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

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... ...