A generator is an iterator function whose body you can slice into many parts by combining a yield / iterator.next() expression.

iteratorFn.js

/*
 * A generator is an iterator function whose body you can slice into many  parts by combining a yield / iterator.next() expression.
 */

function* iteratorFn(value) {
  //body does not execute until first iterator.next() is called
  console.log("before yield!", value); // execute first iterator body slice
  value = yield yieldFn(value); // stop execution & returns iterator value - resumes execution, after second iterator.next() is called & updates value from arg in iterator.next( arg )
  console.log("after yield:", value); // execute second iterator body slice
  return "I am done!" + value + "second part"; // stops execution & returns final iterator value
}

function yieldFn(value) { // executed in yield expression
  console.log("inside yield!!!");
  console.log("Works!!!");
  console.log(value)
  return "Hello World!!!" + value; // iterator value
}

var iterator = iteratorFn("TTT"); // creates iterator object - TTT is passed to first iterator body slide
console.log(iterator);
var iteratorValue = iterator.next("ZZZ"); // executes first iteratorFn body slice - ZZZ is ignored
console.log(iteratorValue);
iteratorValue = iterator.next("PPP"); // executes second iterator body slice - PPP is passed to second iterator body slice
console.log(iteratorValue);