All jQuery Interview Questions

What is the difference between find() and children() in jQuery?

<div id="d"><ul><li>A</li><li>B</li></ul></div>

console.log($('#d').children().length);
console.log($('#d').find('li').length);

See Traversing: Descendants.

What is the difference between parent(), parents() and closest() in jQuery?

<div class="box"><ul><li><span id="s">x</span></li></ul></div>

console.log($('#s').parent().prop('tagName'));
console.log($('#s').parents().length);
console.log($('#s').closest('.box').prop('tagName'));

See Traversing: Ancestors.

What is the difference between append(), prepend(), after() and before()?

<ul id="l"><li>Middle</li></ul>

$('#l').append('<li>End</li>').prepend('<li>Start</li>');
$('#l').after('<p>After</p>').before('<p>Before</p>');

See Add Elements.

How do you make an Ajax call in jQuery?

{ "status": "ok", "items": 3 }

<p>Page</p>

$.ajax({ url: 'data.json', dataType: 'json' })
  .done(function (res) { console.log('Status: ' + res.status + ', items: ' + res.items); })
  .fail(function () { console.log('Request failed'); });

See jQuery Ajax.

What is the difference between $.get(), $.post() and $.ajax() in jQuery?

See jQuery Ajax.

How do you use the animate() method in jQuery?

<div id="box" style="width:100px;">Box</div>

$('#box').animate({ width: '+=100px', opacity: 0.5 }, 500, function () {
  console.log('Done: ' + $(this).css('width'));
});

Only numeric properties can be animated (colors need a plugin). See jQuery animate().

How do you check if an element exists in jQuery?

<p id="a">A</p>

console.log($('#a').length > 0);
console.log($('#missing').length > 0);
if ($('#missing').length) { console.log('found'); } else { console.log('not found'); }

A selector which finds nothing returns an empty jQuery object, and the methods then do nothing. See jQuery Selectors.

What is the difference between this and $(this) in jQuery?

<button id="b" data-name="Save">Click</button>

$('#b').on('click', function () {
  console.log(this.id);                 // DOM property
  console.log($(this).data('name'));    // jQuery method
  console.log(this === $(this)[0]);
});
$('#b').click();

How do you serialize a form in jQuery?

<form id="f"><input name="a" value="1"><input name="b" value="x y"></form>

console.log($('#f').serialize());
console.log($('#f').serializeArray());

See jQuery Forms.

What is jQuery UI and how is it different from jQuery?

jQuery is the core library for the DOM, events, effects and Ajax. jQuery UI is an extra library which needs jQuery, and gives user interface parts: datepicker, dialog, accordion, tabs, sortable and draggable elements, and more effects and animations of colors. You load jQuery first, then jQuery UI. For new projects, many of these widgets are made today with plain HTML, CSS and JavaScript, or with other libraries.

What is the difference between jquery.js, jquery.min.js and the slim build?

If you need $.ajax() or .fadeIn(), do not use the slim file.

Is jQuery still relevant and should you learn it?

Yes, for a few reasons: a lot of existing websites, WordPress themes and plugins, and admin panels use jQuery, and a version 4.0 was released in January 2026, so it is still maintained. If you will work on real projects, you will meet it.

But for a new project, modern JavaScript can do most of the work, and frameworks like React and Vue are used for big applications. A good plan: learn JavaScript first, then learn jQuery to read and fix the existing code. See jQuery vs Vanilla JavaScript.

How do you check the jQuery version?

<p>Page</p>

console.log($.fn.jquery);

Run it in the console of your own page. The output above is from jQuery 3.7.1.

What is event delegation in jQuery and why is it useful?

<ul id="list"><li>A</li></ul>

$('#list').on('click', 'li', function () { console.log('clicked ' + $(this).text()); });
$('#list').append('<li>B</li>');     // added later
$('#list li').eq(1).click();

It saves memory (one handler) and works for dynamic content. See Event Delegation.

What is the difference between bind(), live(), delegate() and on() in jQuery?

<p>Page</p>

console.log('on: ' + typeof $.fn.on + ', live: ' + typeof $.fn.live);

What is a callback function in jQuery and why is it needed in effects?

<div id="b">Box</div>

$('#b').hide(300, function () {
  console.log('2. callback: effect finished');
});
console.log('1. this runs immediately after starting the effect');

See Method Chaining and Callbacks.

What is the difference between stop(), finish() and delay() in jQuery?

Use .stop(true) before a hover animation, so quick hovering does not pile up animations. See jQuery animate().

What are Deferred objects and promises in jQuery?

{ "n": 1 }

{ "n": 2 }

<p>Page</p>

$.when($.getJSON('a.json'), $.getJSON('b.json')).done(function (a, b) {
  console.log('Sum: ' + (a[0].n + b[0].n));
});

See Deferred and Promises.

How do you handle Ajax errors in jQuery?

<p>Page</p>

$.getJSON('missing.json')
  .done(function () { console.log('ok'); })
  .fail(function (xhr, status, error) {
    console.log('Failed: ' + xhr.status + ' ' + error);
  })
  .always(function () { console.log('Request finished'); });

Also set a timeout, show a message to the user and log the details. See jQuery Ajax.

How do you create a jQuery plugin?

<p class="a">One</p><p class="a">Two</p>

$.fn.markAs = function (options) {
  var settings = $.extend({ label: 'done' }, options);
  return this.each(function () {
    $(this).addClass(settings.label);
  });
};

$('.a').markAs({ label: 'checked' });

See jQuery Plugins.