By Mark Szymanski


2010-06-10 00:04:27 8 Comments

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 you do the same in JavaScript?

30 comments

@Shijo Rs 2018-10-09 13:17:03

var array = ['hai','hello','how','are','you']
$(document).ready(function(){
$('#clickButton').click(function(){
for(var i=0;i<array.length;i++){
    alert(array[i])
}
})
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.1/jquery.min.js"></script>
<input id="clickButton" value="click Me" type="button"/>
<div id="show"></div>

@Willem van der Veen 2018-09-25 17:09:08

Summary:

When iterating over an array we often want to achieve one of the following goals:

  1. Create a new array array leave the original intact: use map()
  2. Perform an action with/on every array element and potentially mutate the array: use for..of, forEach(), or a regular for loop

Examples:

const arr1 = [1, 2, 3];

const arr2 = arr1.map(el => el * 2);

// with map we create a new arr2, arr1 is left untouched
console.log(arr2, arr1);


// regular for loop
for (let i = 0; i < arr1.length; i++) {
  console.log(arr1[i]);
}

console.log('\n');

// for of loop
for (let el of arr1) {
  console.log(el);

}

console.log('\n');


// forEach()
arr1.forEach(el => {
  console.log(el)
})

Which one to use?

Most of the time it doens't really matter that much. However there are some edge cases which one type of iteration might be more suited to your needs:

  1. When you want to manipulate an array but leave the old array intact (e.g. functional programming) you can best use map() for iteration. When you dont' need to return a new array don't use map(). map() has the slowest performance of all looping methods.
  2. When performance play a role, when you need to iterate an very amount, a regular for loop:

for(let i=0; i < arr.length; i++) {}

has often (can differ due to differences in JS engine) the best performance because it is the most 'low level' form of iteration.

@colxi 2018-08-20 23:36:01

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

Performamce results

Results :

The traditional for() iterator, is by far the fastest method, specially 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

@Yuci 2018-02-23 11:29:38

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);

enter image description here

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);

enter image description here

The Array.prototype.find() method returns the value of the first element in the array that satisfies the provided testing function.

References

@Mark Reed 2012-04-16 02:03:36

Yes, but only if your implementation includes the for...of feature introduced in ECMAScript 2015 (the "Harmony" release).

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 via let and const:

// REQUIRES ECMASCRIPT 2015+
const myStringArray = ["Hello", "World"];
for (const s of myStringArray) {
  // ... do something with s ...
}
// s is no longer defined here

Many JavaScript developers are still working in an environment that's not there yet, however - especially if writing code to run in web browsers, where the site developers often can't be sure what browser/version their clients will be using.

If you can assume the JavaScript interpreter 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 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 ...
} );

But if even that is too much to assume, and you want something that works in all versions of JavaScript, then you have to use an explicit counting loop. The safest version, which handles sparse arrays properly, is something like this:

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 ...
  }
}

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; using Rhino on my machine, the speedup is 43%.

You will often 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 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 does it iterate over those in numeric order. Therefore, the for...in syntax should not be used for looping through Arrays.

@Phrogz 2012-06-04 16:29:24

Note that some interpreters (e.g. V8) will automatically cache the length of the array if the code is called enough times and it detects that the length is not modified by the loop. While caching the length is still nice, it may not provide a speed boost when your code is being invoked enough times to actually make a difference.

@Denis V 2013-11-28 21:08:37

@mark-reed Could you please explain why you used i in myStringArray in your example? How can that be false?

@Mark Reed 2013-11-28 23:40:37

@DenisV - easily - the array could be sparse. The fact that a.length == n only means that the highest numeric key in the array is n-1 - it doesn't necessarily imply that the rest of the keys 0 through n-2 are actually present in the array. That's the typical case, but it's not by any means guaranteed.

@Denis V 2013-11-29 11:59:25

@MarkReed I would say, it's not true. You cannot delete an array element without changing the length property. length property guarantees that you have length elements in the array. Arrays always have all the IDs (though elements can be undefined). And the only way you can delete an item from the Array is splice and it renumbers the items and changes the length. While delete a[i] preserves the id, but assigns the value of this element to undefined.

