Spec, tests, reference implementation, and docs for ESnext-track Array.prototype.includes
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Latest commit b6671ae Nov 17, 2015 @domenic domenic Advance to stage 4
Closes #13.


Array.prototype.includes Proposal



This proposal is officially in stage 4 of the TC39 process, and is in the process of being integrated into the spec.

This proposal was formerly for Array.prototype.contains, but that name is not web-compatible. Per the November 2014 TC39 meeting, the name of both String.prototype.contains and Array.prototype.contains was changed to includes to dodge that bullet.


When using ECMAScript arrays, it is commonly desired to determine if the array includes an element. The prevailing pattern for this is

if (arr.indexOf(el) !== -1) {

with various other possibilities, e.g. arr.indexOf(el) >= 0, or even ~arr.indexOf(el).

These patterns exhibit two problems:

  • They fail to "say what you mean": instead of asking about whether the array includes an element, you ask what the index of the first occurrence of that element in the array is, and then compare it or bit-twiddle it, to determine the answer to your actual question.
  • They fail for NaN, as indexOf uses Strict Equality Comparison and thus [NaN].indexOf(NaN) === -1.

Proposed Solution

We propose the addition of an Array.prototype.includes method, such that the above patterns can be rewritten as

if (arr.includes(el)) {

This has almost the same semantics as the above, except that it uses the SameValueZero comparison algorithm instead of Strict Equality Comparison, thus making [NaN].includes(NaN) true.

Thus, this proposal solves both problems seen in existing code.

We additionally add a fromIndex parameter, similar to Array.prototype.indexOf and String.prototype.includes, for consistency.


Why includes instead of has?

If you survey existing APIs, has is used for conceptual "keys," whereas includes is used for conceptual "values." That is:

  • Keys inside a key-value map: Map.prototype.has(key), WeakMap.prototype.has(key), Reflect.has(target, propertyKey)
  • Sets, whose elements are conceptually both keys and values: Set.prototype.has(value), WeakSet.prototype.has(value), Reflect.Loader.prototype.has(name)
  • Strings, which are conceptually maps from indices to code points: String.prototype.includes(searchString, position)

The best consistency here is with String, not with Map or Set.

The web has classes like DOMStringList and DOMTokenList which are array-like, and have methods named contains with the same semantics as our includes. Unfortunately, meshing with those is not web-compatible, as explained above; we will have to accept this inconsistency.

But String.prototype.includes works on strings, not characters!?

Yes, that's true. The best way to think about this is that String.prototype.indexOf and String.prototype.includes behave like their Array.prototype counterparts in the special case of a single character. But the string versions can also be used in the more general case of a larger string.

So in this way, the relationship between String.prototype.includes and Array.prototype.includes is the same as the relationship between String.prototype.indexOf and Array.prototype.indexOf.

Why SameValueZero?

There are four equality algorithms in the current ES6 draft:

  • Abstract Equality Comparison (==)
  • Strict Equality Comparison (===): used by Array.prototype.indexOf, Array.prototype.lastIndexOf, and case-matching
  • SameValueZero: used by %TypedArray% and ArrayBuffer constructors, as well as Map and Set operations
  • SameValue: used in all other places

(Note however that most places SameValue is used could be replaced by SameValueZero since those places often never compare primitives, or at least never compare numbers.)

Using Abstract Equality Comparison would be bonkers, of course. Using SameValue is not a good idea for the same reasons it is not used by Map and Set. (Briefly: -0s can sneak into your code fairly easily via arithmetic operations, but you almost always desire -0 to be treated the same as +0, so distinguishing them will just cause spurious failures.) This leaves Strict Equality Comparison and SameValueZero as the two possibilities.

SameValueZero is generally the better choice, as it allows you to detect if an array includes a NaN. The argument for Strict Equality Comparison boils down to "bug compatibility" with Array.prototype.indexOf. But one of the purposes of Array.prototype.includes is to steer users away from creating these sort of bugs.

This introduces a slight refactoring hazard from Array.prototype.indexOf to Array.prototype.includes: they will indeed behave differently for arrays containing NaNs. However, it seems much more likely that code will become less buggy via this refactoring, instead of causing problems. Introducing a new method, and accompanying it with the appropriate messaging around this case, should help.

Typed Arrays

As with all non-mutating array methods, we also install this method on %TypedArray%.prototype.

Illustrative Examples

assert([1, 2, 3].includes(2) === true);
assert([1, 2, 3].includes(4) === false);

assert([1, 2, NaN].includes(NaN) === true);

assert([1, 2, -0].includes(+0) === true);
assert([1, 2, +0].includes(-0) === true);

assert(["a", "b", "c"].includes("a") === true);
assert(["a", "b", "c"].includes("a", 1) === false);