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What is a jQuery Deferred, and how does it relate to Promises?

jQuery AJAX Intermediate 2–3 Yrs3–5 Yrs Conceptual 1 min read

Quick Answer

A jQuery.Deferred is jQuery's promise implementation — you resolve()/reject() it and consumers attach .done()/.fail()/.then(). $.ajax() returns one. $.when(a, b) is jQuery's Promise.all. jQuery 3 made Deferreds Promises/A+ compliant so they interoperate with native promises and async/await.

A jQuery Deferred is jQuery's own version of a promise, from before promises were standardised. You create one, resolve or reject it, and callers hook on with .done, .fail or .then. $.ajax returns a Deferred, and $.when is the equivalent of Promise.all. jQuery 3 made them compliant with the Promises/A+ spec so they play nicely with native promises and you can even await them. In new code I'd just use native promises and fetch.

  • Pre-jQuery 3, Deferred .then didn't behave to spec (swallowed exceptions, wrong resolution semantics) — jQuery 3 fixed this.
  • In new code, use native Promise / fetch; Deferreds only matter when maintaining older jQuery code.
  • You can await a jQuery Deferred in modern browsers because it's thenable.
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