JavaScript Pocket Reference, 3rd Edition

Chapter 14. Client-Side Storage

Web applications can use browser APIs to store data locally on the user’s computer. This client-side storage serves to give the web browser a memory. Web apps can store user preferences, for example, or even store their complete state, so that they can resume exactly where you left off at the end of your last visit. Client-side storage is segregated by origin, so pages from one site can’t read the data stored by pages from another site. But two pages from the same site can share storage and can use it as a communication mechanism. Data input in a form on one page can be displayed in a table on another page, for example. Web applications can choose the lifetime of the data they store: data can be stored temporarily so that it is retained only until the window closes or the browser exits, or it can be saved to the hard drive and stored permanently, so that it is available months or years later. This chapter covers two forms of client-side storage: the modern Web Storage API and the ancient Cookies API.

Storage, Security, and Privacy

Web browsers often offer to remember web passwords for you, and they store them safely in encrypted form on the disk. But none of the forms of client-side data storage described in this chapter involve encryption: anything you save resides on the user’s hard disk in unencrypted form. Stored data is therefore accessible to curious users who share access to the computer and to malicious software (such as spyware) that exists ...