How to check if a variable is set?

Question

I tried to do it that way:

else if (vCodUG === undefined)

and made an error of:

Uncaught Referenceerror: vCodUG is not defined

Answer 1

Instead of comparing values

vCogUG == undefined

use the operator typeof:

typeof vCogUG === "undefined"

Example:

if (typeof vCogUG === "undefined") {
    alert("vCogUG é undefined");  // Será mostrado
} else {
    alert("O valor de vCogUG é " + vCogUG); // Não será mostrado
}

var vCogUG = 3;

if (typeof vCogUG === "undefined") {
    alert("vCogUG é undefined"); // Não será mostrado
} else {
    alert("O valor de vCogUG é " + vCogUG); // Será mostrado
}

Answer 2

Use the operator typeof:

if (typeof nomeVar != 'undefined') {}

Answer 3

The answers of Leofilipe and brandizzi are correct.

There is however an alternative you can use for a similar check:

var vCogUG;
vCogUG === undefined;

If the variable vCogUG already exists and is reachable in the scope, it will not be overwritten. Otherwise, you will have a variable with undefined value. In both cases, you can compare the way you were doing in your code. I prefer this form because we compare the value of the variable with undefined, instead of comparing its type with the literal string "undefined".

editing: Gibson called attention to the fact that, depending on the context, you will rather overwrite the variable. This occurs with closures (see his comment). If you are inside a closure or auto-invoked function, you better turn to typeof even.