Symbol.isConcatSpreadable

¥Value

众所周知的符号 Symbol.isConcatSpreadable。

¥The well-known symbol Symbol.isConcatSpreadable.

¥Description

[Symbol.isConcatSpreadable] 属性可以定义为自身或继承的属性，其值为布尔值。它可以控制数组和类数组对象的行为：

¥The [Symbol.isConcatSpreadable] property can be defined as an own or inherited property and its value is a boolean. It can control behavior for arrays and array-like objects:

• 对于数组对象，默认行为是展开（展平）元素。在这些情况下，Symbol.isConcatSpreadable 可以避免扁平化。
• 对于类似数组的对象，默认行为是不展开或展平。在这些情况下，Symbol.isConcatSpreadable 可以强制展平。

¥Examples

¥Arrays

默认情况下，Array.prototype.concat() 将数组展开（展平）到其结果中：

¥By default, Array.prototype.concat() spreads (flattens) arrays into its result:

const alpha = ["a", "b", "c"];
const numeric = [1, 2, 3];

const alphaNumeric = alpha.concat(numeric);

console.log(alphaNumeric); // Result: ['a', 'b', 'c', 1, 2, 3]

将 Symbol.isConcatSpreadable 设置为 false 时，你可以禁用默认行为：

¥When setting Symbol.isConcatSpreadable to false, you can disable the default behavior:

const alpha = ["a", "b", "c"];
const numeric = [1, 2, 3];

numeric[Symbol.isConcatSpreadable] = false;
const alphaNumeric = alpha.concat(numeric);

console.log(alphaNumeric); // Result: ['a', 'b', 'c', [1, 2, 3] ]

¥Array-like objects

对于类似数组的对象，默认是不扩散。需要将 Symbol.isConcatSpreadable 设置为 true 才能获得展平数组：

¥For array-like objects, the default is to not spread. Symbol.isConcatSpreadable needs to be set to true in order to get a flattened array:

const x = [1, 2, 3];

const fakeArray = {
  [Symbol.isConcatSpreadable]: true,
  length: 2,
  0: "hello",
  1: "world",
};

x.concat(fakeArray); // [1, 2, 3, "hello", "world"]

注意：length 属性用于控制要添加的对象属性的数量。在上面的示例中，length:2 表示必须添加两个属性。

¥Note: The length property is used to control the number of object properties to be added. In the above example, length:2 indicates two properties has to be added.

¥See also

• core-js 中 Symbol.isConcatSpreadable 的 Polyfill
• Array.prototype.concat()