next should be called only to advance state. can skip calls

get_stock_price.es.js

let price = 10;

let stockPrice = function () {
  setTimeout(function() {
    iterator.next(price++);
    console.log(price); //=> 10
    // why is this logged ? next does not behave like return ?
    // and it is always 10. "price++" did not increment value of price
  }, 300);
}

let oops = function () {
  throw new Error("oops!");
}

let stockTicker = function*() {
  price = yield stockPrice();
  console.log("price is " + price); //=> price is 10
  
  price = yield stockPrice();
  console.log("price is " + price); //=> price is 10

  // throw new Error("oops!");
  oops();
};

var iterator = stockTicker();
console.log(iterator.next()); //=> {"done":false}

// calling next here is bad. because next is called from the `stockPrice` function
// calling next here would give the next value, even though stockPrice has a setTimeout callback running
//console.log(iterator.next());
//console.log(iterator.next());

continuation of https://gist.github.com/deepak/8e2f414132bb685e2050