index, item) {
// Do something with `item` (or `this` is also `item` if you like)
});
Have a look this for detailed information or you can also check MDN for looping through an array in JavaScript & using jQuery check jQuery for each.

Array loop:
for(var i = 0; i < things.length; i++){
var thing = things[i];
console.log(thing);
}
Object loop:
for(var prop in obj){
var propValue = obj[prop];
console.log(propValue);
}

I would thoroughly recommend making use of the Underscore.js library. It provides you with various functions that you can use to iterate over arrays/collections.
For instance:
_.each([1, 2, 3], function(num){ alert(num); });
=> alerts each number in turn...

If anybody is interested in the performance side of the multiple mechanisms available for Array iterations, I've prepared the following JSPerf tests:
https://jsperf.com/fastest-array-iterator
Results:
The traditional for() iterator, is by far the fastest method, especially when used with the array length cached.
let arr = [1,2,3,4,5];
for(let i=0, size=arr.length; i<size; i++){
// Do something
}
The Array.prototype.forEach() and the Array.prototype.map() methods are the slowest approximations, probably as a consequence of the function call overhead.

I did not yet see this variation, which I personally like the best:
Given an array:
var someArray = ["some", "example", "array"];
You can loop over it without ever accessing the length property:
for (var i=0, item; item=someArray[i]; i++) {
// item is "some", then "example", then "array"
// i is the index of item in the array
alert("someArray[" + i + "]: " + item);
}
See this JsFiddle demonstrating that: http://jsfiddle.net/prvzk/
This only works for arrays that are not sparse. Meaning that there actually is a value at each index in the array. However, I found that in practice I hardly ever use sparse arrays in JavaScript... In such cases it's usually a lot easier to use an object as a map/hashtable. If you do have a sparse array, and want to loop over 0 .. length-1, you need the for (var i=0; i<someArray.length; ++i) construct, but you still need an if inside the loop to check whether the element at the current index is actually defined.
Also, as CMS mentions in a comment below, you can only use this on arrays that don't contain any falsish values. The array of strings from the example works, but if you have empty strings, or numbers that are 0 or NaN, etc. the loop will break off prematurely. Again in practice this is hardly ever a problem for me, but it is something to keep in mind, which makes this a loop to think about before you use it... That may disqualify it for some people :)
What I like about this loop is:
It's short to write
No need to access (let alone cache) the length property
The item to access is automatically defined within the loop
body under the name you pick.
Combines very naturally with array.push and array.splice to use arrays like lists/stacks
The reason this works is that the array specification mandates that when you read an item from an index >= the array's length, it will return undefined. When you write to such a location it will actually update the length.
For me, this construct most closely emulates the Java 5 syntax that I love:
for (String item : someArray) {
}
... with the added benefit of also knowing about the current index inside the loop

If you're using the jQuery library, consider using
http://api.jquery.com/jQuery.each/
From the documentation:
jQuery.each( collection, callback(indexInArray, valueOfElement) )
Returns: Object
Description: A generic iterator function, which can be used to
seamlessly iterate over both objects and arrays. Arrays and array-like
objects with a length property (such as a function's arguments object)
are iterated by numeric index, from 0 to length-1. Other objects are
iterated via their named properties.
The $.each() function is not the same as $(selector).each(), which is
used to iterate, exclusively, over a jQuery object. The $.each()
function can be used to iterate over any collection, whether it is a
map (JavaScript object) or an array. In the case of an array, the
callback is passed an array index and a corresponding array value each
time. (The value can also be accessed through the this keyword, but
Javascript will always wrap the this value as an Object even if it is
a simple string or number value.) The method returns its first
argument, the object that was iterated.

There are 4 ways of array iteration:
// 1: for
for (let i = 0; i < arr.length; ++i) {
console.log(arr[i]);
}
// 2: forEach
arr.forEach((v, i) => console.log(v));
// 3: for in
for (let i in arr) {
console.log(arr[i]);
}
// 4: for of
for (const v of arr) {
console.log(v);
}
Summary: 1 and 3 solutions create extra variable, 2 - create extra function context. The best way is 4th - "for of".

