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JavaScript How to Make a Promise?

Andreas Beyer
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.a{fill-rule:evenodd;}techdegree
Andreas Beyer
Full Stack JavaScript Techdegree Student 12,474 Points

I just want to chain two functions, why does it not work

function times2(input) {
  setTimeout(function() {
    var number = input*2
    console.log("times2: "+number)
    return number
  },1000)
}

function times4(input) {
  var number = input*4
  console.log("times4: "+number)
  return number
}

function both(input) {
  return new Promise(resolve => {
      console.log(input);
      resolve(input);

  });
}


both(2).then(times2).then(times4)

//it want it to log: 2, 4, 16
//instead it logs: 2, NaN, 4
//WHY?!?!

//i just want to simply chain two functions without changing them, but i do not find any solution to do so in node

1 Answer

Steven Parker
Steven Parker
229,005 Points

For each step in a "then" chain, the result of the previous level is passed to the next. But the "times2" function doesn't return anything. It doesn't help to return something from the "setTimeout" handler, nothing waits for it.

So since "times2" doesn't return anything, "times4" is called with an undefined argument, which results in a calculation returning NaN. Then, 1 second later, the "setTimeout" completes and prints out the "times2" value.

if you make "times2" return a value, you'll see the expected output from "times4" but it will still happen immediately. To have it run in sequence, the "setTimeout" function would need to be converted into a promise, and "times2" would need to return that promise.