2.4.6 HTML5 Canvas Start and Stop an Animation

window.requestAnimFrame = (function(callback){
    return window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function(callback){
        window.setTimeout(callback, 1000 / 60);
    };
})();
 
function drawRect(myRectangle){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
    context.beginPath();
    context.rect(myRectangle.x, myRectangle.y, myRectangle.width, myRectangle.height);
 
    context.fillStyle = "#8ED6FF";
    context.fill();
    context.lineWidth = myRectangle.borderWidth;
    context.strokeStyle = "black";
    context.stroke();
}
 
function animate(lastTime, myRectangle, animProp){
    if (animProp.animate) {
        var canvas = document.getElementById("myCanvas");
        var context = canvas.getContext("2d");
 
        // update
        var date = new Date();
        var time = date.getTime();
        var timeDiff = time - lastTime;
        var linearSpeed = 100;
        // pixels / second
        var linearDistEachFrame = linearSpeed * timeDiff / 1000;
        var currentX = myRectangle.x;
 
        if (currentX < canvas.width - myRectangle.width - myRectangle.borderWidth / 2) {
            var newX = currentX + linearDistEachFrame;
            myRectangle.x = newX;
        }
        lastTime = time;
 
        // clear
        context.clearRect(0, 0, canvas.width, canvas.height);
 
        // draw
        drawRect(myRectangle);
 
        // request new frame
        requestAnimFrame(function(){
            animate(lastTime, myRectangle, animProp);
        });
    }
}
 
window.onload = function(){
    var myRectangle = {
        x: 0,
        y: 50,
        width: 100,
        height: 50,
        borderWidth: 5
    };
 
    /*
     * make the animation properties an object
     * so that it can be modified by reference
     * from an event
     */
    var animProp = {
        animate: false
    };
 
    // add click listener to canvas
    document.getElementById("myCanvas").addEventListener("click", function(){
        if (animProp.animate) {
            animProp.animate = false;
        }
        else {
            animProp.animate = true;
            var date = new Date();
            var time = date.getTime();
            animate(time, myRectangle, animProp);
        }
    });
 
    drawRect(myRectangle);
};