when for Parallel Calls

when for Parallel Calls: free step-by-step lesson with examples, common mistakes, and interview tips — part of jQuery Tutorial on Toolliyo Academy.

8 min read Updated 8/24/2026

On this page

jQuery Tutorial · Lesson 59 of 100

when for Parallel Calls

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

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

What is this?

$.when(a, b, ...) waits for multiple thenables. When all resolve, .done receives combined results; any reject trips .fail. Ideal for parallel GETs.

Why should you care?

QueryVerse headers often need user profile and notification count together before first paint.

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">Load user + todos in parallel</button>
  <pre id="out" class="box muted"></pre>
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<script>
$(function () {
  $('#go').on('click', function () {
    $('#out').text('Loading…');
    $.when(
      $.getJSON('https://jsonplaceholder.typicode.com/users/1'),
      $.getJSON('https://jsonplaceholder.typicode.com/todos?userId=1')
    ).done(function (userRes, todosRes) {
      // each res is [data, statusText, jqXHR]
      var user = userRes[0];
      var todos = todosRes[0];
      $('#out').text(user.name + ' has ' + todos.length + ' todos (sample)');
    }).fail(function () {
      $('#out').text('One or more calls failed');
    });
  });
});
</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">Load user + todos in parallel</button>
  <pre id="out" class="box muted"></pre>
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<script>
$(function () {
  $('#go').on('click', function () {
    $('#out').text('Loading…');
    $.when(
      $.getJSON('https://jsonplaceholder.typicode.com/users/1'),
      $.getJSON('https://jsonplaceholder.typicode.com/todos?userId=1')
    ).done(function (userRes, todosRes) {
      // each res is [data, statusText, jqXHR]
      var user = userRes[0];
      var todos = todosRes[0];
      $('#out').text(user.name + ' has ' + todos.length + ' todos (sample)');
    }).fail(function () {
      $('#out').text('One or more calls failed');
    });
  });
});
</script>
</body>
</html>

Result