Promise[Symbol.species]

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

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

// Hypothetical underlying implementation for illustration
class Promise {
  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 SubPromise extends Promise {}
SubPromise[Symbol.species] === SubPromise; // true

Promise chaining methods — then(), catch(), and finally() — return new promise objects. They get the constructor to construct the new promise through this.constructor[Symbol.species]. If this.constructor is undefined, or if this.constructor[Symbol.species] is undefined or null, the default Promise() constructor is used. Otherwise, the constructor returned by this.constructor[Symbol.species] is used to construct the new promise object.

The Symbol.species property returns the default constructor function, which is the Promise constructor for Promise.

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

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

class MyPromise extends Promise {
  // Override MyPromise species to the parent Promise constructor
  static get [Symbol.species]() {
    return Promise;
  }
}

By default, promise methods would return promises with the type of the subclass.

class MyPromise extends Promise {
  someValue = 1;
}

console.log(MyPromise.resolve(1).then(() => {}).someValue); // 1

By overriding [Symbol.species], the promise methods will return the base Promise type.

class MyPromise extends Promise {
  someValue = 1;
  static get [Symbol.species]() {
    return Promise;
  }
}

console.log(MyPromise.resolve(1).then(() => {}).someValue); // undefined

• Promise
• Symbol.species