I was looking at some snippets of code, and I found multiple elements calling a function over a node list with a forEach applied to an empty array.
For example I have something like:
[].forEach.call( document.querySelectorAll('a'), function(el) {
// whatever with the current node
});
but I can't understand how it works. Can anyone explain me the behaviour of the empty array in front of the forEach and how the call works?
[] is an array.
This array isn't used at all.
It's being put on the page, because using an array gives you access to array prototypes, like .forEach.
This is just faster than typing Array.prototype.forEach.call(...);
Next, forEach is a function which takes a function as an input...
[1,2,3].forEach(function (num) { console.log(num); });
...and for each element in this (where this is array-like, in that it has a length and you can access its parts like this[1]) it will pass three things:
the element in the array
the index of the element (third element would pass 2)
a reference to the array
Lastly, .call is a prototype which functions have (it's a function which gets called on other functions).
.call will take its first argument and replace this inside of the regular function with whatever you passed call, as the first argument (undefined or null will use window in everyday JS, or will be whatever you passed, if in "strict-mode"). The rest of the arguments will be passed to the original function.
[1, 2, 3].forEach.call(["a", "b", "c"], function (item, i, arr) {
console.log(i + ": " + item);
});
// 0: "a"
// 1: "b"
// 2: "c"
Therefore, you're creating a quick way to call the forEach function, and you're changing this from the empty array to a list of all <a> tags, and for each <a> in-order, you are calling the function provided.
EDIT
Logical Conclusion / Cleanup
Below, there's a link to an article suggesting that we scrap attempts at functional programming, and stick to manual, inline looping, every time, because this solution is hack-ish and unsightly.
I'd say that while .forEach is less helpful than its counterparts, .map(transformer), .filter(predicate), .reduce(combiner, initialValue), it still serves purposes when all you really want to do is modify the outside world (not the array), n-times, while having access to either arr[i] or i.
So how do we deal with the disparity, as Motto is clearly a talented and knowledgeable guy, and I would like to imagine that I know what I'm doing/where I'm going (now and then... ...other times it's head-first learning)?
The answer is actually quite simple, and something Uncle Bob and Sir Crockford would both facepalm, due to the oversight:
clean it up.
function toArray (arrLike) { // or asArray(), or array(), or *whatever*
return [].slice.call(arrLike);
}
var checked = toArray(checkboxes).filter(isChecked);
checked.forEach(listValues);
Now, if you're questioning whether you need to do this, yourself, the answer may well be no...
This exact thing is done by... ...every(?) library with higher-order features these days.
If you're using lodash or underscore or even jQuery, they're all going to have a way of taking a set of elements, and performing an action n-times.
If you aren't using such a thing, then by all means, write your own.
lib.array = (arrLike, start, end) => [].slice.call(arrLike, start, end);
lib.extend = function (subject) {
var others = lib.array(arguments, 1);
return others.reduce(appendKeys, subject);
};
Update for ES6(ES2015) and Beyond
Not only is a slice( )/array( )/etc helper method going to make life easier for people who want to use lists just like they use arrays (as they should), but for the people who have the luxury of operating in ES6+ browsers of the relatively-near future, or of "transpiling" in Babel today, you have language features built in, which make this type of thing unnecessary.
function countArgs (...allArgs) {
return allArgs.length;
}
function logArgs (...allArgs) {
return allArgs.forEach(arg => console.log(arg));
}
function extend (subject, ...others) { /* return ... */ }
var nodeArray = [ ...nodeList1, ...nodeList2 ];
Super-clean, and very useful.
Look up the Rest and Spread operators; try them out at the BabelJS site; if your tech stack is in order, use them in production with Babel and a build step.
There's no good reason not to be able to use the transform from non-array into array... ...just don't make a mess of your code doing nothing but pasting that same ugly line, everywhere.
The querySelectorAll method returns a NodeList, which is similar to an array, but it's not quite an array. Therefore, it doesn't have a forEach method (which array objects inherit via Array.prototype).
Since a NodeList is similar to an array, array methods will actually work on it, so by using [].forEach.call you are invoking the Array.prototype.forEach method in the context of the NodeList, as if you had been able to simply do yourNodeList.forEach(/*...*/).
