JavaScript Metro style loading bar class

(function (window, document, lib, undefined) {
    "use strict";
    if (lib == undefined || !lib instanceof Assembly ) throw "LoadingBar must be part of an assembly.";

    lib.LoadingBar = function (args /* { element : document element, numBlocks : integer } */) {
        var args = args == undefined ? { numBlocks: this.numBlocks, element: document.createElement('div')} : args;
        this.numBlocks = args.numBlocks == undefined ? this.numBlocks : typeof args.numBlocks == 'number' ? args.numBlocks : this.numBlocks;
        this.element = args.element == undefined ? document.createElement('div') : args.element;
        this.element.className = 'loading-bar';
        this.element.style.position = 'relative';
        this.element.style.left = '0';
        this.element.style.right = '0';
        this.element.style.overflow = 'hidden';
        this.setDimensions();
        var _this = this;
        window.addEventListener('resize', function () { _this.setDimensions() });
        return this;
    }
    lib.LoadingBar.prototype = {
        constructor: lib.LoadingBar,
        interval: null,
        loops: 0,
        loopPauseDuration: 1000,
        element: null,
        numBlocks: 5,
        animationSpeed: 10,
        blockDisplacemenetSpeed: 6,
        slowDownPercentage: 50,
        slowDownRange: 0,
        slowDownAreaStart: 0,
        slowDownAreaStop: 0,
        setDimensions: function () {
            this.element.style.width = window.innerWidth + 'px';
            this.slowDownRange = Math.floor((parseInt(this.element.style.width) / 3));
            this.slowDownAreaStart = this.slowDownRange;
            this.slowDownAreaStop = this.slowDownRange * 2;
        },
        addBlock: function () {
            var block = document.createElement('div');
            block.className = 'block';
            block.style.position = 'fixed';
            block.style.display = 'block';
            this.element.appendChild(block);
            return block;
        },
        removeBlock: function (index) {
            this.element.removeChild(this.element.getElementsByClassName('block')[index]);
        },
        clearBlocks: function () {
            while (this.element.hasChildNodes()) {
                this.element.removeChild(this.element.lastChild);
            }
        },
        start: function () {
            this.reset();
            for (var i = 0; i < this.numBlocks; i++) {
                this.addBlock().style.left = Math.floor(-1 * (1 + i) * (this.slowDownRange / this.numBlocks)) + 'px';
            }
            var pause = this.loops == 0 ? 0 : this.loopPauseDuration,
                _this = this;
            this.loops++;
            this.interval = setTimeout(function () { _this.animate(); }, pause);
            return this;
        },
        animate: function () {
            var _this = this,
                blocks = this.element.getElementsByClassName('block');
            if (blocks.length == 0) return this.start();
            for (var i in blocks) {
                if (typeof blocks[i] == 'object') {
                    var x = parseInt(blocks[i].style.left),
                    displacementSpeed = this.blockDisplacemenetSpeed;
                    if (x >= this.slowDownAreaStart && x <= this.slowDownAreaStop) {
                        displacementSpeed = Math.floor(displacementSpeed / (100 / this.slowDownPercentage));
                    }
                    blocks[i].style.left = (x + displacementSpeed) + 'px';
                    if (parseInt(blocks[i].style.left) >= parseInt(this.element.style.width)) {
                        this.removeBlock(i)
                    }
                }
            }
            this.interval = setTimeout(function () { _this.animate(); }, _this.animationSpeed);
        },
        reset: function () {
            this.clearBlocks();
            clearTimeout(this.interval);
        },
        stop: function () {
            this.loops = 0;
            this.clearBlocks();
            clearTimeout(this.interval);
        }
    }
})(window, document, Metro)