const array1 = [1, 2, 3];console.log(array1.includes(2));// Expected output: trueconst pets = ["cat", "dog", "bat"];console.log(pets.includes("cat"));// Expected output: trueconsole.log(pets.includes("at"));// Expected output: false

array.includes(searchElement[, fromIndex])

[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

// https://tc39.github.io/ecma262/#sec-array.prototype.includesif (!Array.prototype.includes) {  Object.defineProperty(Array.prototype, "includes", {    value: function (searchElement, fromIndex) {      // 1. Let O be ? ToObject(this value).      if (this == null) {        throw new TypeError('"this" is null or not defined');      }      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);      // 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.        // c. Increase k by 1.        // NOTE: === provides the correct "SameValueZero" comparison needed here.        if (o[k] === searchElement) {          return true;        }        k++;      }      // 8. Return false      return false;    },  });}

Help improve MDN

This page was last modified on⁨17 de fev. de 2025⁩ byMDN contributors.