object

An object that implements the iterable protocol or the iterator protocol.

If object is an iterable, its @@iterator method is called to obtain the iterator. Otherwise, object is assumed to be an iterator. If the iterator is already instanceof Iterator (which means it has Iterator.prototype in its prototype chain), it is returned directly. Otherwise, a new Iterator object is created that wraps the original iterator.

This method exists to convert custom iterators, probably exported by libraries, to proper iterators. All iterator objects returned by Iterator.from() inherit from a common prototype object, which has the following methods:

next()

Calls the underlying iterator's next() method and returns the result.

return()

Calls the underlying iterator's return() method and returns the result, or returns { value: undefined, done: true } if the underlying iterator doesn't have a return() method.

Because obj is already an iterable that returns a proper iterator when its @@iterator method is called, Iterator.from(obj) returns the same iterator.

const iterator = (function* () {
  yield 1;
  yield 2;
  yield 3;
})();

const obj = {
  [Symbol.iterator]() {
    return iterator;
  },
};

const iterator2 = Iterator.from(obj);
console.log(iterator2 === iterator); // true

Because obj2 is an iterable that returns a non-proper iterator when its @@iterator method is called, Iterator.from(obj2) returns a new iterator that wraps the original iterator.

const iterator = {
  current: 0,
  next() {
    return { value: this.current++, done: false };
  },
};

const obj2 = {
  [Symbol.iterator]() {
    return iterator;
  },
};

const iterator2 = Iterator.from(obj2);
console.log(iterator2 === iterator); // false
console.log(iterator2.next()); // { value: 0, done: false }
console.log(iterator.next()); // { value: 1, done: false }

Because obj is already a proper iterator, Iterator.from(obj) returns itself.

const obj = (function* () {
  yield 1;
  yield 2;
  yield 3;
})();

const iterator = Iterator.from(obj);
console.log(iterator === obj); // true

Because obj2 is a non-proper iterator, Iterator.from(obj2) returns a new iterator that wraps the original iterator.

const obj2 = {
  current: 0,
  next() {
    return { value: this.current++, done: false };
  },
};

const iterator = Iterator.from(obj2);
console.log(iterator === obj2); // false
console.log(iterator.next()); // { value: 0, done: false }
console.log(obj2.next()); // { value: 1, done: false }

• Polyfill of Iterator.from in core-js
• Iterator
• Array.from()