Deferred objects simplify callback management in an asynchronous environment, mitigating the "Pyramid of Doom" that results from too many levels of nested callbacks. A deferred object exposes a promise as interface but internally keeps a system for changing the status of the promise.

Example libraries:

jQuery: http://api.jquery.com/jQuery.Deferred/
Node.js / Vanilla JS: https://github.com/kriskowal/q