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

Question

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

Answer 1 · 2018-03-21T23:01:27Z

March 21, 2018 11:01pm

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.

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Andreas Beyer

Andreas Beyer

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

1 Answer

Steven Parker

Steven Parker