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.

TL;DR

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 will soon have two more in ECMAScript 2015 ("ES2015", "ES6"), the latest version of JavaScript that vendors are working on supporting:

Details:

1. Use forEach and related

If you're using an environment that supports the Array features of ES5 (directly or using a shim), you can use the new forEach ( spec | MDN ):

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

forEach accepts an iterator function and, optionally, a value to use as this when calling that iterator function (not used above). The iterator function 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 iterator function 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:

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) {
    console.log(a[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) {
    console.log(a[index]);
}

And/or counting backward:

var index;
var a = ["a", "b", "c"];
for (index = a.length - 1; index >= 0; --index) {
    console.log(a[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); // Would cause "ReferenceError: index is not defined"
//console.log(value); // Would cause "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 => {
        alert("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 does not define that for-in will follow that order. (Details in this other answer.)

Still, it can be useful, particularly for sparse arrays, 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 are explained
        /^0$|^[1-9]d*$/.test(key) &&    // and then hidden
        key <= 4294967294                // away below
        ) {
        console.log(a[key]);
    }
}

Note the two checks:

That's a tiny bit of added overhead per loop iteration on most arrays, but if you have a sparse array, it can be a more efficient way to loop because it only loops for entries that actually exist. Eg, for the array above, we loop a total of three times (for keys "0" , "10" , and "10000" — remember, they're strings), not 10,001 times.

Now, you won't want to write that every time, so you might put this in your toolkit:

function arrayHasOwnIndex(array, prop) {
    return array.hasOwnProperty(prop) && /^0$|^[1-9]d*$/.test(prop) && prop <= 4294967294; // 2^32 - 2
}

And then we'd use it like this:

for (key in a) {
    if (arrayHasOwnIndex(a, key)) {
        console.log(a[key]);
    }
}

Or if you're interested in just a "good enough for most cases" test, you could use this, but while it's close, it's not quite correct:

for (key in a) {
    // "Good enough" for most cases
    if (String(parseInt(key, 10)) === key && a.hasOwnProperty(key)) {
        console.log(a[key]);
    }
}

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

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

var val;
var a = ["a", "b", "c"];
for (val of a) {
    console.log(val);
}

Output:

a
b
c

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:

var a = ["a", "b", "c"];
var it = a.values();
var entry;
while (!(entry = it.next()).done) {
    console.log(entry.value);
}

The iterator is a function (specifically, a generator) that returns a new object each time you call next . The object returned by the iterator has a property, done , telling us whether it's done, and a property value with the value for that iteration.

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

(As of this writing, Firefox 29 supports entries and keys but not values .)

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:

Array.prototype.forEach.call(node.childNodes, function(child) {
    // Do something with `child`
});

// (This is all presumably in some scoping function)
var forEach = Array.prototype.forEach;

// Then later...
forEach.call(node.childNodes, function(child) {
    // Do something with `child`
});

Create a true array

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

var trueArray = Array.prototype.slice.call(arrayLikeObject);

var divs = Array.prototype.slice.call(document.querySelectorAll("div"));

var trueArray = [...iterableObject];

var divs = [...document.querySelectorAll("div")];

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

// 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 ES5 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.

Edit : 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:

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 i.
}

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

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

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

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

EDIT :

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

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

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