Note that the empty array literal is just a shortcut to the expanded version, which you will probably see quite often too:
Array.prototype.forEach.call(/*...*/);
The other answers have explained this code very well, so I'll just add a suggestion.
This is a good example of code that should be refactored for simplicity and clarity. Instead of using [].forEach.call() or Array.prototype.forEach.call() every time you do this, make a simple function out of it:
function forEach( list, callback ) {
Array.prototype.forEach.call( list, callback );
}
Now you can call this function instead of the more complicated and obscure code:
forEach( document.querySelectorAll('a'), function( el ) {
// whatever with the current node
});
It can be better written using
Array.prototype.forEach.call( document.querySelectorAll('a'), function(el) {
});
What is does is document.querySelectorAll('a') returns an object similar to an array, but it does not inherit from the Array type.
So we calls the forEach method from the Array.prototype object with the context as the value returned by document.querySelectorAll('a')
[].forEach.call( document.querySelectorAll('a'), function(el) {
// whatever with the current node
});
It is basically the same as:
var arr = document.querySelectorAll('a');
arr.forEach(function(el) {
// whatever with the current node
});
Want to update on this old question:
The reason to use [].foreach.call() to loop through elements in the modern browsers is mostly over. We can use document.querySelectorAll("a").foreach() directly.
NodeList objects are collections of nodes, usually returned by
properties such as Node.childNodes and methods such as
document.querySelectorAll().
Although NodeList is not an Array, it is possible to iterate over it
with forEach(). It can also be converted to a real Array using
Array.from().
However, some older browsers have not implemented NodeList.forEach()
nor Array.from(). This can be circumvented by using
Array.prototype.forEach() — see this document's Example.
Lots of good info on this page (see answer+answer+comment), but I recently had the same question as the OP, and it took some digging to get the whole picture. So, here's a short version:
The goal is to use Array methods on an array-like NodeList that doesn't have those methods itself.
An older pattern co-opted Array's methods via Function.call(), and used an array literal ([]) rather than than Array.prototype because it was shorter to type:
[].forEach.call(document.querySelectorAll('a'), a => {})
A newer pattern (post ECMAScript 2015) is to use Array.from():
Array.from(document.querySelectorAll('a')).forEach(a => {})
An empty array has a property forEach in its prototype which is a Function object. (The empty array is just an easy way to obtain a reference to the forEach function that all Array objects have.) Function objects, in turn, have a call property which is also a function. When you invoke a Function's call function, it runs the function with the given arguments. The first argument becomes this in the called function.
You can find documentation for the call function here. Documentation for forEach is here.
Just add one line:
NodeList.prototype.forEach = HTMLCollection.prototype.forEach = Array.prototype.forEach;
And voila!
document.querySelectorAll('a').forEach(function(el) {
// whatever with the current node
});
Enjoy :—)
Warning: NodeList is a global class. Don't use this recomendation if you writing public library. However it's very convenient way for increasing self-efficacy when you work on website or node.js app.
Just a quick and dirty solution I always end up using. I wouldn't touch prototypes, just as good practice. Of course, there are a lot of ways to make this better, but you get the idea.
const forEach = (array, callback) => {
if (!array || !array.length || !callback) return
for (var i = 0; i < array.length; i++) {
callback(array[i], i);
}
}
forEach(document.querySelectorAll('.a-class'), (item, index) => {
console.log(`Item: ${item}, index: ${index}`);
});
[] always returns a new array, it is equivalent to new Array() but is guaranteed to return an array because Array could be overwritten by the user whereas [] can not. So this is a safe way to get the prototype of Array, then as described, call is used to execute the function on the arraylike nodelist (this).
Calls a function with a given this value and arguments provided
individually. mdn
Norguard explained WHAT [].forEach.call() does and James Allardice WHY we do it: because querySelectorAll returns a NodeList that doesn't have a forEach method...
Unless you have modern browser like Chrome 51+, Firefox 50+, Opera 38, Safari 10.
If not you can add a Polyfill:
if (window.NodeList && !NodeList.prototype.forEach) {
NodeList.prototype.forEach = function (callback, thisArg) {
thisArg = thisArg || window;
for (var i = 0; i < this.length; i++) {
callback.call(thisArg, this[i], i, this);
}
};
}
let's say you have : const myList= document.querySelectorAll("p");
This will return an list/array of all in your HTML.
