Javascript implements web page shielding Backspace event, input box is not shielded_Basic knowledge

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; 
};