For-each over an array in JavaScript


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

I thought it was something like this:

forEach(instance in theArray)

Where theArray is my array, but this seems to be incorrect.

3/8/2020 2:47:59 PM

Accepted Answer


  • Don't use for-in unless you use it with safeguards or are at least aware of why it might bite you.
  • Your best bets are usually

    • a for-of loop (ES2015+ only),
    • Array#forEach (spec | MDN) (or its relatives some and such) (ES5+ only),
    • a simple old-fashioned for loop,
    • or for-in with safeguards.

But there's lots more to explore, read on...

JavaScript has powerful semantics for looping through arrays and array-like objects. I've split the answer into two parts: Options for genuine arrays, and options for things that are just array-like, such as the arguments object, other iterable objects (ES2015+), DOM collections, and so on.

I'll quickly note that you can use the ES2015 options now, even on ES5 engines, by transpiling ES2015 to ES5. Search for "ES2015 transpiling" / "ES6 transpiling" for more...

Okay, let's look at our options:

For Actual Arrays

You have three options in ECMAScript 5 ("ES5"), the version most broadly supported at the moment, and two more added in ECMAScript 2015 ("ES2015", "ES6"):

  1. Use forEach and related (ES5+)
  2. Use a simple for loop
  3. Use for-in correctly
  4. Use for-of (use an iterator implicitly) (ES2015+)
  5. Use an iterator explicitly (ES2015+)


In any vaguely-modern environment (so, not IE8) where you have access to the Array features added by ES5 (directly or using polyfills), you can use forEach (spec | MDN):

var a = ["a", "b", "c"];
a.forEach(function(entry) {

forEach accepts a callback function and, optionally, a value to use as this when calling that callback (not used above). The callback is called for each entry in the array, in order, skipping non-existent entries in sparse arrays. Although I only used one argument above, the callback is called with three: The value of each entry, the index of that entry, and a reference to the array you're iterating over (in case your function doesn't already have it handy).

Unless you're supporting obsolete browsers like IE8 (which NetApps shows at just over 4% market share as of this writing in September 2016), you can happily use forEach in a general-purpose web page without a shim. If you do need to support obsolete browsers, shimming/polyfilling forEach is easily done (search for "es5 shim" for several options).

forEach has the benefit that you don't have to declare indexing and value variables in the containing scope, as they're supplied as arguments to the iteration function, and so nicely scoped to just that iteration.

If you're worried about the runtime cost of making a function call for each array entry, don't be; details.

Additionally, forEach is the "loop through them all" function, but ES5 defined several other useful "work your way through the array and do things" functions, including:

  • every (stops looping the first time the callback returns false or something falsey)
  • some (stops looping the first time the callback returns true or something truthy)
  • filter (creates a new array including elements where the filter function returns true and omitting the ones where it returns false)
  • map (creates a new array from the values returned by the callback)
  • reduce (builds up a value by repeatedly calling the callback, passing in previous values; see the spec for the details; useful for summing the contents of an array and many other things)
  • reduceRight (like reduce, but works in descending rather than ascending order)

2. Use a simple for loop

Sometimes the old ways are the best:

var index;
var a = ["a", "b", "c"];
for (index = 0; index < a.length; ++index) {

If the length of the array won't change during the loop, and it's in performance-sensitive code (unlikely), a slightly more complicated version grabbing the length up front might be a tiny bit faster:

var index, len;
var a = ["a", "b", "c"];
for (index = 0, len = a.length; index < len; ++index) {

And/or counting backward:

var index;
var a = ["a", "b", "c"];
for (index = a.length - 1; index >= 0; --index) {

But with modern JavaScript engines, it's rare you need to eke out that last bit of juice.

