Deferred Basics

Deferred Basics: free step-by-step lesson with examples, common mistakes, and interview tips — part of jQuery Tutorial on Toolliyo Academy.

8 min read Updated 8/23/2026

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jQuery Tutorial · Lesson 58 of 100

Deferred Basics

Setup & DOM ✓ → Events Effects AJAX ✓ → Perf & Integrate → Ship & Projects

Perf & Integrate · 3 — Harden · ~10 min · Traversal and Plugins

What is this?

$.Deferred creates a controllable thenable: resolve, reject, and notify. jqXHR already implements this pattern — learning Deferred helps you compose non-XHR async work with the same .done/.fail style.

Why should you care?

QueryVerse sometimes waits on both an animation and a token fetch before enabling Save.

See it live — copy this example

Examples include the jQuery 3.7 CDN. Paste into an HTML file or use Run Example to preview.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>QueryVerse</title>
  <style>
    body { font-family: system-ui, sans-serif; margin: 1.25rem; }
    .box { padding: .75rem; border: 1px solid #ccc; border-radius: 8px; margin: .5rem 0; }
    .muted { color: #666; }
    button { margin-right: .35rem; margin-top: .35rem; }
    input, select, textarea { margin: .25rem 0; }
    table { border-collapse: collapse; width: 100%; }
    th, td { border: 1px solid #ddd; padding: .4rem .55rem; text-align: left; }
    .hidden { display: none; }
    .row-odd { background: #f7f7f7; }
    .active { font-weight: 700; }
    .toast { background: #111; color: #fff; padding: .5rem .75rem; border-radius: 6px; }
  </style>
</head>
<body>
  <button type="button" id="go">Run deferred</button>
  <p id="out" class="muted"></p>
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<script>
$(function () {
  function wait(ms) {
    var d = $.Deferred();
    setTimeout(function () { d.resolve(ms); }, ms);
    return d.promise();
  }
  $('#go').on('click', function () {
    $('#out').text('waiting…');
    wait(300).done(function (ms) {
      $('#out').text('resolved after ' + ms + 'ms');
    }).fail(function () {
      $('#out').text('rejected');
    });
  });
});
</script>
</body>
</html>

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>QueryVerse</title>
  <style>
    body { font-family: system-ui, sans-serif; margin: 1.25rem; }
    .box { padding: .75rem; border: 1px solid #ccc; border-radius: 8px; margin: .5rem 0; }
    .muted { color: #666; }
    button { margin-right: .35rem; margin-top: .35rem; }
    input, select, textarea { margin: .25rem 0; }
    table { border-collapse: collapse; width: 100%; }
    th, td { border: 1px solid #ddd; padding: .4rem .55rem; text-align: left; }
    .hidden { display: none; }
    .row-odd { background: #f7f7f7; }
    .active { font-weight: 700; }
    .toast { background: #111; color: #fff; padding: .5rem .75rem; border-radius: 6px; }
  </style>
</head>
<body>
  <button type="button" id="go">Run deferred</button>
  <p id="out" class="muted"></p>
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<script>
$(function () {
  function wait(ms) {
    var d = $.Deferred();
    setTimeout(function () { d.resolve(ms); }, ms);
    return d.promise();
  }
  $('#go').on('click', function () {
    $('#out').text('waiting…');
    wait(300).done(function (ms) {
      $('#out').text('resolved after ' + ms + 'ms');
    }).fail(function () {
      $('#out').text('rejected');
    });
  });
});
</script>
</body>
</html>

Result

What happened?

Return d.promise() so callers cannot resolve your Deferred.
Prefer native Promise in new greenfield code, but Deferred appears in legacy your page heavily.