get Promise[Symbol.species]

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

Warning: The existence of Symbol.species allows execution of arbitrary code and may create security vulnerabilities. It also makes certain optimizations much harder. Engine implementers are investigating whether to remove this feature. Avoid relying on it if possible.

Syntax

Promise[Symbol.species]

Return value

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.

Description

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] === Promise; // true

Promise chaining methods — then(), 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.