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.

Answer

TL;DR

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"):

Use forEach and related (ES5+)

Use a simple for loop

Use for-in correctly

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

Use an iterator explicitly (ES2015+)

Details:

1. Use `forEach` and related

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) {
    console.log(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) {
    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, 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 {
    console.log(index);
} catch (e) {

    console.error(e);   // "ReferenceError: index is not defined"
}
try {
    console.log(value);
} 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);
    });
}

zero


one

two

three

four

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.)

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 sparese 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
        ) {
        console.log(a[key]);
    }
}

Note the three checks:

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

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

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 (hasOwn.call(a, key) &&
            rexNum.test(key) &&
            index <= 4294967294
            ) {
            callback.call(thisArg, 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 adds 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) {
    console.log(val);
}

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 = it.next()).done) {

    console.log(entry.value);
}

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:

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

Use a simple for loop

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

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.

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

for-of will use the iterator provided by the object (if any); we'll have to see how this plays with the various array-like objects, particularly host-provided ones. For instance, the specification for the NodeList from querySelectorAll was updated to support iteration. The spec for the HTMLCollection from getElementsByTagName was not.

Use an iterator explicitly (ES2015+)

See #4, we'll have to see how iterators play out.

Create a true array

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

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 = Array.prototype.slice.call(arrayLikeObject);
```
So for instance, if we want to convert a NodeList into a true array, we could do this:
```
var divs = Array.prototype.slice.call(document.querySelectorAll("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.

Use spread syntax (...)

It's also possible to use ES2015's spread syntax with JavaScript engines that support this feature:
```
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")];
```

Use Array.from (spec) | (MDN)

Array.from (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.)

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Monday, 22 August 2016