C H A P T E R 11

Using the Storage APIs

In this chapter, we will explore what you can do with HTML5 Web Storage—sometimes referred to as DOMStorage—an API that makes it easy to retain data across web requests. Before the Web Storage API, remote web servers needed to store any data that persisted by sending it back and forth from client to server. With the advent of the Web Storage API, developers can now store data directly on the client side in the browser for repeated access across requests or to be retrieved long after you completely close the browser, thus reducing network traffic.

We’ll first look at how Web Storage differs from cookies and then explore how you can store and retrieve data. Next, we will look at the differences between localStorage and sessionStorage, the attributes and functions that the storage interface provides, and how you can handle Web Storage events. We wrap up with a look at Web SQL Database API and a few practical extras.

Overview of Web Storage

To explain the Web Storage API, it is best to review its predecessor, the intriguingly named cookie. Browser cookies—named after an age-old programming technique for passing small data values between programs—are a built-in way of sending text values back and forth from server to browser. Servers can use the values they put into these cookies to track user information across web pages. Cookie values are then transmitted back and forth every time a user visits a domain. For example, cookies can store a session identifier that allows a web server to know which shopping cart belongs to a user by storing a unique ID in a browser cookie that matches the server’s own shopping cart database. Then, as a user moves from page to page, the shopping cart can be updated consistently. Another use for cookies is to store local values into an application so that these values can be used on subsequent page loads.

Cookie values can also be used for operations that are slightly less desirable to users, such as tracking which pages a user visits for the sake of targeted advertising. As such, some users have demanded that browsers include functionality to allow them to block or remove cookies either all of the time or for specific sites.

Love them or hate them, cookies have been supported by browsers since the earliest days of the Netscape browser, back in the mid-1990s. Cookies are also one of the few features that have been consistently supported across browser vendors since the early days of the Web. Cookies allow data to be tracked across multiple requests, as long as that data is carefully coordinated between the server and the browser code. Despite their ubiquity, cookies have some well-known drawbacks:

•Cookies are extremely limited in size. Generally, only about 4KB of data can be set in a cookie, meaning they are unacceptable for large values such as documents or mail.

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