Top JQuery UI Interview Questions for Web Developers

JQuery UI is an open-source widget and interaction library built on top of JQuery. It provides powerful UI interactions, effects, widgets, and themes that can be leveraged by web developers to create feature-rich web applications. As JQuery UI gains mainstream adoption, knowledge of it is becoming an increasingly sought-after skillset for front-end developers. This article will explore some of the most common JQuery UI interview questions that hiring managers frequently ask candidates.

Basics of JQuery UI

Before diving into the interview questions let’s do a quick overview of JQuery UI

  • JQuery UI extends JQuery by providing commonly-needed UI interactions and widgets as plugins For example, it includes plugins for drag and drop, resizing, sorting, autocompleting, tabs, and more

  • It uses a theming system which allows the look and feel to be customized to match a site’s visual branding.

  • The widgets and interactions leverage CSS classes, rather than images for rendering effects. This keeps file sizes small.

  • JQuery UI includes a range of pre-built themes that can be utilized out of the box or customized to create new themes.

  • The library is open source and modular. Developers can utilize the entire library or just include the specific components needed.

Now that we’ve covered the basics, let’s look at some interview questions that are commonly asked about JQuery UI.

Common JQuery UI Interview Questions

Here are some of the top technical JQuery UI questions frequently asked in developer interviews:

Q1. What are some of the key widgets provided by JQuery UI?

Some of the most commonly used JQuery UI widgets include:

  • Accordion – Displays collapsible content panels.

  • Autocomplete – Provides suggestions while typing into a text field.

  • Datepicker – Allows selection of a date from a popup calendar.

  • Dialog – Displays content in a modal dialog box.

  • Progressbar – Displays the progress of a task.

  • Slider – Allows values to be selected from a range via a draggable handle.

  • Tabs – Allows tabbed navigation between content panels.

Q2. How can JQuery UI components be themed?

All JQuery UI components can be themed using the built-in ThemeRoller tool. ThemeRoller provides a GUI interface that allows tweaking theme settings like font family, colors, border radius, and more.

Custom themes can then be downloaded as a ZIP package. The theme can be applied by referencing the relevant CSS file(s). Themeroller also generates sample HTML code showing how to apply the theme classes.

Q3. Explain how JQuery UI’s draggable plugin works.

The draggable plugin allows DOM elements to be draggable by mouse. Here’s how it works:

  • The $(...).draggable() method is called on the element(s) to be made draggable.

  • CSS is used to change the cursor to indicate draggability.

  • During drag, the element follows the mouse movement.

  • drag and stop events are fired to provide hooks for custom behavior.

  • Options like axis constraints, handlers, cursor style, etc. can customize drag behavior.

Q4. How can custom draggable behavior be implemented?

Some ways to customize draggable behavior include:

  • Set axis option to "x" or "y" to constrain dragging to an axis.

  • Set containment option to a parent element to constrain dragging within it.

  • Use start and drag callbacks to implement custom logic during dragging.

  • Return false from start callback to prevent dragging.

  • Use revert option to animate element back to initial position on drag stop.

  • Use cursorAt option to customize the mouse cursor position.

Q5. What are some key benefits of using JQuery UI?

Some key benefits of using JQuery UI include:

  • Saves development time – UI interactions that would take time to build from scratch are available out of the box.

  • Cross-browser support – Supports all major browsers like Chrome, Firefox, Safari, Opera, and IE.

  • Themeable – Customizable look and feel that matches brand styling.

  • Lightweight – Compact file size and modular inclusion of components.

  • Open source – Free to use and extend.

  • Customizable – Interactions and widgets can be customized via options and callbacks.

  • Responsive – Components work well on mobile/tablet screens.

Q6. How can custom JQuery UI widgets be created?

JQuery UI provides a widget factory that can be used to build custom widgets by extending existing widgets. Here is the typical workflow:

  • A plugin is defined via $.widget() by specifying a name and base widget to extend.

  • _create() and _init() methods can initialize the widget DOM contents.

  • Additional methods can be defined to handle interactions.

  • Events can be bound to DOM elements to trigger methods.

