Array.prototype.includes()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since August 2016.

includes() 方法會判斷陣列是否包含特定的元素，並以此來回傳 true 或 false。

嘗試一下

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

語法

arr.includes(searchElement[, fromIndex])

參數

searchElement: 要搜尋的元素。
fromIndex 選擇性: 要於此陣列中開始搜尋 searchElement 的位置。如為負數值，則自 array.length + fromIndex 開始向後搜尋。預設值為 0。

範例

[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` 大於或等於陣列長度

如果 fromIndex大於或等於陣列長度, 會回傳false. 此陣列將不會被搜尋.

var 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 0, 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

`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.

規範

Specification
ECMAScript® 2026 Language Specification # sec-array.prototype.includes