How can I check if a variable exist in JavaScript?

Question

I am not asking if the variable is undefined or if it is null. I want to check if the variable exists or not. Is this possible?

Answer 1

if ('bob' in window) console.log(bob);

Keep in mind with this way, even if you declared a variable with var, that would mean it exists.

Answer 2

The typeof techniques don't work because they don't distinguish between when a variable has not been declared at all and when a variable has been declared but not assigned a value, or declared and set equal to undefined.

But if you try to use a variable that hasn't been declared in an if condition (or on the right-hand side of an assignment) you get an error. So this should work:

var exists = true;
try {
    if (someVar)
        exists = true;
} catch(e) { exists = false; }
if (exists)
   // do something - exists only == true if someVar has been declared somewhere.