Array.prototype.includes() - JavaScript 编辑

The includes() method determines whether an array includes a certain value among its entries, returning true or false as appropriate.

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

arr.includes(valueToFind[, fromIndex])

Parameters

valueToFind

The value to search for.

Note: When comparing strings and characters, includes() is case-sensitive.

fromIndex Optional

The position in this array at which to begin searching for valueToFind.

The first element to be searched is found at fromIndex for positive values of fromIndex, or at arr.length + fromIndex for negative values of fromIndex (using the absolute value of fromIndex as the number of elements from the end of the array at which to start the search).

Defaults to 0.

Return value

A Boolean which is true if the value valueToFind is found within the array (or the part of the array indicated by the index fromIndex, if specified).

Values of zero are all considered to be equal, regardless of sign. (That is, -0 is considered to be equal to both 0 and +0), but false is not considered to be the same as 0.

Note: Technically speaking, includes() uses the sameValueZero algorithm to determine whether the given element is found.

Examples

[1, 2, 3].includes(2)      // true
[1, 2, 3].includes(4)      // false
[1, 2, 3].includes(3, 3)   // false
[1, 2, 3].includes(3, -1)  // true
[1, 2, NaN].includes(NaN)  // true

fromIndex is greater than or equal to the array length

If fromIndex is greater than or equal to the length of the array, false is returned. The array will not be searched.

let arr = ['a', 'b', 'c']

arr.includes('c', 3)    // false
arr.includes('c', 100)  // false

Computed index is less than 0

If fromIndex is negative, the computed index is calculated to be used as a position in the array at which to begin searching for valueToFind. If the computed index is less or equal than -1 * arr.length, the entire array will be searched.

// array length is 3
// fromIndex is -100
// computed index is 3 + (-100) = -97

let arr = ['a', 'b', 'c']

arr.includes('a', -100) // true
arr.includes('b', -100) // true
arr.includes('c', -100) // true
arr.includes('a', -2)   // false

includes() used as a generic method

includes() method is intentionally generic. It does not require this value to be an Array object, so it can be applied to other kinds of objects (e.g. array-like objects).

The example below illustrates includes() method called on the function's arguments object.

(function() {
  console.log(Array.prototype.includes.call(arguments, 'a'))  // true
  console.log(Array.prototype.includes.call(arguments, 'd'))  // false
})('a','b','c') 

Please do not add polyfills on reference articles. For more details and discussion, see https://discourse.mozilla.org/t/mdn-rfc-001-mdn-wiki-pages-shouldnt-be-a-distributor-of-polyfills/24500

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Array.prototype.includes' in that specification.

Browser compatibility

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See also

• TypedArray.prototype.includes()
• String.prototype.includes()
• Array.prototype.indexOf()
• Array.prototype.find()
• Array.prototype.findIndex()