javascript实现网页屏蔽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; };