Thursday, February 9, 2012

setTimeout Setting Mysterious Callback Parameters

I got burned by this twice in the last month.

Say you want to call the following function after 5 seconds.
function doSomething(optionalParam) {
  // check if optional param passed in.
  if (optionalParam) {; // this is where the error sometimes happens.
  // do more stuff.

// delay 5 seconds.
setTimeout(doSomething, 5000);

Periodically I was getting undefined errors when trying to call foo(). After some googling, it turns out Gecko browsers have setTimeout pass back an extra parameter indicating the "actual lateness" of the timeout in milliseconds.

I haven't run into this with other browsers but to avoid the issue it's best to do the following when your callback functions have optional params (Not having optional params would obviously be even safer).
// delay 5 seconds.
setTimeout(function() {
}, 5000);