simple raf

// requestAnimationFrame的向下兼容处理
if (!window.requestAnimationFrame) {
    window.requestAnimationFrame = function(fn) {
        setTimeout(fn, 17);
    };
}
于是，动效绘制大致路数会变成这样：

var canvas = document.querySelector('canvas');
var context = canvas.getContext('2d');

// 画布渲染
var render = function () {
    // 清除画布
    context.clearRect(0, 0, canvas.width, canvas.height);

    // 绘制
    draw();

    // 继续渲染
    requestAnimationFrame(render);
};

render();