generators.js

function* gen() {
  const three = yield 'three';
  const two = yield 'two';
  const sum = yield `${two}, ${three}`;
  
  return sum;
}

const iterable = gen(); // the generator instance folllws the iterable protocol

iterable.next()
// {value: "three", done: false}
iterable.next()
// {value: "two", done: false}
iterable.next() 
// {value: "undefined, undefined", done: false}
iterable.next() 
// {value: undefined, done: true}
iterable.next()
// {value: undefined, done: true}