@Mark Reed 2013-11-29 13:34:36

@DenisV: false. a=[1,2,3,4]; delete a[2]; for (j in a) { console.log(j); } outputs 0, 1, 3, and 4. a.length is still 5.

@Denis V 2013-11-29 14:30:19

@MarkReed but doing for j in a for Array is highly discouraged. And if you access non-existing element in an array, it will not throw an error or produce any other unexpected behavior.

@Denis V 2013-11-29 14:32:43

@MarkReed moreover, if you use console.log(a) you will get the following: [1, 2, undefined, 4], which proves that the 3rd element exists. I agree, that it looks like inconsistency in the standard, but I believe, the check is reduntant in this case.

@Mark Reed 2013-11-30 01:14:07

I'm not suggesting for j in a. I'm demonstrating that the in check is not redundant, as you claimed it was, by showing all the indexes and showing that there is one between 0 and length-1 that is not there. I could also have just printed 2 in a, which is indeed false, despite the fact that you said that was impossible.

@Grijesh Chauhan 2014-01-14 15:37:58

As you says that "restrict yourself to a subset of available JavaScript engines" Is it means forEach is not available in all browsers? Btw your answer is very helpful to me Thanks!

@Mark Reed 2014-01-14 15:40:39

@GrijeshChauhan - correct. For instance, IE through version 8 doesn't support it. See this question.

@Code Bling 2014-09-05 07:42:47

@MarkReed Are you sure that your performance data is correct? I noticed that your loop (set up as for (var i=0; i<len; ++i)), uses ++i, which would cause the first item in the array to be skipped.

@Mark Reed 2014-09-05 13:03:01

@CodeBling: No, it doesn't. The increment (third clause) doesn't get evaluated until after the body of the loop, so there's no difference between ++i and i++ unless there are additional expressions within that clause.

@DrBailey 2015-08-31 20:04:37

It seems like this is rather verbose for something that is extremely common. Is there a reason why there is not a simple implementation of this in standard JS?

@Mark Reed 2015-08-31 20:49:16

Well, Harmony is the new "standard"; it just hasn't been widely adopted yet. As for "why", you could ask Brendan Eich. But JS was designed and implemented in an incredibly small amount of time, by a first-time language designer. It's amazing that we got a language as good as we did. The fact that for...in iterates over indexes seems weird - until you remember that Javascript's "arrays" are really just dictionaries/objects with numeric keys. Iterating over keys makes sense because you can then use keys to extract the values, but you can't go the other way.

@jj_ 2016-03-30 02:37:14

If you want to use jQuery, it has a nice example in its documentation:

 $.each([ 52, 97 ], function( index, value ) {
      alert( index + ": " + value );
 });

@John Slegers 2016-02-29 18:56:30

Intro

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:

  1. The initialization: executed before the look block is executed the first time
  2. The condition: checks a condition every time before the loop block is executed, and quits the loop if false
  3. 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" with 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.js:

_.forEach(myArray, function(value, key) {
    console.log(value);
});

@hasen 2010-06-10 00:09:16

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.

@hasen 2010-06-10 00:14:46

No, but it can be more powerful. check this out: joelonsoftware.com/items/2006/08/01.html

@harto 2010-06-10 00:20:47

That particular example is probably better implemented using Array.forEach. map is for generating a new array.

@CMS 2010-06-10 00:30:08

@hasen, the Array.prototype.map method is part of the ECMAScript 5th Edition Standard, is not yet available on all implementations (e.g. IE lacks of it), also for iterating over an array I think the Array.prototype.forEach method is more semantically correct... also please don't suggest the for-in statement, see my answer for more details :)

@intuited 2010-06-10 03:06:14

It's available in some libraries, eg jQuery... but is not really meant for iterating over things. At least semantically, it doesn't imply that things actually happen in any given order. Thus the name: you are mapping one set of values to another, not processing them iteratively.

@harto 2010-06-10 04:42:30

If you check out the link CMS provided, there's an implementation you can use in browsers that don't natively support it.

@hasen 2010-06-10 08:43:39

@intuited, I'd say the same applies to the foreach loop in java/c#/python; theoretically they can be applied in a random order (i.e. in parallel across multiple processors or even multiple machines). I'd also say that the body of the foreach loop is nothing but the body of the function that's being passed to map.

