Arrayprototypeincludes

Array.prototype.includes is a method of the JavaScript Array prototype that determines whether an array contains a given element, returning true or false. It was introduced in the ECMAScript 2016 standard and is widely supported in contemporary environments.

The method has the signature includes(searchElement[, fromIndex]). The fromIndex parameter is optional and defaults to 0. If fromIndex is negative, it is treated as length + fromIndex, but not less than 0. The search uses the SameValueZero comparison algorithm, meaning it considers NaN to be equal to NaN and does not perform type coercion. This makes includes capable of finding NaN within an array, unlike indexOf, which uses strict equality and cannot locate NaN.

Examples illustrate common usage: [1, 2, 3].includes(2) returns true, while [1, 2, 3].includes(4) returns false. [NaN].includes(NaN) returns true because NaN is treated as equal to NaN. FromIndex can alter behavior with negative values, for example, [1, 2, 3].includes(2, -1) returns false because the search starts from the last element.

Notes and compatibility: includes is primarily used on actual arrays but can be borrowed by array-like objects. It performs a linear scan, so performance is proportional to the array length. Polyfills exist for environments that do not implement ES2016.