Transparent canvas fader

gistfile1.js

/**
 * Fade transparent canvas using 'destination-out' mode.
 * To avoid visible trails call
 * fadeCanvas(canvas, 0.4, 'source-over');
 *
 * @param {element} canvas Reference to a Canvas DOM element
 * @param {number} alpha Alpha percentage of how much to fade
 * @param {string} mode Fading mode
 */
function fadeCanvas(canvas, alpha, mode) {
    mode = mode || 'destination-out';
    ctx = canvas.getContext('2d');
    ctx.globalCompositeOperation = mode;
    ctx.fillStyle = "rgba(255, 255, 255, " + alpha + ")";
    ctx.beginPath();
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    ctx.fill();
    ctx.globalCompositeOperation = 'source-over';
 }