get Promise[@@species]

The Promise[@@species] accessor property returns the constructor used to construct return values from promise methods.

Description

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.

Examples

Species in ordinary objects

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

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

Species in derived objects

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

Specifications

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Browser compatibility

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See also