Is localStorage.getItem('item') better than localStorage.item or localStorage['item']?

Both direct property access (localStorage.item or localStorage['item']) and using the functional interface (localStorage.getItem('item')) work fine. Both are standard and cross-browser compatible.^* According to the spec:

The supported property names on a Storage object are the keys of each key/value pair currently present in the list associated with the object, in the order that the keys were last added to the storage area.

They just behave differently when no key/value pair is found with the requested name. For example, if key 'item' does not exist, var a = localStorage.item; will result in a being undefined, while var a = localStorage.getItem('item'); will result in a having the value null. As you have discovered, undefined and null are not interchangeable in JavaScript/EcmaScript. :)

EDIT: As Christoph points out in his answer, the functional interface is the only way to reliably store and retrieve values under keys equal to the predefined properties of localStorage. (There are six of these: length, key, setItem, getItem, removeItem, and clear.) So, for instance, the following will always work:

localStorage.setItem('length', 2);
console.log(localStorage.getItem('length'));

Note in particular that the first statement will not affect the property localStorage.length (except perhaps incrementing it if there was no key 'length' already in localStorage). In this respect, the spec seems to be internally inconsistent.

However, the following will probably not do what you want:

localStorage.length = 2;
console.log(localStorage.length);

Interestingly, the first is a no-op in Chrome, but is synonymous with the functional call in Firefox. The second will always log the number of keys present in localStorage.

^* _{This is true for browsers that support web storage in the first place. (This includes pretty much all modern desktop and mobile browsers.) For environments that simulate local storage using cookies or other techniques, the behavior depends on the shim that is used. Several polyfills for localStorage can be found here.}

The question is already quite old, but since I have been quoted in the question, I think I should say two words about my statement.

The Storage Object is rather special, it's an object, which provides access to a list of key/value pairs. Thus it's not an ordinary object or array.

For example it has the length attribute, which unlike the array length attribute is readonly and returns the number of keys in the storage.

With an array you can do:

var a = [1,2,3,4];
a.length // => 4
a.length = 2;
a // => [1,2]

Here we have the first reason to use the getters/setters. What if you want to set an item called length?

localStorage.length = "foo";
localStorage.length  // => 0
localStorage.setItem("length","foo");
// the "length" key is now only accessable via the getter method:
localStorage.length  // => 1
localStorage.getItem("length") // => "foo"

With other members of the Storage object it's even more critical, since they are writable and you can accidently overwrite methods like getItem. Using the API methods prevents any of these possible problems and provides a consistent Interface.

Also interesting point is the following paragraph in the spec (emphasized by me):

The setItem() and removeItem() methods must be atomic with respect to failure. In the case of failure, the method does nothing. That is, changes to the data storage area must either be successful, or the data storage area must not be changed at all.

Theoretically there should be no difference between the getters/setters and the [] access, but you never know...