In ES2015 and higher, you can make your index and value variables local to the for loop:

let a = ["a", "b", "c"];
for (let index = 0; index < a.length; ++index) {
    let value = a[index];
    console.log(index, value);
//console.log(index);   // would cause "ReferenceError: index is not defined"
//console.log(value);   // would cause "ReferenceError: value is not defined"

let a = ["a", "b", "c"];
for (let index = 0; index < a.length; ++index) {
    let value = a[index];
    console.log(index, value);
try {
} catch (e) {
    console.error(e);   // "ReferenceError: index is not defined"
try {
} catch (e) {
    console.error(e);   // "ReferenceError: value is not defined"

And when you do that, not just value but also index is recreated for each loop iteration, meaning closures created in the loop body keep a reference to the index (and value) created for that specific iteration:

let divs = document.querySelectorAll("div");
for (let index = 0; index < divs.length; ++index) {
    divs[index].addEventListener('click', e => {
        console.log("Index is: " + index);

let divs = document.querySelectorAll("div");
for (let index = 0; index < divs.length; ++index) {
    divs[index].addEventListener('click', e => {
        console.log("Index is: " + index);

If you had five divs, you'd get "Index is: 0" if you clicked the first and "Index is: 4" if you clicked the last. This does not work if you use var instead of let.

3. Use for-in correctly

You'll get people telling you to use for-in, but that's not what for-in is for. for-in loops through the enumerable properties of an object, not the indexes of an array. The order is not guaranteed, not even in ES2015 (ES6). ES2015+ does define an order to object properties (via [[OwnPropertyKeys]], [[Enumerate]], and things that use them like Object.getOwnPropertyKeys), but it didn't define that for-in would follow that order; ES2020 did, though. (Details in this other answer.)

The only real use cases for for-in on an array are:

  • It's a sparse arrays with massive gaps in it, or
  • You're using non-element properties and you want to include them in the loop

Looking only at that first example: You can use for-in to visit those sparse array elements if you use appropriate safeguards:

// `a` is a sparse array
var key;
var a = [];
a[0] = "a";
a[10] = "b";
a[10000] = "c";
for (key in a) {
    if (a.hasOwnProperty(key)  &&        // These checks are
        /^0$|^[1-9]\d*$/.test(key) &&    // explained
        key <= 4294967294                // below
        ) {

Note the three checks:

  1. That the object has its own property by that name (not one it inherits from its prototype), and

  2. That the key is all decimal digits (e.g., normal string form, not scientific notation), and

  3. That the key's value when coerced to a number is <= 2^32 - 2 (which is 4,294,967,294). Where does that number come from? It's part of the definition of an array index in the specification. Other numbers (non-integers, negative numbers, numbers greater than 2^32 - 2) are not array indexes. The reason it's 2^32 - 2 is that that makes the greatest index value one lower than 2^32 - 1, which is the maximum value an array's length can have. (E.g., an array's length fits in a 32-bit unsigned integer.) (Props to RobG for pointing out in a comment on my blog post that my previous test wasn't quite right.)

You wouldn't do that in inline code, of course. You'd write a utility function. Perhaps:

// Utility function for antiquated environments without `forEach`
var hasOwn = Object.prototype.hasOwnProperty;
var rexNum = /^0$|^[1-9]\d*$/;
function sparseEach(array, callback, thisArg) {
    var index;
    for (var key in array) {
        index = +key;
        if (, key) &&
            rexNum.test(key) &&
            index <= 4294967294
            ) {
  , array[key], index, array);

var a = [];
a[5] = "five";
a[10] = "ten";
a[100000] = "one hundred thousand";
a.b = "bee";

sparseEach(a, function(value, index) {
    console.log("Value at " + index + " is " + value);

4. Use for-of (use an iterator implicitly) (ES2015+)

ES2015 added iterators to JavaScript. The easiest way to use iterators is the new for-of statement. It looks like this:

const a = ["a", "b", "c"];
for (const val of a) {

Under the covers, that gets an iterator from the array and loops through it, getting the values from it. This doesn't have the issue that using for-in has, because it uses an iterator defined by the object (the array), and arrays define that their iterators iterate through their entries (not their properties). Unlike for-in in ES5, the order in which the entries are visited is the numeric order of their indexes.

5. Use an iterator explicitly (ES2015+)

Sometimes, you might want to use an iterator explicitly. You can do that, too, although it's a lot clunkier than for-of. It looks like this:

const a = ["a", "b", "c"];
const it = a.values();
let entry;
while (!(entry = {

The iterator is an object matching the Iterator definition in the specification. Its next method returns a new result object each time you call it. The result object has a property, done, telling us whether it's done, and a property value with the value for that iteration. (done is optional if it would be false, value is optional if it would be undefined.)

