JavaScript generators and their prototype chains

In the section GeneratorFunction Objects, there is a figure of the relationships: generator_objects_relationships

Question 1

Yes.

Question 2

d is an IteratorPrototype. And by the doc:

The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:

Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))

It seems that there is not an Iterator such that Iterator.prototype is equal to d.


Yes. Let's break this down some more:

function* g() {} // a generator function
const GeneratorFunction = Object.getPrototypeOf(g).constructor;
console.assert(Object.getPrototypeOf(GeneratorFunction) == Function);
const Generator = GeneratorFunction.prototype;
console.assert(Object.getPrototypeOf(Generator) == Function.prototype);
const ArrayIteratorPrototype = Object.getPrototypeOf([].values());

const a = g(); // a generator
console.assert(a instanceof g);

const b = Object.getPrototypeOf(a); // a prototype object
console.assert(b == g.prototype);

const c = Object.getPrototypeOf(b); // the Generator.prototype
console.assert(c == Generator.prototype);

const d = Object.getPrototypeOf(c); // the Iterator.prototype
console.assert(d == Object.getPrototypeOf(ArrayIteratorPrototype));

const e = Object.getPrototypeOf(d); // the Object.prototype
console.assert(e == Object.prototype);

const f = Object.getPrototypeOf(e); // null
console.assert(f == null);

The object d you were looking for is the prototype object shared by all iterators (array iterators, map iterators, set iterators, string iterators etc), including generators. There is not global Iterator (yet) just as there are no global Generator or GeneratorFunction.

Tags:

Javascript

Generator

Prototype

Recent Posts