var GF = function(){

    // vars for counting frames/s, used by the measureFPS function

    var frameCount = 0;

    var lastTime;

    var fpsContainer;

    var fps; 

    var measureFPS = function(newTime){

         // test for the very first invocation

         if(lastTime === undefined) {

           lastTime = newTime; 

           return;

         }

        //calculate the difference between last & current frame

        var diffTime = newTime - lastTime; 

​

        if (diffTime >= 1000) {

            fps = frameCount;    

            frameCount = 0;

            lastTime = newTime;

        }

​

        //and display it in an element we appended to the 

        // document in the start() function

       fpsContainer.innerHTML = 'FPS: ' + fps; 

       frameCount++;

    };

    var mainLoop = function(){

        //main function, called each frame 

        measureFPS(+(new Date()));

        // call the animation loop every 1/60th of second

        var img = new Image();

        img.onerror = mainLoop;

        img.src = 'data:image/png,' + Math.random();

    };

​

    var start = function(){

        // adds a div for displaying the fps value

        fpsContainer = document.createElement('div');

        document.body.appendChild(fpsContainer);

​

        requestAnimationFrame(mainLoop);

    };

​

    //our GameFramework returns a public API visible from outside its scope

    return {

        start: start

    };

};

​

var game = new GF();

game.start();