Array.prototype.includes()
==========================

**`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
----

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。

### 回傳值

布林值（`Boolean`）。

範例
--


```js
[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.


```js
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 than 0, the entire array will be searched.


```js
// 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 (en-US) object.


```js
(function () {
  console.log([].includes.call(arguments, "a")); // true
  console.log([].includes.call(arguments, "d")); // false
})("a", "b", "c");
```

Polyfill
--------


```js
// 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 Language Specification # sec-array.prototype.includes |

瀏覽器相容性
------

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參見
--

* `TypedArray.prototype.includes()` (en-US)
* `String.prototype.includes()` (en-US)
* `Array.prototype.indexOf()`
* `Array.prototype.find()`
* `Array.prototype.findIndex()`