Generators - Understanding the basic mechanics of how raw generators works

generator-basics.js

const fakeTimeDelay = () => new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve(10);
  }, 1500);
});

function* getData() {
  console.log(`getData() is running`);
  console.log(`getData() is still running`);
  console.log(`getData() is about to hit the first yield`);
  let data = yield fakeTimeDelay();
  console.log(`getData() is running after the yield has returned`);
  console.log(`data value is ${ data * 5 }`); // logs 50
  return data + 5; // returns 15
}

// Initialises the getData generator
let gen = getData();

// gen.next() runs the getData generator method until the first yield is hit.
// When this happens in this case, the fakeTimeDelay() method is called and
// the results returned to the promise variable
let promise = gen.next();

// The value is stored as an iterable object { value: 10, done: false }
// It is here that the value could be handled but it MUST BE RETURN by
// calling another gen.next() method to RETURN IT BACK to the yield that
// called it within the getData generator function if the method is to
// contine and the function to complete albeit until the next yield
// is hit
promise.value.then((num) => {
  let value = gen.next(num);
});

// Final log of getData generator
// "getData() is running"
// "getData() is still running"
// "getData() is about to hit the first yield"
// "getData() is running after the yield has returned"
// "data value is 50"
// 15