Esoteric
let a= ["Hello", "World"];
while(a.length) { console.log( a.shift() ); }
Performance test
Today (2022-11-13) I perform a test on Chrome 107, Safari 15.2 and Firefox 106 on chosen solutions.
Conclusions
solutions C and D are fast or fastest on all browsers for all arrays.
solution A and B are slowest on all browsers for all arrays
Results
Details
I perform 3 tests:
small - for 2 elements array (like OP) - you can run it here
medium - for 10K elements array and - you can run it here
big - for 100K elements array - you can run it here
The below snippet presents code used in the test.
function A(a) {
let r=0;
while(a.length) r+= a.shift().length;
return r;
}
function B(a) {
let r=0;
for(i in a) r+= a[i].length;
return r;
}
function C(a) {
let r=0;
for(x of a) r+= x.length;
return r;
}
function D(a) {
let r=0;
for (i=0; i<a.length; ++i) r+= a[i].length;
return r;
}
function E(a) {
let r=0;
a.forEach(x=> r+= x.length);
return r;
}
let arr= ["Hello", "World!"];
[A,B,C,D,E].forEach(f => console.log(`${f.name}: ${f([...arr])}`))
Here are example results for Chrome for a medium array:

There's a method to iterate over only own object properties, not including prototype's ones:
for (var i in array) if (array.hasOwnProperty(i)) {
// Do something with array[i]
}
but it still will iterate over custom-defined properties.
In JavaScript any custom property could be assigned to any object, including an array.
If one wants to iterate over sparsed array, for (var i = 0; i < array.length; i++) if (i in array) or array.forEach with es5shim should be used.

The most elegant and fast way
var arr = [1, 2, 3, 1023, 1024];
for (var value; value = arr.pop();) {
value + 1
}
http://jsperf.com/native-loop-performance/8
Edited (because I was wrong)
Comparing methods for looping through an array of 100000 items and do a minimal operation with the new value each time.
http://jsben.ch/#/BQhED
Preparation:
<script src="//code.jquery.com/jquery-2.1.0.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/underscore.js/1.6.0/underscore-min.js"></script>
<script>
Benchmark.prototype.setup = function() {
// Fake function with minimal action on the value
var tmp = 0;
var process = function(value) {
tmp = value; // Hold a reference to the variable (prevent engine optimisation?)
};
// Declare the test Array
var arr = [];
for (var i = 0; i < 100000; i++)
arr[i] = i;
};
</script>
Tests:
<a href="http://jsperf.com/native-loop-performance/16"
title="http://jsperf.com/native-loop-performance/16"
><img src="http://i.imgur.com/YTrO68E.png" title="Hosted by imgur.com" /></a>

There are a couple of ways to do it in JavaScript. The first two examples are JavaScript samples. The third one makes use of a JavaScript library, that is, jQuery making use of the .each() function.
var myStringArray = ["hello", "World"];
for(var i in myStringArray) {
alert(myStringArray[i]);
}
var myStringArray = ["hello", "World"];
for (var i=0; i < myStringArray.length; i++) {
alert(myStringArray[i]);
}
var myStringArray = ["hello", "World"];
$.each(myStringArray, function(index, value){
alert(value);
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

The optimized approach is to cache the length of array and using the single variable pattern, initializing all variables with a single var keyword.
var i, max, myStringArray = ["Hello", "World"];
for (i = 0, max = myStringArray.length; i < max; i++) {
alert(myStringArray[i]);
// Do something
}
If the order of iteration does not matter then you should try reversed loop. It is the fastest as it reduces overhead condition testing and decrement is in one statement:
var i,myStringArray = ["item1","item2"];
for (i = myStringArray.length; i--) {
alert(myStringArray[i]);
}
Or better and cleaner to use a while loop:
var myStringArray = ["item1","item2"],i = myStringArray.length;
while(i--) {
// Do something with fruits[i]
}

In JavaScript, there are so many solutions to loop an array.
The code below are popular ones
/** Declare inputs */
const items = ['Hello', 'World']
/** Solution 1. Simple for */
console.log('solution 1. simple for')
for (let i = 0; i < items.length; i++) {
console.log(items[i])
}
console.log()
console.log()
/** Solution 2. Simple while */
console.log('solution 2. simple while')
let i = 0
while (i < items.length) {
console.log(items[i++])
}
console.log()
console.log()
/** Solution 3. forEach*/
console.log('solution 3. forEach')
items.forEach(item => {
console.log(item)
})
console.log()
console.log()
/** Solution 4. for-of*/
console.log('solution 4. for-of')
for (const item of items) {
console.log(item)
}
console.log()
console.log()