  • The widget is used by calling $(...).pluginName().

Q7. How can JQuery UI and other JS libraries be used together?

If another library uses $ as the main object, it will conflict with JQuery’s usage. To resolve this:

  1. Before JQuery script tag, store JQuery in a variable like jq = $.noConflict();

  2. Use jq instead of $ for JQuery objects.

  3. Other libraries can continue using $ without conflict.

Q8. What is the difference between hide() and remove() methods in JQuery UI?

  • hide() hides the matched element by setting display: none style. The element still exists in the DOM.

  • remove() deletes the element from the DOM. Any attached data and events are deleted as well.

Q9. What are some advantages of JQuery UI over plain JavaScript?

Some advantages of using JQuery UI vs plain JavaScript include:

  • Large library of pre-built widgets and interactions.

  • Themeable look and feel.

  • Concise syntax and chaining.

  • Consistent cross-browser behavior.

  • Simpler event handling.

  • Easier DOM manipulation and Ajax.

  • Improved code organization via widgets.

  • Avoid having to write common utilities from scratch.

So in essence, JQuery UI provides substantial advantages by handling common complexities on behalf of the developer.

Q10. How can JQuery UI elements be dynamically created?

To create JQuery UI elements dynamically:

  1. Start by generating the raw HTML markup first.

  2. Insert this HTML into the DOM (e.g. via .append()).

  3. Call the JQuery UI interaction on the inserted markup.

For example:


// Generate HTMLvar tabHtml = '<div id="tab1">Tab 1 Content</div>'; // Insert HTML $(".tabs").append(tabHtml);// Make tab$("#tab1").tabs(); 

Final Thoughts

JQuery UI is a highly useful library for front-end developers looking to create interactive UIs. Mastering popular JQuery UI widgets like accordions, tabs, modals, sliders, and more can greatly boost your skillset and value as a developer.

Preparing answers for the key JQuery UI interview questions above will help demonstrate your knowledge to potential employers and boost your chances of being hired! The time invested in learning this library will pay off by making you more effective at bringing interactive interfaces to life.

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Questions and answers sent in will be looked over and edited by Toptal, LLC, and may or may not be posted, at their sole discretion.

Toptal sourced essential questions that the best jQuery developers and engineers can answer. Driven from our community, we encourage experts to submit questions and offer feedback.

jquery ui interview questions

Explain what the following code will do:

This code runs a query to get any element whose ID comes first, along with all elements whose class comes first and all elements that are children of the element and have a name attribute that ends in “first.” This is an example of using multiple selectors at once. The function will return a jQuery object containing the results of the query. 2 .

These lines of code are for an app that needs to set a click handler for all buttons on the page, even ones that are added dynamically later.

What’s wrong with this code? How can it be fixed so that it works correctly even if buttons are added later on?

The button that is added dynamically after the call to bind() will not have the click handler attached. This is because the bind() method only connects handlers to elements that are already there when it is called.

This issue can be fixed with functions that use “event bubbling” to match events on elements from the present and the future. In the past, this was done by replacing bind() with live(). live() was deprecated in jQuery 1. 7 though. delegate() is like live(), but it lets you choose how far an event must travel through the DOM.

The best way to do it is to use on(), which, depending on its syntax, can act like bind(), live(), or delegate(). The following code attaches the handler to all current and future buttons:

What selector would I use to find all elements whose IDs end in a certain string? How would I change the selector to only find elements whose IDs end in the same string?

Let’s say you want to retrieve all elements whose IDs end with “txtTitle”. This could be done using the following selector:

To retrieve only

elements whose IDs end with “txtTitle”, the selector would be:

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What’s the deal with the $ in jQuery? What is it and what does it mean?

Also, how can you use jQuery with another JavaScript library that names things with $? You’ll get extra points if you can give two answers.

Since $ has no special meaning in JavaScript, it is free to be used in object naming. In jQuery, it is simply used as an alias for the jQuery object and jQuery() function.

However, jQuery is not the only one that can use $. There may be times when you want to use it with another JS library that also uses $, which would cause a confusing name issue. jQuery provides the jQuery. noConflict() method for just this reason. After calling this method, you have to use the underlying name jQuery whenever you talk about jQuery and its functions.

