Symbol.species

The well-known symbol Symbol.species specifies a function-valued property that the constructor function uses to create derived objects.

Description

The species accessor property allows subclasses to override the default constructor for objects.

Property attributes of Symbol.species
Writable no
Enumerable no
Configurable no

Examples

Using species

You might want to return Array objects in your derived array class MyArray. For example, when using methods such as map() that return the default constructor, you want these methods to return a parent Array object, instead of the MyArray object. The species symbol lets you do this:

class MyArray extends Array {
  // Overwrite species to the parent Array constructor
  static get [Symbol.species]() { return Array; }
}
let a = new MyArray(1,2,3);
let mapped = a.map(x => x * x);

console.log(mapped instanceof MyArray); // false
console.log(mapped instanceof Array);   // true

Specifications

Specification
ECMAScript Language Specification (ECMAScript)
# sec-symbol.species

Browser compatibility

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See also