Weird random value as default interval handler parameter

Question

Hi,

Just came across this. It's not affecting anything really but i'm wondering why it's happening.

If I run the following code in firefox with firebug on:

setInterval(function(param) 
     {
        console.log("param is %o",param)
     },500);

param seems to be assigned a vaguely random value:

   param is -2
    param is -1
    param is -2
    param is 1
    param is -1
    param is 6
    param is -1
    param is 0
    param is -2
    param is 2
    param is 0
    param is 2
    param is 0
    param is 0
    param is 0
[..]
    param is 0
    param is 0
    param is 0
    param is 0
    param is 0
    param is 0
    param is 0
    param is 911
    param is 0
    param is 0
    param is 0
    param is -1

I do appreciate that I'm not passing any argument to setInterval to pass on to the function, but why does javascript chooses to pass this random number ?

I would have expected undefined or something like that...

Cheers

p.s. Haven't tested in other browsers

Answer 1

It appears to be dependent upon Firefox's CPU usage.

I would guess that it's the delay from when the callback should have been called.

EDIT: I was right. It's the number of milliseconds late the callback was called.

Answer 2

it's an interval ID automatically set by window.setInterval. if you store the result, you can clear the interval later (to stop it)

var intID = window.setInterval( function(){ alert("I'm annoying!"); }, 10000 );

// this will kill it before it annoys you, :D
window.clearInterval( intID );

this is also the case with window.setTimeout:

var timeID = window.setTimeout( function(){ alert("I'm annoying!"); }, 10000 );

// this will kill it before it annoys you, :D
window.clearTimeout( timeID );