get Promise[@@species]

The @@species accessor property returns the default constructor for Promise objects. Subclass constructors may override it to change the constructor assignment.

When chaining promise methods like then, finally, etc., the instance's constructor[@@species] will be accessed. The returned constructor will be used to construct the return value of the promise method.

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

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

In an instance of a derived class from Promise (e.g. your custom 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 @@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

• Array
• Symbol.species