Function for doing quick animations

animate.js

var animate = (function(){

    'use strict';
    
    var PI = Math.PI;
    var PI_HALF = Math.PI/2;

    var deltas = {
        linear:function(p) {return p;},
        easeInSine:function(p) {return Math.cos(p * PI_HALF);},
        easeOutSine:function(p) {return Math.sin(p * PI_HALF);},
        easeInOutSine:function(p) {return Math.cos(p * PI) - 1;}
    };

    return function (options, step, complete) {

        var start = 0;
        var delta = deltas[options.ease || 'linear'];
        var duration = options.duration;

        function draw(now) {

            if (!start) {
                start = now;
            }
            var t = now - start;

            var progress = t / duration;
            if (progress > 1) {progress = 1;}

            step(delta(progress));

            if (progress < 1) {
                window.requestAnimationFrame(draw);
            }
            else if (complete) {
                complete();
            }

        }

        window.requestAnimationFrame(draw);

    };

});