A very compact jQuery plugin to run a function when a test passes after repeated tries.
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jQuery.doWhen( testFn, callbackFn, config )

doWhen is a jQuery plugin to solve all your wait-until-ready problems, wash your dishes for you, and blow dry your cat when it falls in the tub. All that for just 382 bytes, minified!

Have you ever called a function before all the things it depends on are ready? The dependencies might be an html node, a plugin, a dynamically generated script finished loading, an image finished loading, a server state checked via ajax... the possibilities are endless. If you've been coding Javascript as long as I have, you've probably created countless bespoke solutions to fit each case.

No more. Let's say you need to do something to an element once it is created:

  return !!document.getElementById('myelement');
}, function(){
  document.getElementById('myelement').innerHTML = "I'm loaded"!

The first function is a test. When it returns a truthy value, the second function is run. It's as simple as that.

If you need either function to run in a specific context (where the "this" keyword points to), use the config parameter:

jQuery.doWhen(..., ..., {
  context: myObject

You can also pass it data:

jQuery.doWhen(..., ..., {
  data: {
    blix: 'blarg'

If you want to use a different interval than the default of 100ms between tests (which you'd definitely do for any test involving ajax):

jQuery.doWhen(..., ..., {
  interval: 60 * 1000 //once every minute

Update 27-9-2012: doWhen is now modularised for AMD, but will load fine without. Thanks to Erin Swenson-Healey for the submission. The context and data options now apply to the test function as well.

Update 11-11-2011: doWhen now has a standard non-jQuery version, just 335 bytes! See the "plain" directory.