The well-known symbol @@species.

The @@species accessor property allows subclasses to override the default constructor for objects. This specifies a protocol about how instances should be copied. For example, when you use copying methods of arrays, such as map(). the map() method uses instance.constructor[Symbol.species] to get the constructor for constructing the new array. For more information, see subclassing built-ins.

All built-in implementations of @@species return the this value, which is the current instance's constructor. This allows copying methods to create instances of derived classes rather than the base class — for example, map() will return an array of the same type as the original array.

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 = a.map((x) => x * x);

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

• Array[@@species]
• ArrayBuffer[@@species]
• Map[@@species]
• Promise[@@species]
• RegExp[@@species]
• Set[@@species]
• TypedArray[@@species]