Here’s an example from the jQuery documentation:

Alternatively, you can also use a closure instead of the $.noConflict() method; e.g.:

Given the following HTML:

and the following CSS:

Write code in jQuery to animate the #expander div, expanding it from 100 x 100 pixels to 200 x 200 pixels over the course of three seconds.

This could be done in jQuery as follows:

What is method chaining in jQuery? Provide an example.

What advantages does it offer?

It is possible to use method chaining in jQuery to run multiple methods on a jQuery selection in a single code statement. For example, the following snippets of code are equivalent:

Without chaining:

With chaining:

If you don’t use chaining, jQuery has to search the whole DOM to find the button before each method is applied. With chaining, the button only needs to be selected once. Thus, in addition to yielding more concise code, method chaining in jQuery offers a potentially powerful performance advantage.

Note: To be exact, it should be said that jQuery method chaining is not the only way to avoid searching the whole DOM over and over again. One could also set a variable equal to the initial DOM search results (i. e. , in the above example, one could set a variable equal to $( “button#play-movie” and then call the on(), css(), and show() methods on that variable). However, chaining is still the shorter and faster choice, and it doesn’t need to store the result in a local variable. 7 .

Explain what the following code does:

This code uses method chaining to accomplish a couple of things. First, it selects all the elements and changes their CSS width to 300px. In the next step, it adds all the elements to the selection. This lets it change the background color of both the and elements to blue. 8 .

What is the difference between jQuery.get() and jQuery.ajax()?

jQuery. ajax() is the all-encompassing Ajax request method provided by jQuery. It lets you make very specific Ajax requests, with choices for how long to wait for a response, what to do if the request fails, whether it blocks (synchronous) or doesn’t block (asynchronous), what format to ask for the response, and a lot more.

jQuery. get() is a shortcut method that uses jQuery. ajax() under the hood, to create an Ajax request that is typical for simple retrieval of information. Other pre-built Ajax requests are provided by jQuery, such as jQuery. post(), jQuery. getScript(), and jQuery. getJSON(). 9 .

Which of the two lines of code below is more efficient? Explain your answer.

The first line of code, which is pure JavaScript without jQuery, is more efficient and faster. Executing the second line of code, which is jQuery, will trigger a call to the JavaScript version.

This scripting language, JavaScript, is used by jQuery to make working with the document tree (DOM), though it slows things down a bit. It is a good idea to remember that jQuery is not always better than plain old JavaScript. Always consider whether using jQuery really provides a useful advantage for your project. 10 .

Explain and contrast the usage of event.preventDefault() and event.stopPropagation(). Provide an example.

event. stopPropagation() stops an event from bubbling up the event chain, whereas event. The browser’s default action on that event is stopped by preventDefault(), but the event still moves up the event chain.

For example, consider the following code snippet:

When the button is clicked, stopPropagation() is called. This means that the event never gets sent to the div, so its click handler never fires. It effectively stops parent elements from knowing about an event on their children.

If you changed the above call to stopPropagation() to a call to preventDefault(), the div’s click handler would still fire, but the browser’s default action would not happen.

The stopPropagation() and preventDefault() methods are mostly used in jQuery event handling, but they can also be used in plain JavaScript. ) 11 .

If (a) a jQuery event handler, (b) a regular JavaScript onclick event handler for an anchor tag, and (c) a regular JavaScript onclick event handler for a non-anchor tag (e) all return false, what does that mean? g. , a div, button, etc. )?.

(a) Returning false from a jQuery event handler is effectively the same as calling both preventDefault() and stopPropagation() on the passed jQuery event object.

b) If the regular JavaScript onclick event handler for an anchor tag returns false, the browser can’t go to the link address, and the event doesn’t spread through the DOM.

(c) Returning false from a regular JavaScript onclick event handler for a non-anchor tag (e. g. , a div, button, etc. ) has absolutely no effect. 12 .

jQuery provides a useful .clone() method to create a deep copy of matching elements.

