Faire attendre la fonction jusqu'à ce que l'élément existe

Question

J'essaie d'ajouter un canevas par-dessus un autre canevas - comment puis-je faire en sorte que cette fonction attende pour démarrer que le premier canevas soit créé ?

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;
    }