requestAnimationFrame

When it comes to animating DOM elements fluidly, we are limited to the following CSS transitions:

  • POSITION - transform: translate (npx, npx);
  • SCALE - transform: scale(n);
  • ROTATION - transform: rotate(ndeg);
  • OPACITY - opacity: 0;

However, using these is no guarantee that your animations will be fluid, because it causes the browser to start new paint cycles, regardless of what else is going on. Basically, paint are made inefficiently and your animation looks "janky" because the frames per second (FPS) suffers.

To guarantee smooth-as-possible DOM animations, requestAnimationFrame must be used in conjunction with the above CSS transitions.

The reason this works, is because the requestAnimationFrame API lets the browser know that you want an animation to happen at the next paint cycle, as opposed to interrupting what's going on to force a new paint cycle in when a non-RAF animation is called.

ReferencesURL
What is jank?http://jankfree.org/
High Performance Animationshttp://www.html5rocks.com/en/tutorials/speed/high-performance-animations/.
R.A.I.L.https://developers.google.com/web/tools/chrome-devtools/profile/evaluate-performance/rail?hl=en
Analyzing Critical Rendering Pathhttps://developers.google.com/web/fundamentals/performance/critical-rendering-path/analyzing-crp?hl=en
Rendering Performancehttps://developers.google.com/web/fundamentals/performance/rendering/?hl=en
Analyzing Paint Timeshttps://developers.google.com/web/updates/2013/02/Profiling-Long-Paint-Times-with-DevTools-Continuous-Painting-Mode?hl=en
Identifying Paint Bottleneckshttps://developers.google.com/web/fundamentals/performance/rendering/simplify-paint-complexity-and-reduce-paint-areas?hl=en

Use requestAnimationFrame to fade in element

<html>
    <body>
        <h1>This will fade in at 60 frames per second (or as close to possible as your hardware allows)</h1>
        
        <script>
            // Fade in over 2000 ms = 2 seconds.
            var FADE_DURATION = 2.0 * 1000; 
            
            // -1 is simply a flag to indicate if we are rendering the very 1st frame
            var startTime=-1.0; 
            
            // Function to render current frame (whatever frame that may be)
            function render(currTime) { 
                var head1 = document.getElementsByTagName('h1')[0]; 
            
                // How opaque should head1 be?  Its fade started at currTime=0.
                // Over FADE_DURATION ms, opacity goes from 0 to 1
                var opacity = (currTime/FADE_DURATION);
                head1.style.opacity = opacity;
            }
            
            // Function to 
            function eachFrame() {
                // Time that animation has been running (in ms)
                // Uncomment the console.log function to view how quickly 
                // the timeRunning updates its value (may affect performance)
                var timeRunning = (new Date()).getTime() - startTime;
                //console.log('var timeRunning = '+timeRunning+'ms');
                if (startTime < 0) {
                    // This branch: executes for the first frame only.
                    // it sets the startTime, then renders at currTime = 0.0
                    startTime = (new Date()).getTime();
                    render(0.0);
                } else if (timeRunning < FADE_DURATION) {
                    // This branch: renders every frame, other than the 1st frame,
                    // with the new timeRunning value.
                    render(timeRunning);
                } else {
                    return;
                }
            
                // Now we're done rendering one frame.
                // So we make a request to the browser to execute the next
                // animation frame, and the browser optimizes the rest.
                // This happens very rapidly, as you can see in the console.log();
                window.requestAnimationFrame(eachFrame);
            };
            
            // start the animation
            window.requestAnimationFrame(eachFrame);    
        </script>
    </body>
</html>

Cancelling an Animation

To cancel a call to requestAnimationFrame, you need the id it returned from when it was last called. This is the parameter you use for cancelAnimationFrame. The following example starts some hypothetical animation then pauses it after one second.

// stores the id returned from each call to requestAnimationFrame
var requestId;

// draw something
function draw(timestamp) {
    // do some animation
    // request next frame
    start();
}

// pauses the animation
function pause() {
    // pass in the id returned from the last call to requestAnimationFrame
    cancelAnimationFrame(requestId);
}

// begin the animation
function start() {
    // store the id returned from requestAnimationFrame
    requestId = requestAnimationFrame(draw);
}

// begin now
start();

// after a second, pause the animation
setTimeout(pause,1000);

Keeping Compatability

Of course, just like most things in browser JavaScript, you just can't count on the fact that everything will be the same everywhere. In this case, requestAnimationFrame might have a prefix on some platforms and are named differently, such as webkitRequestAnimationFrame. Fortunately, there's a really easy way to group all the known differences that could exist down to 1 function:

window.requestAnimationFrame = (function(){
    return window.requestAnimationFrame ||
        window.webkitRequestAnimationFrame ||
        window.mozRequestAnimationFrame ||
        function(callback){
            window.setTimeout(callback, 1000 / 60);
        };
})();

Note that the last option (which fills in when no existing support was found) will not return an id to be used in cancelAnimationFrame. There is, however an efficient polyfill that was written which fixes this.