JS animation loop with duration

gistfile1.js

_animateSection: function(duration) {

    if (duration === undefined) duration = 1;

    var self = this;

    _animationLoop();

    function _animationLoop(_startTime, _timestamp) {

        if (_startTime === undefined || _timestamp === undefined) {
            requestAnimationFrame(function (timestamp) {
                _animationLoop(timestamp, timestamp);
            });
            return;
        }

        var timeElapsed = _timestamp - _startTime;
        var percent = ((timeElapsed / (duration * 1000)));

        if (timeElapsed < duration * 1000) {

            /**
             * DO STUFF HERE
             */

            // keep the animation loop going
            requestAnimationFrame(function (timestamp) {
                if (_startTime === undefined) _startTime = timestamp;
                _animationLoop(_startTime, timestamp);
            });
        }
    }
}