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

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

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


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

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


ECMAScript Language Specification
# sec-symbol.species

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See also