The meaning of value varies depending on the iterator; arrays support (at least) three functions that return iterators:

  • values(): This is the one I used above. It returns an iterator where each value is the array entry for that iteration ("a", "b", and "c" in the example earlier).
  • keys(): Returns an iterator where each value is the key for that iteration (so for our a above, that would be "0", then "1", then "2").
  • entries(): Returns an iterator where each value is an array in the form [key, value] for that iteration.

For Array-Like Objects

Aside from true arrays, there are also array-like objects that have a length property and properties with numeric names: NodeList instances, the arguments object, etc. How do we loop through their contents?

Use any of the options above for arrays

At least some, and possibly most or even all, of the array approaches above frequently apply equally well to array-like objects:

  1. Use forEach and related (ES5+)

    The various functions on Array.prototype are "intentionally generic" and can usually be used on array-like objects via Function#call or Function#apply. (See the Caveat for host-provided objects at the end of this answer, but it's a rare issue.)

    Suppose you wanted to use forEach on a Node's childNodes property. You'd do this:, function(child) {
        // Do something with `child`

    If you're going to do that a lot, you might want to grab a copy of the function reference into a variable for reuse, e.g.:

    // (This is all presumably in some scoping function)
    var forEach = Array.prototype.forEach;
    // Then later..., function(child) {
        // Do something with `child`
  2. Use a simple for loop

    Obviously, a simple for loop applies to array-like objects.

  3. Use for-in correctly

    for-in with the same safeguards as with an array should work with array-like objects as well; the caveat for host-provided objects on #1 above may apply.

  4. Use for-of (use an iterator implicitly) (ES2015+)

    for-of uses the iterator provided by the object (if any). That includes host-provided objects. For instance, the specification for the NodeList from querySelectorAll was updated to support iteration. The spec for the HTMLCollection from getElementsByTagName was not.

  5. Use an iterator explicitly (ES2015+)

    See #4.

Create a true array

Other times, you may want to convert an array-like object into a true array. Doing that is surprisingly easy:

  1. Use the slice method of arrays

    We can use the slice method of arrays, which like the other methods mentioned above is "intentionally generic" and so can be used with array-like objects, like this:

    var trueArray =;

    So for instance, if we want to convert a NodeList into a true array, we could do this:

    var divs ="div"));

    See the Caveat for host-provided objects below. In particular, note that this will fail in IE8 and earlier, which don't let you use host-provided objects as this like that.

  2. Use spread syntax (...)

    It's also possible to use ES2015's spread syntax with JavaScript engines that support this feature. Like for-of, this uses the iterator provided by the object (see #4 in the previous section):

    var trueArray = [...iterableObject];

    So for instance, if we want to convert a NodeList into a true array, with spread syntax this becomes quite succinct:

    var divs = [...document.querySelectorAll("div")];
  3. Use Array.from

    Array.from (spec) | (MDN) (ES2015+, but easily polyfilled) creates an array from an array-like object, optionally passing the entries through a mapping function first. So:

    var divs = Array.from(document.querySelectorAll("div"));

    Or if you wanted to get an array of the tag names of the elements with a given class, you'd use the mapping function:

    // Arrow function (ES2015):
    var divs = Array.from(document.querySelectorAll(".some-class"), element => element.tagName);
    // Standard function (since `Array.from` can be shimmed):
    var divs = Array.from(document.querySelectorAll(".some-class"), function(element) {
        return element.tagName;

Caveat for host-provided objects

If you use Array.prototype functions with host-provided array-like objects (DOM lists and other things provided by the browser rather than the JavaScript engine), you need to be sure to test in your target environments to make sure the host-provided object behaves properly. Most do behave properly (now), but it's important to test. The reason is that most of the Array.prototype methods you're likely to want to use rely on the host-provided object giving an honest answer to the abstract [[HasProperty]] operation. As of this writing, browsers do a very good job of this, but the 5.1 spec did allow for the possibility a host-provided object may not be honest. It's in §8.6.2, several paragraphs below the big table near the beginning of that section), where it says:

Host objects may implement these internal methods in any manner unless specified otherwise; for example, one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always generates false.

