ECMAScript 2015 Generators

Generators are functions that can “yeild” (i.e. return) multiple values each time they are called. They run until they reach a yield statement, and will carry on from there when they are next iterated.

Note that generator functions have a * after function.

function* myABCsGeneratorFunction() {
  yield 'A';
  yield 'B';
  yield 'C';
}

const myABCsGenerator = myABCsGeneratorFunction();

// logs "{value: 'A', done: false}"
console.log(myABCsGenerator.next());

// logs "{value: 'B', done: false}"
console.log(myABCsGenerator.next());

// logs "{value: 'C', done: false}"
console.log(myABCsGenerator.next());

// logs "{value: undefined, done: true}"
console.log(myABCsGenerator.next());

Technically speaking myABCsGeneratorFunction - the “Generator Function” - returns the “Generator” itself when called.

Looping until generator is done

function* myABCsGeneratorFunction() {
  yield 'A';
  yield 'B';
  yield 'C';
}

const myABCsGenerator = myABCsGeneratorFunction();

let value = myABCsGenerator.next();
while (!value.done) {
  console.log(value.value);
  value = myABCsGenerator.next();
}

Stopping a generator early

You can make a generator “stop” early by calling return(). Further calls to the generator will return an undefined value and done will be true.

function* myABCsGeneratorFunction() {
  yield 'A';
  yield 'B';
  yield 'C';
}

const myABCsGenerator = myABCsGeneratorFunction();

// logs "{value: 'A', done: false}"
console.log(myABCsGenerator.next());

// logs "{value: 'Z', done: true}"
console.log(myABCsGenerator.return('Z'));

// logs "{value: undefined done: true}"
console.log(myABCsGenerator.next());

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ECMAScript 2015 Generators

Looping until generator is done

Stopping a generator early