jQuery is a JavaScript library that simplified DOM manipulation, event handling, animations, and AJAX development. Although modern JavaScript APIs have reduced the need for jQuery in many new applications, it remains important when maintaining legacy applications, enterprise systems, and projects built with jQuery-based plugins.
For experienced developers, jQuery interviews often focus on DOM performance, event delegation, AJAX, asynchronous behavior, selectors, plugins, and integration with existing applications.
Below are the top 10 jQuery interview questions with answers and explanations.
1. What is jQuery and why was it widely used?
Answer:
jQuery is a JavaScript library designed to simplify common client-side development tasks.
It provides APIs for:
DOM manipulation
Event handling
AJAX requests
Animations
Element selection
Form handling
Plugin development
Example:
$('#username').val('John');
The equivalent modern JavaScript is:
document.getElementById('username').value = 'John';
Explanation:
jQuery became popular because it provided a consistent API and simplified browser compatibility issues that were particularly significant in older browsers.
Today, many tasks can be performed using native JavaScript APIs, but jQuery remains relevant in existing applications and ecosystems.
2. What is the difference between $(document).ready() and [removed]?
Answer:
$(document).ready() executes when the DOM has been loaded and is ready to be manipulated.
$(document).ready(function () {
console.log('DOM is ready');
});
[removed] waits until the entire page and its dependent resources, such as images, have loaded.
[removed] = function () {
console.log('Everything loaded');
};
Explanation:
For DOM manipulation, waiting for the complete loading of images and other resources is often unnecessary.
Therefore, $(document).ready() is commonly used when code only needs the DOM structure.
Modern JavaScript provides:
document.addEventListener('DOMContentLoaded', function () {
// DOM ready
});
3. What is event delegation in jQuery and why is it useful?
Answer:
Event delegation attaches an event handler to a parent element instead of attaching individual handlers to every child element.
Example:
$('#userList').on('click', '.delete-user', function () {
const id = $(this).data('id');
console.log(id);
});
Here, the handler is attached to #userList, not every .delete-user element.
Explanation:
Event delegation is particularly useful for dynamically generated elements.
For example, if AJAX adds new rows to a table:
User List
|
+-- Delete
+-- Delete
+-- Delete
A delegated event handler can handle existing and subsequently added .delete-user elements.
It can also reduce the number of event handlers attached to the page.
4. What is the difference between .on(), .click(), and .bind()?
Answer:
.on() is the preferred modern jQuery event API.
$('#save').on('click', function () {
console.log('Saved');
});
Older code may use:
$('#save').click(function () {
console.log('Saved');
});
And:
$('#save').bind('click', function () {
console.log('Saved');
});
Explanation:
.bind() is deprecated in modern jQuery versions.
.on() is preferred because it supports both direct event handling and event delegation.
For example:
$('#users').on('click', '.edit-user', function () {
// Works for dynamically added elements
});
5. How do you make an AJAX request using jQuery?
Answer:
jQuery provides $.ajax() for asynchronous HTTP requests.
Example:
$.ajax({
url: '/api/users',
method: 'GET',
dataType: 'json',
success: function (response) {
console.log(response);
},
error: function (xhr) {
console.error(xhr.status);
}
});
For POST:
$.ajax({
url: '/api/users',
method: 'POST',
data: {
name: 'John',
email: 'john@example.com'
},
success: function (response) {
console.log(response);
}
});
Explanation:
In production applications, AJAX code should also handle:
Loading states
Validation errors
Authentication failures
Network failures
Server errors
Timeouts
Response validation
Modern applications may instead use fetch() or libraries such as Axios.
6. What is the difference between .prop() and .attr()?
Answer:
.attr() works with HTML attributes, while .prop() is generally used for DOM properties.
For example:
$('#remember').attr('checked');
versus:
$('#remember').prop('checked');
For a checkbox's current state, .prop('checked') is appropriate.
if ($('#remember').prop('checked')) {
console.log('Remember me selected');
}
Explanation:
The HTML attribute represents markup, while the DOM property represents the current state of the element.