If you want to use jQuery, it has a nice example in its documentation:
$.each([ 52, 97 ], function( index, value ) {
alert( index + ": " + value );
});

The best way in my opinion is to use the Array.forEach function. If you cannot use that I would suggest to get the polyfill from MDN. To make it available, it is certainly the safest way to iterate over an array in JavaScript.
Array.prototype.forEach()
So as others has suggested, this is almost always what you want:
var numbers = [1,11,22,33,44,55,66,77,88,99,111];
var sum = 0;
numbers.forEach(function(n){
sum += n;
});
This ensures that anything you need in the scope of processing the array stays within that scope, and that you are only processing the values of the array, not the object properties and other members, which is what for .. in does.
Using a regular C-style for loop works in most cases. It is just important to remember that everything within the loop shares its scope with the rest of your program, the { } does not create a new scope.
Hence:
var sum = 0;
var numbers = [1,11,22,33,44,55,66,77,88,99,111];
for(var i = 0; i<numbers.length; ++i){
sum += numbers[i];
}
alert(i);
will output "11" - which may or may not be what you want.
A working jsFiddle example:
https://jsfiddle.net/workingClassHacker/pxpv2dh5/7/

It's not 100% identical, but similar:
var myStringArray = ['Hello', 'World']; // The array uses [] not {}
for (var i in myStringArray) {
console.log(i + ' -> ' + myStringArray[i]); // i is the index/key, not the item
}

For example, I used in a Firefox console:
[].forEach.call(document.getElementsByTagName('pre'), function(e){
console.log(e);
})
You can use querySelectorAll to get same result
document.querySelectorAll('pre').forEach( (e) => {
console.log(e.textContent);
})
<pre>text 1</pre>
<pre>text 2</pre>
<pre>text 3</pre>

Student trying to understand callback functions

Hi I'm trying to learn how to implement callback functions. My teacher has helped me out multiple times but I still can't pass data through the following equation below. I'm trying to get certain elements of array to get pushed into a new function if only they pass a test within the function. Please have a look and thank you for your input. An explanation as to why I get an empty array and resources to further my understanding would be appreciated.
// EACH DEFINITION
function each (collection, callback) {
for(var i = 0; i < collection.length; i ++){
callback(collection[i]);
}
}
// VARIABLE DECLARATION
var myArray = [1,2,3,4,5,6];
var isEven = function (num) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
test(item);
});
if(test(array)){
arr.push(array);
}
return arr;
}
// IMPLEMENT INVOCATION
implement(myArray, isEven);

You are building arr outside the each() loop.
I would think your code would be like this:
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
if(test(item)) {
arr.push(item);
}
});
return arr;
}
Though in this case there is no reason for your implement() filtering function at all, since javascript Array prototype already has a filter method. You could simplify your call to this:
var filteredArray = myArray.filter(isEven);
Though you might also then want to change your isEven definition to be more correct as:
var isEven = function (num, index, array) {
In your case you don't need to work with the last two parameters.

// EACH DEFINITION
function each (collection, callback, results) {
for(var i = 0; i < collection.length; i ++){
callback(collection[i]);
}
console.log(results);
}
// VARIABLE DECLARATION
var myArray = [1,2,3,4,5,6];
var isEven = function (num, array) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
function filter (item) {
if (test(item)) {
arr.push(item);
}
}
each(array, filter, arr);
// If you return arr here, it will still be empty. You must pass it to functions it is being operated on.
}
// IMPLEMENT INVOCATION
implement(myArray, isEven);
Not only are you trying to push to arr outside of your loop, but you're trying to return arr before it has gained any values.

