Map[Symbol.species]

The value of the constructor (this) on which get [Symbol.species] was called. The return value is used to construct copied Map instances.

The [Symbol.species] accessor property returns the default constructor for Map objects. Subclass constructors may override it to change the constructor assignment.

The [Symbol.species] property returns the default constructor function, which is the Map constructor for Map.

Map[Symbol.species]; // function Map()

In an instance of a custom Map subclass, such as MyMap, the MyMap species is the MyMap constructor. However, you might want to overwrite this, in order to return parent Map objects in your derived class methods:

class MyMap extends Map {
  // Overwrite MyMap species to the parent Map constructor
  static get [Symbol.species]() {
    return Map;
  }
}

• Map
• Symbol.species