Array.prototype.includes()

searchElement

The value to search for.

fromIndex Optional

Zero-based index at which to start searching, converted to an integer.

• Negative index counts back from the end of the array — if -array.length <= fromIndex < 0, fromIndex + array.length is used. However, the array is still searched from front to back in this case.
• If fromIndex < -array.length or fromIndex is omitted, 0 is used, causing the entire array to be searched.
• If fromIndex >= array.length, the array is not searched and false is returned.

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

The includes() method compares searchElement to elements of the array using the SameValueZero algorithm. Values of zero are all considered to be equal, regardless of sign. (That is, -0 is equal to 0), but false is not considered to be the same as 0. NaN can be correctly searched for.

When used on sparse arrays, the includes() method iterates empty slots as if they have the value undefined.

The includes() method is generic. It only expects the this value to have a length property and integer-keyed properties.

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

const arr = ["a", "b", "c"];

arr.includes("c", 3); // false
arr.includes("c", 100); // false

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 than or equal to 0, the entire array will be searched.

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

const arr = ["a", "b", "c"];

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

You can search for undefined in a sparse array and get true.

console.log([1, , 3].includes(undefined)); // true

The includes() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
  3: 1, // ignored by includes() since length is 3
};
console.log(Array.prototype.includes.call(arrayLike, 2));
// true
console.log(Array.prototype.includes.call(arrayLike, 1));
// false

• Polyfill of Array.prototype.includes in core-js
• Indexed collections guide
• Array
• Array.prototype.indexOf()
• Array.prototype.find()
• Array.prototype.findIndex()
• TypedArray.prototype.includes()
• String.prototype.includes()