网页屏蔽Backspace事件，输入框不屏蔽

document.onkeydown = function (e) {     var code;        if (!e){ var e = window.event;}        if (e.keyCode){ code = e.keyCode;}     else if (e.which){ code = e.which;}     //BackSpace 8;     if (       (event.keyCode == 8)       && ((event.srcElement.type != "text" && event.srcElement.type != "textarea" && event.srcElement.type != "password")         ||  event.srcElement.readOnly == true         )            ) {               event.keyCode = 0;              event.returnValue = false;         }     return true;};