@intuited 2010-06-10 16:04:12

@hasen j: that's not been my understanding, mostly based on the fact that the foreach loop is derived from the for loop rather than from functional programming concepts. Also the fact that it's called a "loop" :)

@ebruchez 2014-05-20 23:04:42

@Mark Reed 2014-09-10 17:00:23

The difference between forEach and map is that the former doesn't return the results of the iteration. map (sometimes a.k.a. collect, but very different from apply) is expressly for transforming each element of an array into a corresponding result; it's a 1-to-1 mapping, hence the name. It's part of a whole family of operations that include reduce (which produces a single result from the whole array) and filter (which produces a subset of the original array) and so on. Whereas forEach just does something with each element, semantics unspecified.

@Mark Reed 2016-03-23 15:32:18

The callback function passed to map actually gets called with three arguments: the first is the current item in the array, the second is the index of that item, and the third is a reference to the array itself. You only need to declare as many as you want as parameters, because Javascript, but they're all available.

@gengkev 2016-04-26 23:47:36

Downvote because if you're not actually mapping something, then using [].map is misleading. [].forEach makes semantic sense and also passes the same three arguments to the function.

@Alireza 2017-05-27 02:57:41

Yes, you can do the same in JavaScript using loop, but not limited to that, many ways to do loop over arrays in JavaScrip, 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

For loop is a common way looping through arrays in JavaScript, but 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

While loop considered as the fastest way to loop through long arrays, but usually less used in the JavaScript:

let i=0;

while (arr.length>i) {
    console.log(arr[i]);
    i++;
}

3) Do while
Do while 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 few more ways to do that.

Also we use for in loop for looping over objects in javascript.

Also look at map(), filter(), reduce() etc functions on Array in JavaScript. They may do things much faster and better than using while and for.

This is good article if you like to learn more about the async 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:

@shea 2017-06-29 08:33:56

Is there really a performance difference before a for loop and a while loop when iterating through an array? I was under the impression the differences were primarily syntactical

@Muhammad Alvin 2012-04-18 14:46:06

It's not 100% identical, but similar:

   var myStringArray = ['Hello', 'World']; // array uses [] not {}
    for (var i in myStringArray) {
        console.log(i + ' -> ' + myStringArray[i]); // i is the index/key, not the item
    }

@Kzqai 2012-04-18 15:34:45

It seems that this would run up against similar problems as other for in usages with an array object, in that prototype member variables would be caught by the for in as well.

@BILAL AHMAD 2017-09-13 22:11:03

Just a simple one line solution

arr = ["table", "chair"];

// solution
arr.map((e) => {
  console.log(e);
  return e;
});

@Michel Jung 2017-10-06 11:09:28

You'd rather want to use .forEach() and drop the return e;

@marpme 2017-11-17 22:28:46

as map implies, the function map is for mapping a certain value to something else, hence I would not suggest using that one for this certain example.

@Andrew 2017-09-02 20:35:32

It seems that listed all the variants except forEach by lodash:

_.forEach([1, 2], (value) => {
  console.log(value);
});

@bzim 2016-08-01 21:18:18

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);
}

@Alongkorn Chetasumon 2016-10-14 10:31:58

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()

@Espen 2016-10-26 08:51:39

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 i available, it is certainly the safest way to iterate over an array in JavaScript.

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/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 it's 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.

Working jsFiddle example: https://jsfiddle.net/workingClassHacker/pxpv2dh5/7/

@Marlon Bernardes 2013-08-11 15:54:05

for (var 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 ECMA Script  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), but it is not yet widely supported by the browsers.

It currently works with Firefox 13+, Chrome 37+ and it does not natively work with other browsers (see browser compatibility below). Luckily we have JS compilers (such as Babel) that allow us to use next-generation features today.

It also works on Node (I tested it on version 0.12.0).

Iterating an array

// You could also use "let" instead of "var" for block scope.
for (var letter of ["a", "b", "c"]) { 
   console.log(letter); 
}

Iterating an array of objects

var band = [
  {firstName : 'John', lastName: 'Lennon'}, 
  {firstName : 'Paul', lastName: 'McCartney'}
];

for(var 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 (let n of fibonacci()) {
  console.log(n);
  // truncate the sequence at 1000
  if (n >= 1000) {
    break;
  }
}

Compatibility table: http://kangax.github.io/es5-compat-table/es6/#For..of loops

Spec: http://wiki.ecmascript.org/doku.php?id=harmony:iterators

}

@joeytwiddle 2018-02-10 06:26:01

If you're using ES6, I would suggest const s instead of var s

@molokoloco 2014-03-08 02:06:39

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.

