Why does declaring var timer = setTimeout(etc.) automatically execute the function?

Question

Why does declaring var timer = setTimeout(etc.) automatically execute the setTimeout() function? And how would one pass a setTimeout() function as a variable without having the setTimeout() automatically execute?

var do_this = function(params){console.log("executes automatically, but function is only declared")};
var delay = 50;
var timeoutID = setTimeout(do_this(), delay) //executes automatically

You're *calling* `do_this()`! Parentheses are calling functions. — deceze, Apr 14 '22 at 11:30

score -1 · Answer 1 · answered Apr 14 '22 at 11:16

-1

Functions such as setTimeout require a callback-function that can be called the moment the timer expires. When you just provide a function, it is called immediately because it is never added to the callback-queue.

Thus, you should pass it a callback-function:

setTimeout( () => ... )

answered Apr 14 '22 at 11:16

Bart Barnard

1,128
8
17

When you "just provide a function", how's that different from passing a "callback-function"?! – deceze Apr 14 '22 at 11:19
I meant the difference between `setTimeout( function() )` and setTimeout( () => function() )` – Bart Barnard Apr 14 '22 at 11:21
OK. But the simpler thing to do then is just `setTimeout(function)`… – deceze Apr 14 '22 at 11:22

Why does declaring var timer = setTimeout(etc.) automatically execute the function?

1 Answers1