modified implementing fadeIn and fadeOut without jQuery

FX.js

/*!
 * implementing fadeIn and fadeOut without jQuery
 * gist.github.com/englishextra/baaa687f6ae9c7733d560d3ec74815cd
 * modified jsfiddle.net/LzX4s/
 * changed options.complete(); to:
 * function"==typeof options.complete && options.complete();
 * usage:
 * FX.fadeIn(document.getElementById('test'),
 * {duration:2000,complete:function(){alert('Complete');}});
 */
var FX = (function () {
	var e = {
		easing: {
			linear: function (a) {
				return a;
			},
			quadratic: function (a) {
				return Math.pow(a, 2);
			},
			swing: function (a) {
				return 0.5 - Math.cos(a * Math.PI) / 2;
			},
			circ: function (a) {
				return 1 - Math.sin(Math.acos(a));
			},
			back: function (a, b) {
				return Math.pow(a, 2) * ((b + 1) * a - b);
			},
			bounce: function (a) {
				for (var b = 0, c = 1; ; b += c, c /= 2) {
					if (a >= (7 - 4 * b) / 11) {
						return -Math.pow((11 - 6 * b - 11 * a) / 4, 2) + Math.pow(c, 2);
					}
				}
			},
			elastic: function (a, b) {
				return Math.pow(2, 10 * (a - 1)) * Math.cos(20 * Math.PI * b / 3 * a);
			}
		},
		animate: function (a) {
			if (a) {
				var b = new Date(),
				c = setInterval(function () {
						var d = (new Date() - b) / a.duration;
						if (1 < d) {
							d = 1;
						}
						a.progress = d;
						var e = a.delta(d);
						a.step(e);
						if (1 == d) {
							clearInterval(c);
							if ("function" === typeof a.complete) {
								a.complete();
							}
						}
					}, a.delay || 10);
			}
		},
		fadeOut: function (a, b) {
			if (a) {
				this.animate({
					duration: b.duration,
					delta: function (a) {
						a = this.progress;
						return e.easing.swing(a);
					},
					complete: b.complete,
					step: function (b) {
						a.style.opacity = 1 - b;
					}
				});
			}
		},
		fadeIn: function (a, b) {
			if (a) {
				this.animate({
					duration: b.duration,
					delta: function (a) {
						a = this.progress;
						return e.easing.swing(a);
					},
					complete: b.complete,
					step: function (b) {
						a.style.opacity = 0 + b;
					}
				});
			}
		}
	};
	return e;
})();

var a = document.getElementById('test');
FX.fadeIn(a, {
	duration: 2000,
	complete: function () {
		alert('Complete');
	}
});
setTimeout(function () {
	FX.fadeOut(a, {
		duration: 4000,
		complete: function () {
			alert('Complete');
		}
	})
}, 2000);