Callbacks and Deferred Objects

$.Callbacks(flags) is a list of functions fired together, the engine behind jQuery 561 events and Deferreds. Flags combine: once, memory (late additions receive the last fired arguments), unique (no duplicates) and stopOnFalse. Lists offer add(), remove(), fire(), fireWith(), disable(), lock() and fired().

$.Deferred() is jQuery's pre-ES2015 promise (Promises and Asynchrony). The owner calls resolve(), reject() or notify(); consumers attach done(), fail(), always(), progress() or then(). Hand out d.promise(), which cannot change state. $.when() works like Promise.all(), and $.ajax() returns a Deferred.

Memory callbacks, a Deferred with progress, $.when() and awaitHTMLLive

<script src="https://code.jquery.com/jquery-4.0.0.min.js"></script>
<pre id="log" style="margin:0"></pre>
<script>
  const log = (msg) => $('#log').append(`${msg}\n`);
  $.Callbacks('memory').fire('Ann').add((who) => log(`late listener got ${who}`));
  const d = $.Deferred();
  d.progress((pct) => log(`progress ${pct}%`)).done((v) => log(`done(): ${v}`));
  d.then((v) => log(`then(): ${v}, asynchronously`));
  d.notify(50).resolve('data').reject('ignored');
  $.when(d, 2).done((a, b) => log(`state ${d.state()}, $.when(): ${a} and ${b}`));
  (async () => log(`await: ${await d.promise()}`))();
</script>

Browser output of Listing 8.66 — Browser output of 66

done() runs synchronously, but since jQuery 3 then() follows Promises/A+: it runs asynchronously, returns a new promise and turns thrown errors into rejections. A Deferred is a thenable, so await accepts it. A settled Deferred ignores later resolve() and reject() calls. The slim build omits both modules; use native Promises.

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Callbacks and Deferred Objects