JsAccount is a technology that allows message account types to be created in Mozilla Mailnews code using JavaScript. Although this is primarily targeted at allowing extensions to create new accounts, it might also be useful as a bridge to convert existing account types from being C++ based to JavaScript based.
In mailnews code, an account type is a set of classes that allow implementation of a messaging particular protocol. The account type is given a short string identifier ("imap", "news", "pop3") and is then used to create objects of the appropriate type by appending that string to the end of a base XPCOM contractID. So, for example, to create an imap server, you generate a contractID using a base ID, NS_MSGINCOMINGSERVER_CONTRACTID_PREFIX "@mozilla.org/messenger/server;1?type=", then append "imap" to get:
@mozilla.org/messenger/server;1?type=imap
In the C++ code, there is a base object implementing shared functionality. An account-specific class inherits that base functionality, then extends it to represent the account-specific behavior that is needed. This same basic concept is used to represent a whole series of classes that are necessary to implement a specific mailnews account type.
For the server example, there is a base class named nsMsgIncomingServer.cpp that implements that base interface nsIMsgIncomingServer.idl. For imap, there is a specific class nsImapIncomingServer.cpp that inherits from nsMsgIncomingServer.cpp, overrides some of the methods in nsIMsgIncomingServer.idl, and also implements an imap-specific interface nsIImapIncomingServer.idl. All of this works fine using C++ inheritance and polymorphism.
Although JsAccount is intended mostly for mailnews accounts, the same basic method of using a base class extended for specific types is also used in other ways in mailnews code, including for addressbook types and views. The technology may also be applied to those other object types as well.
The JavaScript class system works very differently than the C++ system, and you cannot use normal language constructs to override a C++ class with a JavaScript class. What JsAccount allows you to do is to create XPCOM objects in JavaScript, and use those objects to override or extend the methods from the C++ base class in a way that will function correctly whether those objects are executed from within C++ code or JavaScript code. This allows you to create a new account using JavaScript code, while using the same base class functionality that is used by the core C++ account types. Thus a new account type may be created in JavaScript-based extension. The technology may also be used to create JavaScript versions of existing account types in an incremental manner, slowly converting methods from C++ to JavaScript.