Use Try-catch to determine whether a variable is declared undeclared or unassigned

This article mainly introduces the use of Try-catch to determine whether the variable is declared unassigned or not declared, need friends can refer to the purpose is if a variable is declared unassigned, you can directly assign a value, and can not change the scope of the variable     if not stated, it is declared again,     Search on the Internet, the common method is if (typeof (a) = = ' undefined ') {var a= ' ss ';},    But this method returns true for a variable that is not declared or declared to be unassigned. And if so,:    code is as follows: Var a;  function f () {  if (typeof (a) = = ' undefined ')   {var a=1;}  }  F ();  Console.log (a);    will display undefined because the F () only declares a local variable with the same name.     But if an unassigned variable is declared: if (novaluev==null), it returns true;    undeclared variable: if (nodeclarev==null), an error is made.     So you can do this:    code is as follows: function f () {  if (typeof (v) = = ' undefined ') {  try{  if (v==null)// Description V is declared unassigned   v=1; If V is a global variable, this does not change its scope  }  catch (Err) {//description V is not declared   var v;v=2; } }  Console.log (v);  }  F ();    This is also wrong because JS has a ' Declaration of Advance ' feature, that is, variables declared within the function are visible in the function and in the function's child function, regardless of where it is declared in the function.     So because of the Var v above, it causes either of the cases to go only try.     Modify:    code as follows: function f () {  if (typeof (v) = = ' undEfined ') {  try{  if (v==null)/= V is declared unassigned   v=1;//If V is a global variable, this does not change its scope  }  catch (Err) {// Description V is not declared   eval (' Var v '); v=2; It's not like this.  } }  Console.log (v); }  F ();    This is OK.     Write a judgment function that returns ' Nodeclare ' indicating that the variable is not declared, ' Novalue ' indicates that the variable has been declared unassigned, ' HasValue ' indicates that the variable has been declared assigned:    code as follows: function f ( V) {  if (typeof (v) = = ' undefined ') {  try{  if (v==null)   return ' Novalue '; }  catch (err) {& nbsp Return ' Nodeclare '; } }  else return ' HasValue '; }  var a;  console.log (F (a));  a=0 ;  Console.log (f (a));  Console.log (f (b));    again wrong ... console.log (f (b)); Error ......