Sometimes, you may want to check if a variable is already defined e.g. to not override definitions made by other scripts. However, there are many ways to check if a variable is or isn't defined in JavaScript. Here, I'll try to cover some of them.

The first one is checking if the value of the variable is identical to undefined:

var x; // x is declarated, but doesn't has a value i.e. undefined
if (x === undefined) { // True
    // x is undefined
}

This approach only works if the variable is, at least, declarated. If it isn't, a ReferenceError happens. To avoid this, use typeof:

var x;
if (typeof x === "undefined") { // True
    // ...
}
if (typeof y === "undefined") { // True, with y not declarated.
    // ...
}

Another way to check if a variable is declarated is to check on the scope, or, if the variable is global, on the global object.

var x;
if ("x" in this) { // True
    // ...
}

if ("y" in this) { // Nope, but look, no error
    // ...
}

var Klazz = function() {
    this.something = 0;
    if ("something" in this) { // Also true
        // ...
    }
}

As a last note, this, in the global scope, on most browsers, is the same as the window object.

Sources:

Mozilla Developer Network - Undefined

Stack Overflow