JavaScript condition always fails

In the below code, the if statement always fails. New to JavaScript, couldn't figure out.

I believe the code is self explanatory. What am I doing wrong?

if (localStorage.user_name === null || localStorage.user_name === 'undefined') {
    registerUser(userName);
} else {
    login(localStorage.user_name);  // Gets executed always... Even if there is no user_name in localStorage.
}