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

Another potential "solution" is to use the window object. It avoids the reference error problem when in a browser.

if (window.x) {
    alert('x exists and is truthy');
} else {
    alert('x does not exist, or exists and is falsy');
}