The Array[@@species] static accessor property returns the constructor used to construct return values from array methods.
Warning: The existence of
@@speciesallows execution of arbitrary code and may create security vulnerabilities. It also makes certain optimizations much harder. Engine implementers are investigating whether to remove this feature. Avoid relying on it if possible. Modern array methods, such as toReversed(), do not use@@speciesand always return a newArraybase class instance.
Syntax
Array[Symbol.species]
Return value
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
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;
}
}
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
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
The @@species property returns the default constructor function, which is the Array constructor for Array.
Array[Symbol.species]; // [Function: Array]
Species in derived objects
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;
}
}