What is it

function* creates a binding of a new generator function to a given name

• Generator function can be exited and re-entered, with its context (variable bindings) saved across re-entrances

function* generator(i) {
  yield i;
  yield i + 10;
}

const gen = generator(10);

console.log(gen.next().value); // 10
console.log(gen.next().value); // 20
console.log(gen.next().value); // undefined

A function* declaration creates a GeneratorFunction object

• Each time a generator function is called, it returns a new Generator object, which conforms to the iterator protocol
• When the iterator's next() method is called, the generator function's body is executed until the first yield expression, which specifies the value to be returned from the iterator or, with yield*, delegates to another generator function.
• The next() method returns an object with a value property containing the yielded value and a done property which indicates whether the generator has yielded its last value, as a boolean.
• Calling the next() method with an argument will resume the generator function execution, replacing the yield expression where an execution was paused with the argument from next().

function* anotherGenerator(i) {
  yield i + 1;
  yield i + 2;
  yield i + 3;
}

function* generator(i) {
  yield i;
  yield* anotherGenerator(i);
  yield i + 10;
}

const gen = generator(10);

console.log(gen.next().value); // 10
console.log(gen.next().value); // 11
console.log(gen.next().value); // 12
console.log(gen.next().value); // 13
console.log(gen.next().value); // 20
console.log(gen.next().value); // undefined