JavaScript Cookbook

Chapter 20. Persistence

20.0. Introduction

In the early years of web development, to keep a value around as users moved from page to page, you’d have to make use of session persistence functionality built into the environment, attach the data to the web page URL, or provide it in a hidden form field. Of course, this persisted the data from page to page, but once you closed the browser, the data was lost.

JavaScript added a third approach, a cookie, which could not only persist from page to page, but cause the data to persist beyond the current session. With cookies, you could not only access data such as login information throughout a website, but you could close the browser and reopen it another day and still have access to the same data.

These techniques still exist, but now, because of the needs of more complex web applications, including applications that support offline functionality, browsers support a variety of sophisticated data storage and persistence techniques.

No area of JavaScript has gone through more upheaval recently than persistence. Whether it’s storing data between sessions or enabling access to information while offline, complex new ideas have been spawned, supported by web pages and implementations, only to quietly fade away when something new and shiny appeared.

Probably one of the more famous examples of an idea that came and went is Google Gears—an offline storage mechanism that generated considerable excitement in 2007 when it debuted, only to die a fairly quick ...