Two points:
First, your implementation of callback functions is correct. As far as the concept of callbacks goes, you are calling and passing the functions correctly.
However, your implement() function probably has a bug. You are not pushing to arr until after each() has already been called:
function implement(array, test) { // array = myArray, test = isEven
var arr = [];
each(array, function(item) {
result = test(item);
});
// This block should be in the loop itself
// It should also refer to item, not array
if (test(array)) {
arr.push(array);
}
return arr;
}
Try this fix based on the code you provided:
// EACH DEFINITION
function each(collection, callback) {
for (var i = 0; i < collection.length; i++) {
callback(collection[i]);
}
}
// VARIABLE DECLARATION
var myArray = [1, 2, 3, 4, 5, 6];
var isEven = function(num) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test) { // array = myArray, test = isEven
var arr = [];
each(array, function(item) {
if (test(item)) {
arr.push(item)
}
});
if (test(array)) {
arr.push(array);
}
return arr;
}
// IMPLEMENT INVOCATION
var result = implement(myArray, isEven);
console.log(result); // For snippet results

Your callback, as you defined it, is
function(item){
test(item);
}
this will only call test on each item and that's it. Since you want to take it further and add item to arr if test returns true, you should put that checking code inside the callback as well, making it
function(item){
if (test(item)) {
arr.push(item);
}
}
so that this function will be called for each of the item.
Also, this part
if(test(array)){
arr.push(array);
}
is incorrect because you are passing a whole array into isEven when isEven is expecting a number. test(array) will always return false; that's why your arr is empty.
Modifying your code to work as you wanted, it would be
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
if (test(item)) {
arr.push(item);
}
});
return arr;
}
Resources wise, there are callbacks tutorial widely available online, as well as best practices. You can easily find one that suits you best by googling.

It looks to me like the entire issue here is in the implementation section you denote. All of the other code looks adequate.
each(array, function(item){
test(item);
});
Alright, first let's examine this piece of code. You are making a call to your each function, which will use the callback anonymous function defined here as shown.
However, if you were to look at the each function itself, there is no return (which means it returns undefined by default). There is also no modification being done in each. As a result, this set of code has no effect on the execution of the code, and from certain advanced compilation technique may actually be removed by the V8 engine in chrome if that was being used.
This means the only aspect of your code which is executing is
var arr = [];
if(test(array)){
arr.push(array);
}
return arr;
At this point, test is still the isEven function, so you are basically asking this
if(array % 2 === 0) arr.push(array);
Arrays in JavaScript behave interestingly when used in conditional statements, and in this situation the array essentially has toString called on it (more in depth here: https://stackoverflow.com/a/10556035/1026459 , but basically when you have object === number then it will attempt to use toPrimitive on the object which results in a string), which makes it
if("1,2,3" % 2 === 0)
which is false. As a result arr is unchanged, and returned in its original state of [].

Fastest way to repeat an array in Javascript

I have been trying to figure out a fast way of creating an array based on repeating an element a given number of times. For my purposes, generally the element I'd be repeating is another array, and I'd be making a long repeated (2-dimensional) array out of it, so I'd prefer it to be fast. Note that in each example, c=[element] since that's how it naturally occurs in my code.
There are a few options here that I've found. At most basic is option 1:
function repeatSimple(c, n) {
var arr = [];
for (var i=0; i<n; i++) {
arr = arr.concat(c);
};
return arr;
};
Then from this question (Concatenate array to itself in order to duplicate it), gdbdmdb has another option:
function repeatApply(c, n) {
return [].concat.apply([], Array.apply(0, Array(n)).map(function() { return c }));
};
My thinking was (a) I don't really understand the second option well enough to know whether it's a good solution, and (b) it seems silly to have to call concat a whopping n times as in the first option. So I came up with one more:
function repeatBinary(c, n) {
var arr = [];
var r = 0;
while (n>0) {
r = n%2;
if (r>0) {
arr = arr.concat(c);
};
n = (n-r)/2;
if (n>0) {
c = c.concat(c);
};
};
return arr
};
This way, I only have to call concat at most 2log_2(n) times.
So, my question is, what is the fastest way to do this? Is it one of the options I'm looking at here or is there something else that blows them out of the water? Or will all of these options work at such similar speeds that it really doesn't make a difference?

I think the fastest way to do this witout even using the concat function :
function repeatSimple(element, times)
{
var result = [];
for(var i=0;i<times;i++)
result.push(element);
return result;
}
And if you don't want to use the push function , you can assign the value directly to the item on the array like this
function repeatSimple(element, times)
{
var result = Array(times);
for(var i=0;i<times;i++)
result[i] = element;
return result;
}
Another clever way to do this is to join an array and split it like this
function repeatSimple(element, times)
{
(result = Array(times).join("," + element).split(',')).shift()
return result;
}
But this last function returns an array of strings, you have to convert every element to the type you need, but anyway, these functions may help you