yield*

So far aforementioned generator objects only generate a single value using yield expression one at a time. We can also use a yield* expression to generate a sequence of values. When a yield* expression is encountered, sequence generation of current generator object is delegated to another generator object or iterable object.

yield* & iterable objects

In the code below, generator function oneToThree uses yield* [1, 2, 3] to generate three values: 1, 2 and 3, which has the same result as generator function sample in basic generators. Using yield* expression is more concise and easier to read.

function *oneToThree() {
  yield* [1, 2, 3];
}

debug(oneToThree());
// -> Output 1, 2, 3

We can use multiple yield* expressions in a generator function, then values from each yield* expression are generated in order.

function *multipleYieldStars() {
  yield* [1, 2, 3];
  yield 'x';
  yield* 'hello';
}

debug(multipleYieldStars());
// -> Output 1, 2, 3, 'x', 'h', 'e', 'l', 'l', 'o'

yield* & generator objects

We can also use other generator objects in yield* expressions.

function *moreValues() {
  yield* oneToThree();
}

debug(moreValues())
// -> Output 1, 2, 3

Value of yield*

yield* is also an expression, so it's evaluated to a value. The value of yield* expression depends on its target, i.e. the expression after yield*. The value is the last value generated by the iterable object or generator object, i.e. the value property with done set to true.

If yield* is used with iterable objects, then the evaluated value is always undefined, because the last generated value is always {value: undefined, done: true}.

var result;

function loop(iterable) {
  for (let value of iterable) {
    //ignore
  }
}

function *oneToThree() {
  result = yield* [1, 2, 3];
}

loop(oneToThree());
console.log(result);
// -> undefined

If yield* is used with generator objects, we can control the last generated value using return inside of the generator functions.

var result;

function loop(iterable) {
  for (let value of iterable) {
    //ignore
  }
}

function *abc() {
  yield* 'abc';
  return 'd';
}

function *generator() {
  result = yield* abc();
}

loop(generator());
console.log(result);
// -> "d"