Array[Symbol.species]
TheArray[Symbol.species]
static accessor property returns the constructor used to construct return values from array methods.
Warning:The existence of[Symbol.species]
allows execution of arbitrary code and may create security vulnerabilities. It also makes certain optimizations much harder. Engine implementers areinvestigating whether to remove this feature.Avoid relying on it if possible. Modern array methods, such astoReversed()
,do not use[Symbol.species]
and always return a newArray
base class instance.
Syntax
Array[Symbol.species]
Return value
The value of the constructor (this
) on whichget [Symbol.species]
was called. The return value is used to construct return values from array methods that create new arrays.
Description
The[Symbol.species]
accessor property returns the default constructor forArray
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,[Symbol.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()
andmap()
), the array'sconstructor[Symbol.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[Symbol.species]
property returns the default constructor function, which is theArray
constructor forArray
.
Array[Symbol.species]; // [Function: Array]
Species in derived objects
In an instance of a customArray
subclass, such asMyArray
,theMyArray
species is theMyArray
constructor. However, you might want to overwrite this, in order to return parentArray
objects in your derived class methods:
class MyArray extends Array {
// Overwrite MyArray species to the parent Array constructor
static get [Symbol.species]() {
return Array;
}
}
Specifications
Specification |
---|
ECMAScript Language Specification #sec-get-array-%symbol.species% |
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