Now Array.prototype.forEach.call(myList, myCallback)
is equivalent to [].forEach.call(myList, myCallback)
where 'myCallback' is a callback function.
You are basically running the callback function on each element of myList.
Hope this helped you!
I don't know if there is any restriction, but it works.
I turned the nodeList into an iterator object using the spread operator and mapped it:
let _btns = document.querySelectorAll('.btn');
[..._btns].map(function(elem, i) {
elem.addEventListener('click', function (e) {
console.log(elem.textContent);
})
})
.btn {
padding: 5px;
color:#fff;
background-color: darkred;
text-align:center;
color: white;
}
<button class="btn">button 1</button>
<button class="btn">button 2</button>
Related
I have a code like below and it is working fine.
for(let i=0;i< this.Array.length ; i++){
if(this.Array[i].propertyObject.hasOwnProperty('header'))
this.Array[i].ColumnName = this.Array[i].propertyObject.header;
}
May i know how to achieve the same with Map. Thanks in advance.
May i know how to achieve the same with Map
I assume you mean map. map isn't the right tool for doing exactly what that loop does, because that loop modifies the array in place, but map creates a new array instead.
If you want a new array, perhaps also with new objects (e.g., functional programming or immutable programming):
// Replace `this.Array` with a new array
this.Array = this.Array.map(element => {
// If we need to change this element...
if (element.propertyObject.hasOwnProperty("header")) {
// ...do a shallow copy along with the replacement
element = {...element, ColumnName: element.propertyObject.header};
}
return element;
});
Note that that assumes the elements are simple objects. If they aren't, you'll need to handle constructing the replacement differently than just using {...original}.
But if you want to keep the same array as your current code does, your loop is just fine. You have other options (like forEach or for-of), but what you have is also fine. for-of is well-suited to what you're doing:
for (const element of this.Array) {
if (element.propertyObject.hasOwnProperty("header")) {
element.ColumnName = element.propertyObject.header;
}
}
Side note: In new code, you might want to use Object.hasOwn rather than Object.prototype.hasOwnProperty (with a polyfill if needed for older environments; recent versions of all modern browsers support it natively, though).
this.Array.forEach((item, index, arr)=> {
item.ColumnName = "header" in item ? item.header : item.ColumnName;
});
I am trying to create a function that mimics Array.prototype.push.
It takes a variable number of arguments and pushes them into a specific array.
I have managed to do this with the following code:
var array=[];
function append(){
for(var i=0;i<arguments.length;i++)
array.push(arguments[i]);
}
Now my question is:Can I rewrite the append function without using "for loop"?
Thanks in advance.
If you need to get arguments array, you should use Array's slice function on an arguments object, and it will convert it into a standard JavaScript array:
var array = Array.prototype.slice.call(arguments);
You could use Array.prototype.push.apply
function append(){
// make arguments an array
var args = Array.prototype.slice.call(arguments);
// return the number of elements pushed in the array
return Array.prototype.push.apply(array, args);
}
So, what's happening here with args? We use Array.prototype.slice.call with arguments, the purpose being to make arguments an array, because it is a special object. Function.prototype.call is used to call a function with a specific context (aka this), and then the arguments to call the function with (comma separated). Conveniently, it appears that slice() looks at the length property of the this context, and arguments has one too, and when not empty, has properties from 0 to length -1, which allows slice to copy arguments in a new array.
You can rewrite this without a for loop, but you have to use a loop of some sort (you're working with multiple items, it's a necessity).
If you have access to ES6 or Babel, I would use something like:
function append(...args) {
return array.concat(args);
}
Without ES6, you need to work around the fact that arguments isn't a real array. You can still apply most of the array methods to it, by accessing them through the Array prototype. Converting arguments into an array is easy enough, then you can concat the two:
function append() {
var args = Array.prototype.map.call(arguments, function (it) {
return it;
});
return array.concat(args);
}
Bear in mind that neither of these will modify the global array, but will return a new array with the combined values that can be used on its own or assigned back to array. This is somewhat easier and more robust than trying to work with push, if you're willing to array = append(...).