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>

@Deniz Ozger 2014-03-26 15:36:11

This loop doesn't seem to follow order of items in the array.

@molokoloco 2014-03-27 16:41:40

My test was wrong. It's correct, showing all LOOPS now. jsperf.com/native-loop-performance/16

@Bergi 2014-03-30 14:49:57

-1 for modifying the array, which a plain loop should not do. Probably affecting the performance test as well.

@Stijn de Witt 2014-05-15 17:34:36

@bergi is right. This loop wipes out the array as it loops through it. Not what you want in most cases.

@njzk2 2014-07-29 14:59:25

breaks on falsey items.

@kennebec 2010-06-10 02:43:29

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;
})();

@rwitzel 2015-04-17 15:12:25

Addition: IE supports forEach since version 9, see forEach Method MSDN

@Daniel K. 2015-12-12 03:11:44

Sure it's inefficient and many despise it, but it's one of the closest to the mentioned:

var myStringArray = ["Hello","World"];
myStringArray.forEach(function(f){
    // Do something
})

@user22a6db72d7249 2015-12-12 03:27:28

This exact functionality is already part of Mark Reed's answer.

@Juanjo Salvador 2016-03-17 10:13:33

Short answer: yes. You can do with this:

var myArray = ["element1", "element2", "element3", "element4"];

for (i = 0; i < myArray.length; i++) {
  console.log(myArray[i]);
}

In a browser console, you can see something like "element1", "element2", etc., printed.

@user6139250 2016-03-31 11:28:21

It is better to use a sequential for loop:

for (var i = 0; i < myStringArray.length; i++) {
    // Do something
}

@Shubham Khatri 2016-04-21 16:03:42

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>

@brk 2016-12-05 05:25:56

for...in should be avoided for Array-like objects

@Dan Chill 2016-05-26 19:11:06

var obj = ["one","two","three"];

for(x in obj){
    console.log(obj[x]);
}

@Amit Jamwal 2016-04-29 09:05:04

var myStringArray = ["hello", "World"];
myStringArray.forEach(function(val, index){
   console.log(val, index);
})

@Phrogz 2012-06-04 16:26:03

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.

@Stijn de Witt 2013-03-01 12:09:38

For the people that don't get what is so ingenious: The i-- expression is first evaluated and allows the loop to continue when it's not falsish... Afterwards the counter is decremented. As soon as i becomes zero it will break out of the loop as zero is a falsish value in Javascript.

@danwellman 2013-04-27 07:33:43

falsish? You mean falsey. Let's all stick the proper terminology to avoid confusion ;)

@Stijn de Witt 2014-05-15 17:23:26

I've seen the term falsish being used by people I consider gurus. If it's good enough for them it's good enough for me. Also a but disappointed to see that my comment that is actually ontopic and adds explanation/insight gets 0 upvotes, but the comment that nitpicks on a term in my comment gets 4. Ah well just a matter of priorities I guess.

@Mouscellaneous 2016-01-28 08:54:38

"Caching the length"? The length is stored as an integer in the array, it's not measured every time you access it. There's no benefit here in copying the value of length into another variable.

@Phrogz 2016-01-28 18:33:12

@Mouscellaneous These days there certainly is not; in years past iterating JavaScript arrays caching the length on the JavaScript side (instead of reaching across the implementation) was a clear perf gain (when microoptimizing). For example, for (var i=0,len=array.length;i<len;++i) was a common, sensible loop to write.

@ZiTAL 2016-08-11 10:25:00

but the order is in reverse mode

@justingordon 2012-10-21 06:20:48

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.

@Exception 2013-02-07 19:03:29

jQuery for everything?

@Stijn de Witt 2013-03-31 19:04:06

Agreed with Exception. Do not underestimate the impact of extra dependencies. I would advice against this except in code that is already heavily using jQuery anyway.

@Stijn de Witt 2017-06-05 10:50:41

Update: These days, you can use Array.forEach to get much of the same effect with native arrays.

