Examples

Using yield

The following code is the declaration of an example generator function.

function* countAppleSales() {
  const saleList = [3, 7, 5];
  for (let i = 0; i < saleList.length; i++) {
    yield saleList[i];
  }
}

Once a generator function is defined, it can be used by constructing an iterator as shown. 

const appleStore = countAppleSales(); // Generator { }
console.log(appleStore.next()); // { value: 3, done: false }
console.log(appleStore.next()); // { value: 7, done: false }
console.log(appleStore.next()); // { value: 5, done: false }
console.log(appleStore.next()); // { value: undefined, done: true }

You can also send a value with next(value) into the generator. step evaluates as a return value in this syntax rv = yield expression - although a value passed to the generator's next() method is ignored the first time next() is called. 

function* counter(value) {
  let step;

  while (true) {
    step = yield value++;

    if (step) {
      value += step;
    }
  }
}

const generatorFunc = counter(0);
console.log(generatorFunc.next().value);   // 0
console.log(generatorFunc.next().value);   // 1
console.log(generatorFunc.next().value);   // 2
console.log(generatorFunc.next().value);   // 3
console.log(generatorFunc.next(10).value); // 14
console.log(generatorFunc.next().value);   // 15
console.log(generatorFunc.next(10).value); // 26