Make function wait until element exists

Depending on which browser you need to support, there's the option of MutationObserver.

EDIT: All major browsers support MutationObserver now.

Something along the lines of this should do the trick:

// callback executed when canvas was found
function handleCanvas(canvas) { ... }

// set up the mutation observer
var observer = new MutationObserver(function (mutations, me) {
  // `mutations` is an array of mutations that occurred
  // `me` is the MutationObserver instance
  var canvas = document.getElementById('my-canvas');
  if (canvas) {
    handleCanvas(canvas);
    me.disconnect(); // stop observing
    return;
  }
});

// start observing
observer.observe(document, {
  childList: true,
  subtree: true
});

N.B. I haven't tested this code myself, but that's the general idea.

You can easily extend this to only search the part of the DOM that changed. For that, use the mutations argument, it's an array of MutationRecord objects.

If you have access to the code that creates the canvas - simply call the function right there after the canvas is created.

If you have no access to that code (eg. If it is a 3rd party code such as google maps) then what you could do is test for the existence in an interval:

var checkExist = setInterval(function() {
   if ($('#the-canvas').length) {
      console.log("Exists!");
      clearInterval(checkExist);
   }
}, 100); // check every 100ms

But note - many times 3rd party code has an option to activate your code (by callback or event triggering) when it finishes to load. That may be where you can put your function. The interval solution is really a bad solution and should be used only if nothing else works.