Simple javascript animate function for old browsers

animate.js

function animate(elem, options, duration, callback, callbackTimeout) {
    var start = +new Date,
        prop;

    for (prop in options) {
        var style        = getComputedStyle(elem, ''),
            value        = options[prop],
            px           = style[prop].slice(-2),
            currentValue = px == 'px' ? parseInt(style[prop]) : +style[prop],
            unit         = px == 'px' ? 'px' : '';

        (function (a, b, c) {
            return function _animate() {
                var now      = +new Date - start,
                    progress = now / duration,
                    result   = (a - b) * progress + b;

                elem.style[c] = progress < 1 ? result + unit : a + unit;

                if (progress < 1) {
                    setTimeout(_animate, 10);
                }
            };
        })(value, currentValue, prop)();
    }

    if (callback) {
        setTimeout(callback, callbackTimeout || duration);
    }
}