Use requestAnimationFrame to fade in element

Using CSS transitions is still inefficient at the hardware level. Using requestAnimationFrame, we can fade in HTML elements as smoothly as possible.

by Chris

HTML

<h1>This will fade in at 60 frames per second (or as close to possible as your hardware allows)</h1>

JavaScript

// Fade in over 2000 ms = 2 seconds.
var FADE_DURATION = 10 * 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);
    console.log('opacity', opacity);
    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);