Make function wait until element exists

Question

I'm trying to add a canvas over another canvas – how can I make this function wait to start until the first canvas is created?

function PaintObject(brush) {

    this.started = false;

    // get handle of the main canvas, as a DOM object, not as a jQuery Object. Context is unfortunately not yet
    // available in jquery canvas wrapper object.
    var mainCanvas = $("#" + brush).get(0);

    // Check if everything is ok
    if (!mainCanvas) {alert("canvas undefined, does not seem to be supported by your browser");}
    if (!mainCanvas.getContext) {alert('Error: canvas.getContext() undefined !');}

    // Get the context for drawing in the canvas
    var mainContext = mainCanvas.getContext('2d');
    if (!mainContext) {alert("could not get the context for the main canvas");}

    this.getMainCanvas = function () {
        return mainCanvas;
    }
    this.getMainContext = function () {
        return mainContext;
    }

    // Prepare a second canvas on top of the previous one, kind of second "layer" that we will use
    // in order to draw elastic objects like a line, a rectangle or an ellipse we adjust using the mouse
    // and that follows mouse movements
    var frontCanvas = document.createElement('canvas');
    frontCanvas.id = 'canvasFront';
    // Add the temporary canvas as a second child of the mainCanvas parent.
    mainCanvas.parentNode.appendChild(frontCanvas);

    if (!frontCanvas) {
        alert("frontCanvas null");
    }
    if (!frontCanvas.getContext) {
        alert('Error: no frontCanvas.getContext!');
    }
    var frontContext = frontCanvas.getContext('2d');
    if (!frontContext) {
        alert("no TempContext null");
    }

    this.getFrontCanvas = function () {
        return frontCanvas;
    }
    this.getFrontContext = function () {
        return frontContext;
    }

Answer 1

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.