数组[@@物种]

¥Syntax

Array[Symbol.species]

¥Return value

调用 get @@species 的构造函数 (this) 的值。返回值用于从创建新数组的数组方法构造返回值。

¥The value of the constructor (this) on which get @@species was called. The return value is used to construct return values from array methods that create new arrays.

¥Description

@@species 访问器属性返回 Array 对象的默认构造函数。子类构造函数可以重写它以更改构造函数赋值。默认实现基本上是：

¥The @@species accessor property returns the default constructor for Array objects. Subclass constructors may override it to change the constructor assignment. The default implementation is basically:

// Hypothetical underlying implementation for illustration
class Array {
  static get [Symbol.species]() {
    return this;
  }
}

由于这种多态实现，派生子类的 @@species 也会默认返回构造函数本身。

¥Because of this polymorphic implementation, @@species of derived subclasses would also return the constructor itself by default.

class SubArray extends Array {}
SubArray[Symbol.species] === SubArray; // true

当调用不改变现有数组但返回新数组实例（例如 filter() 和 map()）的数组方法时，将访问该数组的 constructor[@@species]。返回的构造函数将用于构造数组方法的返回值。这使得在技术上可以使数组方法返回与数组无关的对象。

¥When calling array methods that do not mutate the existing array but return a new array instance (for example, filter() and map()), the array's constructor[@@species] will be accessed. The returned constructor will be used to construct the return value of the array method. This makes it technically possible to make array methods return objects unrelated to arrays.

class NotAnArray {
  constructor(length) {
    this.length = length;
  }
}

const arr = [0, 1, 2];
arr.constructor = { [Symbol.species]: NotAnArray };
arr.map((i) => i); // NotAnArray { '0': 0, '1': 1, '2': 2, length: 3 }
arr.filter((i) => i); // NotAnArray { '0': 1, '1': 2, length: 0 }
arr.concat([1, 2]); // NotAnArray { '0': 0, '1': 1, '2': 2, '3': 1, '4': 2, length: 5 }

¥Examples

¥Species in ordinary objects

@@species 属性返回默认构造函数，即 Array 的 Array 构造函数。

¥The @@species property returns the default constructor function, which is the Array constructor for Array.

Array[Symbol.species]; // [Function: Array]

¥Species in derived objects

在自定义 Array 子类（例如 MyArray）的实例中，MyArray 种类是 MyArray 构造函数。但是，你可能想要覆盖它，以便在派生类方法中返回父 Array 对象：

¥In an instance of a custom Array subclass, such as MyArray, the MyArray species is the MyArray constructor. However, you might want to overwrite this, in order to return parent Array objects in your derived class methods:

class MyArray extends Array {
  // Overwrite MyArray species to the parent Array constructor
  static get [Symbol.species]() {
    return Array;
  }
}

¥See also

• Array[@@species] 的 Polyfill 和 core-js 中所有受影响的 Array 方法中对 @@species 的支持
• 索引集合 指南
• Array
• Symbol.species