Symbol.species

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

Description

The species accessor property allows subclasses to over-ride the default constructor for objects.

Examples

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; }
}
var a = new MyArray(1,2,3);
var mapped = a.map(x => x * x);

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

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Symbol.species' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of 'Symbol.species' in that specification.	Draft

Browser compatibility

See also

• Map[@@species]
• Set[@@species]