array.includes

The includes() method determines whether an array includes a certain element, returning true or false as appropriate.

Syntax

arr.includes(searchElement[, fromIndex])

Parameters

searchElement

The element to search for.

fromIndex Optional

The position in this array at which to begin searching for searchElement. A negative value searches from the index of array.length - fromIndex by asc. Defaults to 0.

Return value

A Boolean which is true if the value searchElement is found within the array (or the part of the array beginning at 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.

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.

var 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 searchElement. If the computed index is less or equal than -1 * array.length, the entire array will be searched.

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

var 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([].includes.call(arguments, 'a')); // true
  console.log([].includes.call(arguments, 'd')); // false
})('a','b','c');

Polyfill

// https://tc39.github.io/ecma262/#sec-array.prototype.includes
if (!Array.prototype.includes) {
  Object.defineProperty(Array.prototype, 'includes', {
    value: function(searchElement, fromIndex) {

      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // 3. If len is 0, return false.
      if (len === 0) {
        return false;
      }

      // 4. Let n be ? ToInteger(fromIndex).
      //    (If fromIndex is undefined, this step produces the value 0.)
      var n = fromIndex | 0;

      // 5. If n ≥ 0, then
      //  a. Let k be n.
      // 6. Else n < 0,
      //  a. Let k be len + n.
      //  b. If k < 0, let k be 0.
      var k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

      function sameValueZero(x, y) {
        return x === y || (typeof x === 'number' && typeof y === 'number' && isNaN(x) && isNaN(y));
      }

      // 7. Repeat, while k < len
      while (k < len) {
        // a. Let elementK be the result of ? Get(O, ! ToString(k)).
        // b. If SameValueZero(searchElement, elementK) is true, return true.
        if (sameValueZero(o[k], searchElement)) {
          return true;
        }
        // c. Increase k by 1. 
        k++;
      }

      // 8. Return false
      return false;
    }
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty(), it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

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