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21.2. Restricted Features

As I've already mentioned, the first line of defense against malicious scripts in client-side JavaScript is that the language simply omits certain capabilities. The second line of defense is that JavaScript imposes restrictions on certain features that it does support. For example, client-side JavaScript supports a close( ) method for the Window object, but most (hopefully all) web-browser implementations restrict this method so that a script can close only a window that was opened by a script from the same web server. In particular, a script cannot close a window that the user opened; if it tries to do so, the user is presented with a confirmation box asking if he really wants to close the window.

The most important of these security restrictions is known as the same-origin policy and is described in the next section. The following is a list of the other security restrictions found in most implementations of client-side JavaScript. This is not a definitive list. Each browser may have a slightly different set of restrictions, and the proprietary features of each browser may well have proprietary security restrictions to go along with them.

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