Why does this code execute instantly when I add in Promise code?

Question

In the code below I would expect two 4 second delays. But if you run the code you will see it executes instantly. Obviously I don't understand something very fundamental or I am a stupid error I can't see.

function calculatesomething(resolve) {
  console.log("calculating");

  setTimeout(results(resolve,3),4000);
}

var answer=0;
function results(resolve,n) {
  console.log("got results");

  answer=1;
  resolve(answer+n);
}



function doingstuff() {
  console.log("starting");

  var promise1 = new Promise(function(resolve) {
    setTimeout(calculatesomething(resolve),4000);
  });

  promise1.then(function(value) {
    console.log("done: "+value);
  });

  console.log("regular execution");
}

doingstuff();

(sorry about the indenting here. I have no idea why its not respecting the tabs.)

score 6 · Answer 1 · answered Feb 14 '18 at 20:16

6

The very fundamental thing is this:

 setTimeout(results(resolve,3),4000);

You dont need to call the function directly and pass the returned value, but rather pass the function reference:

 setTimeout(results, 4000, /*args:*/ resolve, 4);

answered Feb 14 '18 at 20:16

Jonas Wilms

106,571
13
98
120

1

TIL you can pass the parameters in with `setTimeout`. – Derek 朕會功夫 Feb 14 '18 at 20:18
Pretty elegant solution, upvote – Commercial Suicide Feb 14 '18 at 20:23
@derek just spent 10 minutes each in writing [this](https://stackoverflow.com/questions/48780927/javascript-when-and-when-not-to-use-this/48790252#48790252) and [this](https://stackoverflow.com/questions/48794471/general-solution-for-pre-emptive-background-work-scheduling-on-javascript/48795175#48795175) answer and i get 5 upvotes for such a common mistake, that is well documented everywhere... :/ And you didnt know this? That surprises me... – Jonas Wilms Feb 14 '18 at 20:26
@commercial suicide thanks :) – Jonas Wilms Feb 14 '18 at 20:26
@JonasW. Even as an experienced developer, JavaScript still gives me surprises every now and then ;) – Derek 朕會功夫 Feb 14 '18 at 21:29

Commercial Suicide · Answer 2 · 2018-02-14T22:28:30.847

1

setTimeout accepts a callback function as first parameter and one of possible solutions will be to wrap your function invocation inside an anonymous function like this:

function calculatesomething(resolve) {
  console.log("calculating");

  setTimeout(function() {
    results(resolve,3);
  }, 4000)
}

var answer=0;
function results(resolve,n) {
  console.log("got results");

  answer=1;
  resolve(answer+n);
}



function doingstuff() {
  console.log("starting");

  var promise1 = new Promise(function(resolve) {
    setTimeout(function() {
      calculatesomething(resolve)
    }, 4000);
  });

  promise1.then(function(value) {
    console.log("done: "+value);
  });
        
  console.log("regular execution");
}

doingstuff();

edited Feb 14 '18 at 22:28

answered Feb 14 '18 at 20:15

Commercial Suicide

13,616
13
48
72

Wow it works now!! I will study this more to understand. I have used setTimeout many times without this problem. Thanks so much! – user1254723 Feb 14 '18 at 20:21
@user1254723, cheers! – Commercial Suicide Feb 14 '18 at 20:22
`Jonas W. provided more elegant way, I upvote him, but my example is working too, but not so elegant.` :) ... i appreciate that, however this is not really mandatory just saying... – Jonas Wilms Feb 14 '18 at 20:31
@Jonas W., got it, the answer is updated =) – Commercial Suicide Feb 14 '18 at 22:30

Why does this code execute instantly when I add in Promise code?

2 Answers2