函数防抖 主要用于限制高频触发事件函数的执行间隔

var debounce = function (func, wait, lossless) {
    var lastTimeout, alreadyDo = false;
    if (typeof lossless == ‘undefined‘)
        lossless = true;
    return function () {
        var context = this,
              args = arguments;
        if (!alreadyDo) {
            alreadyDo = true;
            setTimeout(function () { alreadyDo = false }, wait);
            clearTimeout(lastTimeout);
            func.apply(context, args);
        } else if (lossless) {
            clearTimeout(lastTimeout);
            lastTimeout = setTimeout(function () {
                func.apply(context, args);
            }, wait);
        }
    };
}