(I couldn't find the equivalent verbiage in the ES2015 spec, but it's bound to still be the case.) Again, as of this writing the common host-provided array-like objects in modern browsers [NodeList instances, for instance] do handle [[HasProperty]] correctly, but it's important to test.)

5/13/2020 8:43:07 AM

Note: This answer is hopelessly out-of-date. For a more modern approach, look at the methods available on an array. Methods of interest might be:

  • forEach
  • map
  • filter
  • zip
  • reduce
  • every
  • some

The standard way to iterate an array in JavaScript is a vanilla for-loop:

var length = arr.length,
    element = null;
for (var i = 0; i < length; i++) {
  element = arr[i];
  // Do something with element

Note, however, that this approach is only good if you have a dense array, and each index is occupied by an element. If the array is sparse, then you can run into performance problems with this approach, since you will iterate over a lot of indices that do not really exist in the array. In this case, a for .. in-loop might be a better idea. However, you must use the appropriate safeguards to ensure that only the desired properties of the array (that is, the array elements) are acted upon, since the will also be enumerated in legacy browsers, or if the additional properties are defined as enumerable.

In ECMAScript 5 there will be a forEach method on the array prototype, but it is not supported in legacy browsers. So to be able to use it consistently you must either have an environment that supports it (for example, Node.js for server side JavaScript), or use a "Polyfill". The Polyfill for this functionality is, however, trivial and since it makes the code easier to read, it is a good polyfill to include.


If you’re using the jQuery library, you can use jQuery.each:

$.each(yourArray, function(index, value) {
  // do your stuff here


As per question, user want code in javascript instead of jquery so the edit is

var length = yourArray.length;   
for (var i = 0; i < length; i++) {
  // Do something with yourArray[i].

Loop backwards

I think the reverse for loop deserves a mention here:

for (var i = array.length; i--; ) {
     // process array[i]


  • You do not need to declare a temporary len variable, or compare against array.length on each iteration, either of which might be a minute optimisation.
  • Removing siblings from the DOM in reverse order is usually more efficient. (The browser needs to do less shifting of elements in its internal arrays.)
  • If you modify the array while looping, at or after index i (for example you remove or insert an item at array[i]), then a forward loop would skip the item that shifted left into position i, or re-process the ith item that was shifted right. In a traditional for loop, you could update i to point to the next item that needs processing - 1, but simply reversing the direction of iteration is often a simpler and more elegant solution.
  • Similarly, when modifying or removing nested DOM elements, processing in reverse can circumvent errors. For example, consider modifying the innerHTML of a parent node before handling its children. By the time the child node is reached it will be detached from the DOM, having been replaced by a newly created child when the parent's innerHTML was written.
  • It is shorter to type, and read, than some of the other options available. Although it loses to forEach() and to ES6's for ... of.


  • It processes the items in reverse order. If you were building a new array from the results, or printing things on screen, naturally the output will be reversed with respect to the original order.
  • Repeatedly inserting siblings into the DOM as a first child in order to retain their order is less efficient. (The browser would keep having to shift things right.) To create DOM nodes efficiently and in order, just loop forwards and append as normal (and also use a "document fragment").
  • The reverse loop is confusing to junior developers. (You may consider that an advantage, depending on your outlook.)

Should I always use it?

Some developers use the reverse for loop by default, unless there is a good reason to loop forwards.

Although the performance gains are usually insignificant, it sort of screams:

"Just do this to every item in the list, I don't care about the order!"

However in practice that is not actually a reliable indication of intent, since it is indistinguishable from those occasions when you do care about the order, and really do need to loop in reverse. So in fact another construct would be needed to accurately express the "don't care" intent, something currently unavailable in most languages, including ECMAScript, but which could be called, for example, forEachUnordered().

If order doesn't matter, and efficiency is a concern (in the innermost loop of a game or animation engine), then it may be acceptable to use the reverse for loop as your go-to pattern. Just remember that seeing a reverse for loop in existing code does not necessarily mean that the order irrelevant!

