Use try-catch to determine whether a variable has been declared, undeclared or unassigned

Use try-catch to determine whether a variable has been declared, undeclared or unassigned_javascript skills

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Release： 2016-05-16 16:55:39

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The purpose is that if a variable has been declared but not assigned, it can be assigned a value directly; and the scope of the variable cannot be changed

If it is not declared, re-declare it.

I searched on the Internet, A common method is if(typeof(a)=='undefined'){var a='ss';},

But this method will return true for undeclared or declared unassigned variables. And if so:

Copy the code The code is as follows:

 
var a; 
function f(){ 
if(typeof(a)=='undefined') 
{var a=1;} 
} 
f(); 
console.log(a); 

will display undefined because f() only declares a local variable with the same name.

But if it is a declared unassigned variable: if (noValueV==null), it will return true;

Undeclared variable: if (noDeclareV==null), an error will be reported.

So it works like this:

Copy the code The code is as follows:

 
function f ( ){ 
if(typeof(v)=='undefined'){ 
try{ 
if(v==null)//Indicate that v is declared but not assigned 
v=1; //If v is a global variable, this will not change its scope
} 
catch(err){//Indicate that v is undeclared
var v;v=2; 
} 
} 
console.log(v); 
} 
f( ); 

This is also wrong, because js has the feature of 'declaration in advance', that is, functions Variables declared within a function are visible in this function and in subfunctions of this function, regardless of where it is declared within the function.

So due to the var v; above, no matter what the situation, only try is used.

Modify it:

Copy the code The code is as follows:

 
function f( ){ 
if(typeof(v)=='undefined'){ 
try{ 
if(v==null)//Indicate that v is declared but not assigned 
v=1; / /If v is a global variable, this will not change its scope
} 
catch(err){//Indicate that v is undeclared
eval('var v');v=2; / /It’s different here 
} 
} 
console.log(v); 
} 
f( ); 

That’s it.

is written as a judgment function, returning 'noDeclare' means the variable has not been declared, 'noValue' means the variable has been declared but has not been assigned a value, and 'hasValue' means the variable has been declared and assigned a value:

Copy code The code is as follows:

 
function f(v){ 
if(typeof(v)=='undefined'){ 
try{ 
if(v==null) 
return 'noValue'; 
} 
catch(err){ 
return 'noDeclare'; 
} 
 } 
else return 'hasValue'; 
} 
var a; 
console.log(f(a)); 
a=0; 
console.log(f(a) )); 
console.log(f(b)); 

Wrong again... console.log(f(b)); will report an error... ...