How do I know when a function was called in javascript?

Question

I don't know when a function was called. I'm trying to change the text of a p element with an id of downloads, that I know how to do. But how do you figure out when a function was called? The function is named move. Something like this? I just need when a function was called. Maybe an event listener?

    if (move.wasCalled) {
        // Some code
    }

If you did a google search, you would've saved yourself time rather than writing this question https://stackoverflow.com/questions/3332265/how-to-detect-a-function-was-called-with-javascript — Abdul Ahmad, Jul 26 '18 at 16:03
Do you want to execute some code _every time_ the function called - if yes, insert the code inside the function - or do you want to know if a function called? — Chayim Friedman, Jul 26 '18 at 16:03
Do you want 'when' or 'if' function was called? 'when' ---> new Date().getTime() then update a global scope var with this time whereas 'if' ---> use a Boolean flag, set global scope var to true when func called — CSSBurner, Jul 26 '18 at 16:07
I voted to reopen this as the dupe was not exact, although the title suggested that — Jonas Wilms, Jul 26 '18 at 16:16
Possible duplicate of [how to detect a function was called with javascript](https://stackoverflow.com/questions/3332265/how-to-detect-a-function-was-called-with-javascript) — Reza Saadati, Jul 26 '18 at 16:27
@RezaSaadati No, that quastion doesn't have the answer i'm looking for. — Brandon, Jul 26 '18 at 16:32

Jonas Wilms · Answer 1 · 2018-08-04T21:23:25.447

0

Seems as if you want to add an eventlistener for when move is called:

 move = (function(old) {
     var handlers = [];
     function move() {
        old.apply(this, arguments);
        handlers.forEach(function(handler) { handler(); });
     }
     move.wasCalled = function(handler) { handlers.push(handler) };
     return move;
   };
})(move);

So you can use it as:

 move.wasCalled(function() {
   //...
 });

 move();

If you however want to determine synchronously if the function was added already, just set a property when move was called, it can be rewritten as:

move = (old => (...args) => {
  move.wasCalled = true;
  old(...args);
})(move);

Or for older environments:

move = (function(old) { 
  return function() {
    move.wasCalled = true;
    old.apply(null, arguments);
  };
})(move);

So now

 if(move.wasCalled) { /*...*/ }

actually works

edited Aug 04 '18 at 21:23

answered Jul 26 '18 at 16:07

Jonas Wilms

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Whats args? And what should be before args? Sorry, I'm kind of a noob at js – Brandon Jul 26 '18 at 16:10
@sadieKoe if you add this code block after the declaration of `function move() { }` and before you call it the first time, the `move.wasCalled` will be set. Now `(...args)` takes all arguments that are passed to the function that replaces the original function and will then call the original function with that, so `move(1,2,3)` will still work – Jonas Wilms Jul 26 '18 at 16:12
When i put it in the code, it gives an error, "Unexpected token >" – Brandon Jul 26 '18 at 16:16
I just did! Sorry! – Brandon Jul 26 '18 at 16:18
@sadieKoe no worries, its just that a 2 year old version is likely to be vulnerable to a lot of attacks, so its not just that newer code won"t work its also a security thing – Jonas Wilms Jul 26 '18 at 16:20
Ok. Thanks for telling me that. – Brandon Jul 26 '18 at 16:21
Sorry, but what is the ... for? Also, I got an error, "Unexpected token >" – Brandon Jul 26 '18 at 16:23
@sadieKoe okay then forget about your browser, its KhanAcademys fault. They didn't expect that you would write new code :). I added a version that should work there – Jonas Wilms Jul 26 '18 at 16:27
Thank you so much! But for some reason, its not doing anything. I added a `console.log("Hi");`, but nothing happened – Brandon Jul 26 '18 at 16:28
You still here? – Brandon Jul 26 '18 at 16:37
I did this. `if (move.isCalled) { console.log("Hi"); }`, but nothing happened – Brandon Jul 26 '18 at 18:55
Hello! You here? – Brandon Jul 26 '18 at 19:37
I think you are actually looking for some kind of event listener, but thats hard to tell. I guess its the best if you just continue your course without this, and get back here at a later point, then we can continue this :) – Jonas Wilms Jul 26 '18 at 20:06

score 0 · Answer 2 · answered Jul 26 '18 at 16:09

You can add console.log("move function was called."); inside the if statement move.wasCalled like this:

if (move.wasCalled) {
    // Some code
    console.log("move function was called.");
}

So when the if statement is executed it will be logged in the console e.g.

Check in the Google Chrome dev tools console tab.

How do I know when a function was called in javascript?

2 Answers2