HTML5 LocalStorage: Checking if a key exists

Question

Why this does not work ?

if(typeof(localStorage.getItem("username"))=='undefined'){
    alert('no');
};

The goal is to redirect the user from the index page to the login page if not already logged. Here the localStorage.getItem("username")) variable is not defined for the moment.

It's for an ios phonegap app.

Answer 1

Quoting from the specification:

The getItem(key) method must return the current value associated with the given key. If the given key does not exist in the list associated with the object then this method must return null.

You should actually check against null.

if (localStorage.getItem("username") === null) {
  //...
}