Top jQuery Interview Questions and Answers

This list includes common jQuery interview questions that can help beginners, intermediate, and advanced developers prepare for job interviews. Each question is followed by a detailed explanation or example.

Basic Level Questions

1. What is jQuery? Why is it used?

Answer:
jQuery is a lightweight, fast, and feature-rich JavaScript library. It simplifies tasks like DOM manipulation, event handling, animations, and AJAX interactions, making JavaScript code more concise and easier to write.

Key Benefits:

• Simplifies JavaScript code.
• Cross-browser compatibility.
• Extensive plugin support.

2. How do you include jQuery in a web page?

Answer:
You can include jQuery by either downloading it or using a CDN (Content Delivery Network).
Example with a CDN:

<script src="https://code.jquery.com/jquery-3.6.4.min.js"></script>

3. What is the difference between $(this) and this?

Answer:

• this: Refers to the DOM element.
• $(this): Converts the DOM element into a jQuery object, enabling access to jQuery methods.

Example:

<button id="btn">Click Me</button>
<script>
    $("#btn").click(function() {
        console.log(this);       // Logs the DOM element
        console.log($(this));    // Logs the jQuery object
    });
</script>

4. What is the difference between $(document).ready() and window.onload?

Answer:

• $(document).ready(): Executes when the DOM is fully loaded, even if images or external resources are still loading.
• window.onload: Executes only after the entire page, including images and resources, is fully loaded.

5. How can you hide an HTML element in jQuery?

Answer:
You can use the .hide() method:

$("#myElement").hide();

Intermediate Level Questions

6. What is the difference between empty(), remove(), and detach() in jQuery?

Answer:

• empty(): Removes all child elements from the selected element but keeps the parent.
• remove(): Removes the element and its child elements from the DOM.
• detach(): Similar to remove() but retains the data and events for re-insertion.

Example:

<div id="parent">
    <p>Child 1</p>
    <p>Child 2</p>
</div>

<script>
    $("#parent").empty(); // Removes child elements but keeps <div id="parent">
    $("#parent").remove(); // Removes the entire <div id="parent">
    $("#parent").detach(); // Removes but keeps events and data
</script>

7. What are jQuery selectors?

Answer:
Selectors in jQuery are used to select and manipulate HTML elements. They are based on CSS selectors.
Example:

• $("#id"): Selects an element by ID.
• $(".class"): Selects elements by class.
• $("div"): Selects all <div> elements.

8. What is the difference between .bind(), .on(), and .off() in jQuery?

Answer:

• .bind(): Attaches an event handler to elements (deprecated in jQuery 3.x).
• .on(): A replacement for .bind(). It can delegate events to dynamically added elements.
• .off(): Removes event handlers attached with .on() or .bind().

Example:

$("#btn").on("click", function() {
    alert("Button clicked!");
});

9. What is the purpose of the jQuery.noConflict() method?

Answer:
jQuery.noConflict() is used to avoid conflicts with other JavaScript libraries using the $ variable.
Example:

jQuery.noConflict();
jQuery("#myElement").hide(); // Use `jQuery` instead of `$`

10. How do you perform an AJAX request in jQuery?

Answer: Use the $.ajax() method.

Example:

$.ajax({
    url: "example.php",
    method: "GET",
    success: function(response) {
        console.log(response);
    }
});

Advanced Level Questions

11. How can you optimize jQuery performance?

Answer:

• Use ID selectors instead of class selectors for better performance.
• Cache jQuery objects (e.g., var $element = $("#id");).
• Use event delegation with .on() for dynamic elements.
• Minimize DOM manipulations by batching changes.
• Use native JavaScript where possible for critical performance.

12. What are jQuery deferred objects?

Answer:
Deferred objects are used to handle asynchronous operations in jQuery. They work with promises.
Example:

var deferred = $.Deferred();

deferred.done(function() {
    console.log("Operation successful!");
}).fail(function() {
    console.log("Operation failed.");
});

// Simulate success
deferred.resolve();

13. What is the difference between .prop() and .attr()?

Answer:

• .prop(): Used to get or set DOM properties.
• .attr(): Used to get or set HTML attributes.

Example:

<input type="checkbox" id="chk" checked>
<script>
    console.log($("#chk").prop("checked")); // true
    console.log($("#chk").attr("checked")); // "checked"
</script>

14. How do you create a jQuery plugin?

Answer:

jQuery.fn.myPlugin = function() {
    this.css("color", "red");
    return this;
};

// Usage
$("p").myPlugin();

15. What is the use of jQuery.each()?

Answer:
$.each() is used to iterate over arrays or objects.
Example:

$.each([1, 2, 3], function(index, value) {
    console.log("Index:", index, "Value:", value);
});

Scenario-Based Questions

16. How would you delegate a click event to dynamically created elements?

Answer: Use .on() to bind events to parent elements and delegate them to child elements.

Example:

<div id="container"></div>
<script>
    $("#container").on("click", ".child", function() {
        alert("Child clicked!");
    });

    // Dynamically adding a child element
    $("#container").append('<div class="child">Click me</div>');
</script>

17. How would you stop an animation in jQuery?

Answer: Use the .stop() method.

$("#box").stop(true, true); // Stops the animation immediately

Key Tips for Interview Success

• Practice writing jQuery code for real-world scenarios.
• Be clear on differences between properties, methods, and events.
• Understand performance optimization techniques.
• Prepare for integration-related questions (e.g., using jQuery with AJAX or other libraries).