This distinction becomes important with dynamic form controls.
7. How can you improve jQuery performance?
Answer:
Several techniques can improve performance.
Cache frequently used selectors
Instead of:
$('#userTable').find('.user');
$('#userTable').find('.delete');
use:
const $table = $('#userTable');
$table.find('.user');
$table.find('.delete');
Avoid unnecessary DOM manipulation
Instead of repeatedly modifying the DOM inside a loop, construct the required content and update the DOM efficiently.
Use event delegation
For large or dynamic lists:
$('#userTable').on('click', '.delete', function () {
// ...
});
Avoid overly broad selectors
Prefer specific selectors where practical.
Explanation:
At senior level, performance optimization should be based on actual measurements. Excessive DOM operations, inefficient selectors, large event-handler counts, and unnecessary AJAX requests can all affect application performance.
8. What is $.each() and how is it different from JavaScript forEach()?
Answer:
$.each() is a jQuery iteration utility.
$.each(users, function (index, user) {
console.log(user.name);
});
Native JavaScript provides:
users.forEach(function (user) {
console.log(user.name);
});
Explanation:
For modern JavaScript arrays, forEach() is usually sufficient.
jQuery's $.each() can work with both arrays and objects, which was useful before modern JavaScript iteration APIs became widely available.
Example:
const user = {
name: 'John',
age: 30
};
$.each(user, function (key, value) {
console.log(key, value);
});
9. How do you create a jQuery plugin?
Answer:
A simple plugin can be created by extending $.fn.
Example:
$.fn.highlightText = function () {
return this.each(function () {
$(this).css('font-weight', 'bold');
});
};
Usage:
$('.important').highlightText();
Explanation:
Returning this allows the plugin to support jQuery's chaining behavior.
A production-quality plugin should also consider:
Configuration options
Default values
Multiple element handling
Namespacing
Cleanup
Event management
Avoiding global variables
10. How would you debug and maintain a large jQuery application?
Answer:
For a large legacy jQuery application, I would focus on understanding dependencies and gradually improving structure rather than rewriting everything immediately.
I would review:
1. Event handlers
Identify duplicated handlers and excessive global event listeners.
2. AJAX calls
Centralize common behavior such as:
Authentication handling
Error handling
Loading indicators
CSRF handling
Common response processing
3. DOM manipulation
Look for repeated or unnecessarily expensive DOM operations.
4. Plugin dependencies
Identify outdated or conflicting plugins.
5. Global variables
Reduce global state and isolate functionality into modules where practical.
6. Security
Check:
XSS risks
Unsafe HTML insertion
CSRF protection
Untrusted AJAX responses
Input/output handling
For example, avoid blindly inserting untrusted content using:
$('#message').html(userInput);
Prefer text insertion when HTML is not required:
$('#message').text(userInput);
7. Migration strategy
For a legacy application, I would gradually replace functionality with modern JavaScript where it provides a clear benefit, while maintaining compatibility with existing functionality.
Explanation:
The biggest challenge with large jQuery applications is often not writing jQuery code but managing complexity.
A senior developer should be able to identify technical debt, isolate modules, reduce coupling, improve security and performance, and migrate incrementally without unnecessarily breaking existing functionality.
Quick Revision
Before a jQuery interview, prepare these topics:
jQuery fundamentals
DOM manipulation
Selectors
Event handling
Event delegation
.on()vs.click()AJAX
Promises and asynchronous operations
.attr()vs.prop().val(),.text(),.html()DOM performance
$.each()jQuery plugins
Form handling
AJAX error handling
CSRF considerations
XSS prevention
jQuery and REST APIs
Legacy application maintenance
Migration from jQuery to modern JavaScript
Final Interview Tip
For an experienced developer, don't focus only on remembering jQuery methods.
Be prepared to explain why a particular approach was selected, how you handled dynamically generated elements, how you optimized DOM operations, how you handled AJAX failures, and how you maintained or migrated a large legacy jQuery application.
That demonstrates practical development experience rather than simply knowing jQuery syntax.
