jQuery Interview Questions: A Comprehensive Guide

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.

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. Why is this? 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 propagate 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 when talking about jQuery and its functions from now on.

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. First, 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.

Normally:

• 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. It’s important to remember to change the id of the clone before adding it to the DOM when you copy an element with an id attribute. 13 .

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

Consider the code snippet below. What’s 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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