Answer the following questions:

  • What is meant by a “deep copy”?
  • When a copy is made, what is usually left out? How can some of this behavior be controlled?
  • What is a potential “gotcha” when using the . clone() method? (HINT: Name an attribute of an element that you do not want to copy.)

The . The clone() method makes a deep copy of the set of matched elements. This means that it copies not only the matched elements but also their child elements and text nodes.


  • Arrays and objects in element data are not copied; the original element and the cloned element will still share them. You have to copy each file “by hand” if you want to deep copy all of them.
  • Any event handlers that are attached to the original element are not copied to the copy.

It’s possible to make copies of all the event handlers that are bound to the new copy of the element by setting the withDataAndEvents parameter to true.

As of jQuery 1. 4, all element data (attached by the . data() method) is also copied to the new copy.

As of jQuery 1. 5, deepWithDataAndEvents can be added to withDataAndEvents if you want to copy the events and data for all children of the cloned element.

Using . Clone() can create elements with duplicate id attributes, which are supposed to be unique. This could be a problem. When cloning an element with an id attribute, it’s important to remember to change the clone’s id before adding it to the DOM. 13 .

Explain the .promise() method in jQuery, including how and why it would be used.

Consider the code snippet below. What is the difference between the start and end times shown if there are 5 things on the page?

The .promise() method returns a dynamically generated Promise that is resolved once all actions of a certain type bound to the collection, queued or not, have ended.

It takes two optional arguments:

  • type: The type that is returned by default is “fx,” which means that the promise is resolved when all of the animations for the selected elements are done.
  • target – If a target object is specified, . promise() will connect to it and return this object instead of making a new one.

In the code sample provided, the difference between the start and end times displayed will be 10 seconds. This is because . In this case, promise() will wait for all animations (all fadeOut() calls) to finish. The last one will finish 10 seconds after the first one. e. , 5 * 2 seconds) after the animation starts. 14 .

What is the right way to get rid of an element from the DOM in jQuery before its Promise is over?

A returned Promise in jQuery is linked to a Deferred object stored on the .data() for an element. Since the .remove() method removes the element’s data as well as the element itself, it will prevent any of the element’s unresolved Promises from resolving.

Therefore, if it is necessary to remove an element from the DOM before its Promise is resolved, use .detach() instead and follow with .removeData() after resolution. 15 .

Explain the difference between the .detach() and .remove() methods in jQuery.

The .detach() and .remove() methods are the same, except that .detach() retains all jQuery data associated with the removed elements and .remove() does not. .detach() is therefore useful when removed elements may need to be reinserted into the DOM at a later time. 16 .

What’s the difference between document.ready() and window.onload()?

The document. ready() event occurs when all HTML documents have been loaded, but window. onload() occurs when all content (including s) has been loaded. So generally the document. ready() event fires first. 17 .

What’s the difference between prop() and attr()?

You can use both prop() and attr() to get or set the value of a property of an element attribute. However, attr() returns the property’s default value, while prop() returns the property’s current value.

There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every good candidate for the job will be able to answer all of them, and answering all of them doesn’t mean they are a good candidate. At the end of the day, hiring remains an art, a science — and a lot of work.

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What is the difference between JSON and jQuery?

JavaScript is a programming language that is used to create interactive and dynamic web pages, jQuery is a JavaScript library that simplifies the process of working with JavaScript, and JSON is a lightweight data-interchange format that is used to transmit data between web applications.

What is the primary purpose of jQuery?

The purpose of jQuery is to make it much easier to use JavaScript on your website. jQuery takes a lot of common tasks that require many lines of JavaScript code to accomplish, and wraps them into methods that you can call with a single line of code.

Does jQuery HTML work for both HTML and XML documents?

Does jQuery work for both HTML and XML documents? Yes, jQuery can work with both HTML and XML documents.

Is it possible to use jQuery together with Ajax?

jQuery provides several methods for AJAX functionality. With the jQuery AJAX methods, you can request text, HTML, XML, or JSON from a remote server using both HTTP Get and HTTP Post – And you can load the external data directly into the selected HTML elements of your web page!

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