Dojo: The Definitive Guide

Deferreds

JavaScript doesn't currently support the concept of threads, but it does offer the ability to perform asynchronous requests via the XMLHttpRequest object and through delays with the setTimeout function. However, it doesn't take too many asynchronous calls running around before matters get awfully confusing. Base provides a class called Deferred to help manage the complexity often associated with the tedious implementation details of asynchronous events. Like other abstractions, Deferred s allow you to hide away tricky logic and/or boilerplate into a nice, consistent interface.

If the value of a Deferred was described in one sentence, however, it would probably be that it enables you to treat all network I/O uniformly regardless of whether it is synchronous or asynchronous. Even if a Deferred is in flight, has failed, or finished successfully, the process for chaining callbacks and errbacks is the exact same. As you can imagine, this behavior significantly simplifies bookkeeping.

Tip

Dojo's implementation of a Deferred is minimally adapted from MochiKit's implementation, which in turn is inspired from Twisted's implementation of the same. Some good background on MochiKit's implementation is available at http://www.mochikit.com/doc/html/MochiKit/Async.html#fn-deferred. Twisted's implementation of Deferred s is available at http://twistedmatrix.com/projects/core/documentation/howto/defer.html.

Some key features of Deferred s are that they allow you to chain together multiple ...