The value of the constructor (this) on which get [Symbol.species] was called. The return value is used to construct return values from array methods that create new arrays.

The [Symbol.species] accessor property returns the default constructor for Array objects. Subclass constructors may override it to change the constructor assignment. The default implementation is basically:

/ Hypothetical underlying implementation for illustration
class Array {
  static get [Symbol.species]() {
    return this;
  }
}

Because of this polymorphic implementation, [Symbol.species] of derived subclasses would also return the constructor itself by default.

class SubArray extends Array {}
SubArray[Symbol.species] === SubArray; / true

When calling array methods that do not mutate the existing array but return a new array instance (for example, map()), the array's constructor[Symbol.species] will be accessed. The returned constructor will be used to construct the return value of the array method. This makes it technically possible to make array methods return objects unrelated to arrays.

class NotAnArray {
  constructor(length) {
    this.length = length;
  }
}

const arr = [0, 1, 2];
arr.constructor = { [Symbol.species]: NotAnArray };
arr.map((i) => i); / NotAnArray { '0': 0, '1': 1, '2': 2, length: 3 }
arr.filter((i) => i); / NotAnArray { '0': 1, '1': 2, length: 0 }
arr.concat([1, 2]); / NotAnArray { '0': 0, '1': 1, '2': 2, '3': 1, '4': 2, length: 5 }

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

Array[Symbol.species]; / [Function: Array]

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

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

• Polyfill of Array[Symbol.species] and support of [Symbol.species] in all affected Array methods in core-js
• Indexed collections guide
• Array
• Symbol.species