The core code

//Does the specified function exist
function isExitsFunction(funcName) {
  try {
    if (typeof(eval(funcName)) == "function") {
      return true;
    }
  } catch(e) {}
  return false;
}
//Whether the specified variable exists
function isExitsVariable(variableName) {
  try {
    if (typeof(variableName) == "undefined") {
      //alert("value is undefined");
      return false;
    } else {
      //alert("value is true");
      return true;
    }
  } catch(e) {}
  return false;
}

More judgment can be found in this article: (link: #)