Actually i honestly believe that push must be redefined for the functional JS since it's returning value is the length of the resulting array and it's most of the time useless. Such as when it's needed to push a value and pass an array as a parameter to a function you cant do it inline and things get messy. Instead i would like it to return a reference to the array it's called upon or even a new array from where i can get the length information anyway. My new push proposal would be as follows;
Array.prototype.push = function(...args) {
return args.reduce(function(p,c) {
p[p.length] = c;
return p
}, this)
};
It returns a perfect reference to the array it's called upon.
So what is the difference between this two functions?
They both create new Array object. Only difference I found so far is that Array.from supports ArrayLike parameters. I don't see any reason why they just haven't added ArrayLike support for Array.prototype.map function.
Am I missing something?
The purpose of Array.from() is to take a non-array (but array-like) object and make a copy of it into an actual array. This then allows you to use ALL array methods on the copy including things beyond just iterating it such as .splice(), .sort(), .push(), .pop(), etc... which is obviously much more capable than just make .map() work with array-like things.
Array.map seems to be a bit more performant as well:
var a = () => [{"count": 3},{"count": 4},{"count": 5}].map(item => item.count);
var b = () => Array.from([{"count": 3},{"count": 4},{"count": 5}], x => x.count);
var iterations = 1000000;
console.time('Function #1');
for(var i = 0; i < iterations; i++ ){
b();
};
console.timeEnd('Function #1')
console.time('Function #2');
for(var i = 0; i < iterations; i++ ){
a();
};
console.timeEnd('Function #2')
Running this code using Chrome (Version 65.0.3325.181) on this page gave me the follow results:
Function #1: 520.591064453125ms
Function #2: 33.622802734375ms
Static method vs instance method
I know a lot of time has passed since the question was asked. A lot of good things have been said. But I would like to add some more. If we try to determine the nature of the two methods we can say that Array.from
has no relation to any instance of Array. It is static method like Array.isArray or Array.of. You also have static properties like length for the Array object. As a static method Array.from can not be Called from instance.
For example:
var indexes=[0,1,2,3]
index.from()
>>> index.from is not a function
In the other hand if you write
Array.map() you will end up with a Array.map is not a function. It is because Array.prototype.map Exist for the instance of array. In our little example indexes is an instance of Array then we use map on it.
Example
var indexes=[0,1,2,3]
function doubleIt(x){
return 2*x;
}
indexes.map(doubleIt);
With array.from it shoud be something like
Array.from(indexes, doubleIt)
I used quokka plugin on vscode to evaluate performance on vs code in a windows machine. It is not real case of performance benchmarking. But it can help to have an idea. I came up with the same conclusion as #rileynet map seem more performant but only for large array.
var N=10
var tabIndex=[ ...Array(N).keys()]
function doubleIt(x){
return 2*x;
}
tabIndex.map(doubleIt);/*?.*/ 0.040ms
Array.from(tabIndex, doubleIt)/*?.*/ 0.009ms
if N=100
tabIndex.map(doubleIt);/*?.*/ 0.052ms
Array.from(tabIndex, doubleIt)/*?.*/ 0.041ms
if N=1000
tabIndex.map(doubleIt);/*?.*/ 0.228ms
Array.from(tabIndex, doubleIt)/*?.*/ 0.339ms
if N=10000
tabIndex.map(doubleIt);/*?.*/ 2.662ms
Array.from(tabIndex, doubleIt)/*?.*/ 1.847ms
N=100000
tabIndex.map(doubleIt);/*?.*/ 3.538ms
Array.from(tabIndex, doubleIt)/*?.*/ 11.742ms
Making Array.prototype the prototype object for every single array-like "Class" in JS (more importantly, in DOM, where most of the 'array-like' objects live) would be a potential mistake.
What would a .reduce( ) on a list of HTML elements/attributes look like?
Array.from is the official version of [].slice.call(arrayLike); with the added benefit of not having to create an unused array, just to create an array.
So really, Array.from can be polyfilled with function (arrLike) { return [].slice.call(arrLike); }, and minus native-implementation speed/memory improvements, it's the same result.
This has little to do with map|reduce|filter|some|every|find, which are the keys to living a long and happy life, without the need of micromanaging loops to get things done.
