//防止退格键返回    $("#txtMsg").keydown(function (e) {        var doPrevent;        if (e.keyCode == 8) {            var d = e.srcElement || e.target;            if (d.tagName.toUpperCase() == ‘INPUT‘ || d.tagName.toUpperCase() == ‘TEXTAREA‘) {                doPrevent = d.readOnly || d.disabled;            }            else                doPrevent = true;        }        else            doPrevent = false;        if (doPrevent)            e.preventDefault();    });