Use requestAnimationFrame to fade in element

View jsFiddle: https://jsfiddle.net/HimmatChahal/jb5trg67/
Copy + Pasteable code below:
<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>
Found a mistake? Have a question or improvement idea? Let me know.
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