I have many functional methods i need to use it and i need to publish a library with this methods to share it with JavaScript developers it helps very much so for instance i need to add a Method named duplicates will return to me the duplicates of the Array
as you can see this method is not officially published by ECMA so i dont know the best form to put the script
1-
Array.prototype.duplicate = function (){
//script here its usefull to use `this` refer to the Array
}
Using it like
[1,2,2].duplicates();
2-
var Ary = function(a){
if(!(this instanceOf Ary))
return new Ary(a)
if(Object.prototype.toString.call(a) != '[object Array]')
return new Error(a + 'is not an Array')
else
{
for(var i =0 ; i<a.length; i++)
{
this.push(a[i]);
}
}
}
Ary.prototype = new Array();
Ary.prototype.constructor = Ary;
Ary.prototype.duplicates = function(){
//script here its usefull to use `this` refer to the Array
};
Using it like
Ary([1,2,2]).duplicates();
i need to know is it more like it to use prototype directly to Array JavaScript Class to add functionality if it is not officialy published with ECMA and instead we do inherit from Array Class and then play with it ???
or its ok do prototype it ??
and whats the consequences
Regards
For your own code, it's fine to add a duplicates method to Array.prototype but you do need to be prepared for what may happen if you use code (either your own, or something you're using) that incorrectly uses for..in to loop through arrays like this:
for (var i in myArray) { // <==== Wrong without safeguards
}
...because i will get the value "duplicates" at some point, since for..in loops through the enumerable properties of an object and its prototype(s), it does not loop through array indexes. It's fine to use for..in on arrays if you handle it correctly, more in this other answer on SO.
If you're only going to work in an ES5-enabled environment (modern browsers, not IE8 and earlier), you can avoid that by adding your duplicates via Object.defineProperty, like this:
Object.defineProperty(Array.prototype, "duplicates", {
value: function() {
// ...the code for 'duplicates' here
}
});
A property defined that way is not enumerable, and so does not show up in for..in loops, so code that fails to correctly handle for..in on arrays isn't impacted.
Unfortunately, it's currenty impossible in JavaScript to correctly derive from Array.prototype (your second option), because Array has special handling of properties whose names are all digits (called "array indexes") and a special length property. Neither of these can currently be correctly provided in a derived object. More about those special properties in my blog article A Myth of Arrays.
As a general rule: don't monkey patch the native Javascript object prototypes. It may appear harmless, but if you're including third party code in your site/application, it can cause all kinds of subtle bugs.
Modifying Array prototype is particularly evil, because the internet is rife with buggy, incorrect code that iterates arrays using the for ... in construct.
Check it out:
for(var i in [1,2,3]) {
console.log(i);
}
Outputs:
1
2
3
But if you've modified the Array prototype as follows:
Array.prototype.duplicates = function() { }
It outputs
1
2
3
duplicates
See for yourself.
I am iterating over an array in MooTools but seeing additional items when iterating through the array using the shorthand for..in loop. It works fine when I use the regular for loop. Is this a problem with MooTools polluting the global namespace or am I doing something wrong here?
There is a createTabs() function that iterates over an array and creates a tab for each value in the array:
function createTabs() {
var myTabs = [["First", "a.png"], ["Second", "b.png"]];
for(var i in myTabs) {
var tab = new Tab(myTabs[i][0], myTabs[i][1]);
console.log(i);
}
}
This is the output of console.log(i):
0
1
$family
each
clean
associate
link
contains
extend
getLast
getRandom
include
combine
erase
empty
flatten
hexToRgb
rgbToHex
toJSON
I understand the first 2 indexes, but where is the rest coming from?
Edit: Thanks for the quick answers Chetan and k Prime. That makes sense, and the Array.each addition by MooTools is much cleaner way to iterate!
Looks a lot better now:
myTabs.each(function(item) {
var tab = new Tab(item[0], item[1]);
console.log(item);
});
As Chetan pointed out, for .. in is meant for object property iteration, not arrays. however, you can iterate over the current members (and not the inherited members set by MooTools), by using hasOwnProprty, like so:
for (i in array)
if (array.hasOwnProperty(i))
{
//.. do stuff ...
}
Orr, better still, since you're using MooTools, just use the Array.each method:
array.each (function (item, index)
{
// ... do stuff ...
});
for..in is not meant for array iteration. It iterates over all the properties of an object that are not built-in. Since MooTools has added more functions to Array prototype, they are now array properties as well. See this https://developer.mozilla.org/En/Core_JavaScript_1.5_Reference/Statements/For...in
Just use a basic for loop for array iteration.