scrolly a window scroll listener

gistfile1.js

var scrolly = (function () {

    var isDrag = false;
    var lag = 100;
    var isMouseDown = false;
    var listeners = {};
    var previous;

    var Bounce = function (delegate, lag) {
        var that = this;
        return function () {
            if (that.timer) {
                clearTimeout(that.timer);
            }
            that.timer = setTimeout(delegate, lag);
        }
    };

    var onMouseDown = function () {
        isMouseDown = true;
    };

    var onMouseUp = function () {
        isMouseDown = false;
        if (isDrag) {
            isDrag = false;
            trigger('stop');
        }
    };

    var onScroll = function () {
        if (isDrag) {
            previous = window.pageYOffset;
            return;
        }

        if (isMouseDown) {
            isDrag = isMouseDown;
            trigger('start');
            previous = window.pageYOffset;
            return;
        }

        if (window.pageYOffset == previous) {
            return;
        }

        trigger('start');
        trigger('stop');

        previous = window.pageYOffset;
    };

    var forEachListener = function (type, delegate) {
        if (!listeners[type]) {
            return;
        }
        for (var i = 0; i < listeners[type].length; i++) {
            if (delegate.call(listeners[type][i], i) === false) {
                break;
            }
        }
    };

    var trigger = function (type) {
        var that = this;
        forEachListener(type, function () {
            this(window.pageYOffset);
        });
    };

    var on = function (type, delegate) {
        if (!listeners[type]) {
            listeners[type] = [];
        }
        listeners[type].push(delegate);
    };

    var off = function (type, delegate) {
        forEachListener(type, function (i) {
            if (this === delegate) {
                delete listeners[type][i];
            }
        });
    };

    var toggle = function (threshold, inside, outside, isAuto) {
        on('stop', function (y) {
            var inThreshold = y < threshold;
            if ((isAuto !== false && (previous > y || inThreshold)) || (!isAuto && inThreshold)) {
                inside(y);
            } else {
                outside(y);
            }
        })
    };

    var init = function () {
        window.onscroll = new Bounce(onScroll, lag);
        document.onmousedown = onMouseDown;
        document.onmouseup = onMouseUp;
    };

    init();

    return{
        on: on,
        off: off,
        toggle: toggle
    }
})();

scrolly.toggle(200, function () {
document.body.className = 'header';
}, function () {
document.body.className = 'header idle';
});

scrolly.on('start', function(){
console.log('start 1')
});