@staticd 2013-08-06 08:03:25

var x = [4, 5, 6];
for (i = 0, j = x[i]; i < x.length; j = x[++i]) {
    console.log(i,j);
}

A lot cleaner...

@Matiaan 2015-01-07 12:26:49

did you mean "x =" on the first line?

@RizN81 2014-07-23 12:59:27

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.

@Pete 2014-12-30 02:33:04

It's a shame the ES5 forEach isn't at the top of the answers because it most closely matches what the OP was asking for.

@victorq10 2014-10-21 07:32:00

For example, I used in a Firefox console:

[].forEach.call(document.getElementsByTagName('pre'), function(e){ 
   console.log(e);
})

@CMS 2010-06-10 00:07:12

Use a sequential for loop:

var myStringArray = ["Hello","World"];
var arrayLength = myStringArray.length;
for (var i = 0; i < arrayLength; i++) {
    alert(myStringArray[i]);
    //Do something
}

@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 be also enumerated.

For example:

Array.prototype.foo = "foo!";
var array = ['a', 'b', 'c'];

for (var i in array) {
  alert(array[i]);
}

The above code will alert, "a", "b", "c" and "foo!".

That be particularly a problem if you use some library that relies heavily on native prototypes augmention (such as MooTools for example).

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...
    alert("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:

@CMS 2010-06-10 00:10:25

Why the down-vote? for...in should be avoided for Array-like objects!

@OscarRyz 2010-06-10 00:13:31

This is the reason ( by CMS him self ) stackoverflow.com/questions/1885317/…

@Shelby Moore III 2012-02-08 09:03:41

Yours enumerates undefined elements, and for(in) doesn't. Official foreach algorithm checks if(in). I agree that iterating only numeric indices is correct for element enumeration (hasOwnProperty will allow non-numeric properties), note it is superior design to not depend on iteration order, so as to enable parallelism, assuming the iterated callback doesn't have side-effects.

@Gras Double 2012-04-17 18:41:30

@Gabriel and @YuriKolovsky are right, accessing the length property has some cost in JS and you should always cache it, like so: for (var i = 0, l = myStringArray.length; i < l; i++) { ... }

@Matthijs Wessels 2012-08-14 16:41:30

@DoubleGras, I think that is an opinion that is not shared by everyone. See: stackoverflow.com/questions/5752906/… or groups.google.com/forum/?fromgroups#!topic/jsmentors/…

@Stijn de Witt 2013-02-28 14:03:14

Anyone thinking you need to cache the length... Please see my answer, you don't even need to access it a single time, let alone cache it: for (var i=0, item; item=myStringArray[i]; i++) { /* use item here */ }

@Phrogz 2013-04-27 13:32:43

@StijndeWitt No, because that breaks if you have any "falsey" values in your array: false, undefined, 0, "", NaN.

@Enrico 2013-08-07 07:26:21

jsperf.com/caching-array-length/4 Here is a test to see if it is worth caching the length of an array in a Javascript loop

@jasop 2013-08-16 01:35:13

Enrico's link to a performance comparison between caching and not caching shows that 'not caching' is.... 0.02% slower! negligible really

@techie_28 2013-08-20 08:39:13

Can I add something to JS array while iterating over it?Will that add on to the iterations?

@Timo Huovinen 2013-09-23 18:04:57

The only reason I can think of for not caching the length is if the array changes while the loop is in progress

@user890167 2013-11-08 22:48:15

Why is this preferable to someArray.forEach(function(item){}), that it should have so many upvotes?

@slicedtoad 2014-09-10 13:57:44

I understand that for...in loops are undesirable for arrays but forEach is a viable alternative a lot of the time. It looks cleaner, encourages code seperation and reuse, and is supported by IE9+ (EMCA 5.1 standard). It would be really nice if it was edited into this answer as an alternative approach.

@Alex Couper 2015-11-27 19:17:47

In ES6 you can do: for (let i of arr) { } See: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

@4castle 2016-06-16 19:58:32

Another reason not to use for..in with an array, the counter variable will be a STRING!!

@Mohammad Usman 2017-11-08 08:00:08

@CMS I see you have a very big profile on SO. But why you have stopped answering? Your last answer on SO was about 5 years ago I guess?

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