How to check a not-defined variable in JavaScript

Question

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error

alert( x );

How can I cat

Answer 1

The void operator returns undefined for any argument/expression passed to it. so you can test against the result (actually some minifiers change your code from undefined to void 0 to save a couple of characters)

For example:

void 0
// undefined

if (variable === void 0) {
    // variable is undefined
}