HTML5 STORAGE

What I will refer to as “HTML5 Storage” is a specification named Web Storage, which was at one time part of the HTML5 specification proper, but was split out into its own specification for uninteresting political reasons. Certain browser vendors also refer to it as “Local Storage” or “DOM Storage.” The naming situation is made even more complicated by some related, similarly-named, emerging standards that I’ll discuss later in this chapter So what is HTML5 Storage? Simply put, it’s a way for web pages to store named key/value pairs locally, within the client web browser.

USING HTML5 STORAGE

HTML5 Storage is based on named key/value pairs. You store data based on a named key, then you can retrieve that data with the same key. The named key is a string. The data can be any type supported by JavaScript, including strings, Booleans, integers, or floats. However, the data is actually stored as a string. If you are storing and retrieving anything other than strings, you will need to use functions like parseInt() or parseFloat() to coerce your retrieved data into the expected JavaScript datatype.

Calling setItem() with a named key that already exists will silently overwrite the previous value. Calling getItem() with a non-existent key will return null rather than throw an exception.

Like other JavaScript objects, you can treat the localStorage object as an associative array. Instead of using the getItem() and setItem() methods, you can simply use square brackets

TRACKING CHANGES TO THE HTML5 STORAGE AREA

If you want to keep track programmatically of when the storage area changes, you can trap the storage event. The storage event is fired on the window object whenever setItem(), removeItem(), or clear() is called and actually changes something. For example, if you set an item to its existing value or call clear() when there are no named keys, the storage event will not fire, because nothing actually changed in the storage area.

The storage event is supported everywhere the localStorage object is supported, which includes Internet Explorer 8. IE 8 does not support the W3C standard addEventListener (although that will finally be added in IE 9). Therefore, to hook the storage event, you’ll need to check which event mechanism the browser supports. (If you’ve done this before with other events, you can skip to the end of this section. Trapping the storage event works the same as every other event you’ve ever trapped. If you prefer to use jQuery or some other JavaScript library to register your event handlers, you can do that with the storage event, too.