It was better to use forEach()

In general for higher level code where clarity and safety are greater concerns, I previously recommended using Array::forEach as your default pattern for looping (although these days I prefer to use for..of). Reasons to prefer forEach over a reverse loop are:

  • It is clearer to read.
  • It indicates that i is not going to be shifted within the block (which is always a possible surprise hiding in long for and while loops).
  • It gives you a free scope for closures.
  • It reduces leakage of local variables and accidental collision with (and mutation of) outer variables.

Then when you do see the reverse for loop in your code, that is a hint that it is reversed for a good reason (perhaps one of the reasons described above). And seeing a traditional forward for loop may indicate that shifting can take place.

(If the discussion of intent makes no sense to you, then you and your code may benefit from watching Crockford's lecture on Programming Style & Your Brain.)

It is now even better to use for..of!

There is a debate about whether for..of or forEach() are preferable:

  • For maximum browser support, for..of requires a polyfill for iterators, making your app slightly slower to execute and slightly larger to download.

  • For that reason (and to encourage use of map and filter), some front-end style guides ban for..of completely!

  • But the above concerns is not applicable to Node.js applications, where for..of is now well supported.

  • And furthermore await does not work inside forEach(). Using for..of is the clearest pattern in this case.

Personally, I tend to use whatever looks easiest to read, unless performance or minification has become a major concern. So these days I prefer to use for..of instead of forEach(), but I will always use map or filter or find or some when applicable. (For the sake of my colleagues, I rarely use reduce.)

How does it work?

for (var i = 0; i < array.length; i++) { ... }   // Forwards

for (var i = array.length; i--; )    { ... }   // Reverse

You will notice that i-- is the middle clause (where we usually see a comparison) and the last clause is empty (where we usually see i++). That means that i-- is also used as the condition for continuation. Crucially, it is executed and checked before each iteration.

  • How can it start at array.length without exploding?

    Because i-- runs before each iteration, on the first iteration we will actually be accessing the item at array.length - 1 which avoids any issues with Array-out-of-bounds undefined items.

  • Why doesn't it stop iterating before index 0?

    The loop will stop iterating when the condition i-- evaluates to a falsey value (when it yields 0).

    The trick is that unlike --i, the trailing i-- operator decrements i but yields the value before the decrement. Your console can demonstrate this:

    > var i = 5; [i, i--, i];

    [5, 5, 4]

    So on the final iteration, i was previously 1 and the i-- expression changes it to 0 but actually yields 1 (truthy), and so the condition passes. On the next iteration i-- changes i to -1 but yields 0 (falsey), causing execution to immediately drop out of the bottom of the loop.

    In the traditional forwards for loop, i++ and ++i are interchangeable (as Douglas Crockford points out). However in the reverse for loop, because our decrement is also our condition expression, we must stick with i-- if we want to process the item at index 0.


Some people like to draw a little arrow in the reverse for loop, and end with a wink:

for (var i = array.length; i --> 0 ;) {

Credits go to WYL for showing me the benefits and horrors of the reverse for loop.


Some C-style languages use foreach to loop through enumerations. In JavaScript this is done with the loop structure:

var index,
for (index in obj) {
    value = obj[index];

There is a catch. will loop through each of the object's enumerable members, and the members on its prototype. To avoid reading values that are inherited through the object's prototype, simply check if the property belongs to the object:

for (i in obj) {
    if (obj.hasOwnProperty(i)) {
        //do stuff

Additionally, ECMAScript 5 has added a forEach method to Array.prototype which can be used to enumerate over an array using a calback (the polyfill is in the docs so you can still use it for older browsers):

arr.forEach(function (val, index, theArray) {
    //do stuff

It's important to note that Array.prototype.forEach doesn't break when the callback returns false. jQuery and Underscore.js provide their own variations on each to provide loops that can be short-circuited.


If you want to loop over an array, use the standard three-part for loop.

for (var i = 0; i < myArray.length; i++) {
    var arrayItem = myArray[i];

You can get some performance optimisations by caching myArray.length or iterating over it backwards.


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