1.4 What JavaScript Can't Do

JavaScript has an impressive list of capabilities. Note, however, that they are confined to browser-related and HTML-related tasks. Since JavaScript is used in a limited context, it does not have features that would be required for standalone languages:

• JavaScript does not have any graphics capabilities, except for the ability to format and display HTML (which, however, does include images, tables, frames, forms, fonts, and other user-interface elements).

• For security reasons, client-side JavaScript does not allow the reading or writing of files. Obviously, you wouldn't want to allow an untrusted program from any random web site to run on your computer and rearrange your files!

• JavaScript does not support networking of any kind, except--an important exception!--that it can cause a web browser to download the contents of arbitrary URLs.

• Finally, JavaScript doesn't have any multithreading capabilities, except whatever comes implicitly from the web browser's internal use of threads.