Coding Interview QuestionsInterview Questions and Answers

Latest 60 jQuery Interview Questions

Table of Contents

Introduction

jQuery is a popular JavaScript library used to simplify and enhance web development. When preparing for a jQuery interview, it’s essential to have a good understanding of its core concepts. Some common interview questions include: “What is jQuery?”, “What are the advantages of using jQuery?”, “How do you select elements using jQuery?”, “What is the difference between .find() and .children() methods?”, and “How can you handle events in jQuery?”. Demonstrating knowledge of jQuery’s features, syntax, and functionality will help you showcase your expertise and readiness to develop interactive and dynamic web applications.

Basic Questions

1. What is jQuery and why is it used?

jQuery is a popular and lightweight JavaScript library that simplifies DOM manipulation, event handling, animation, and AJAX interactions for web development. It allows developers to write less code and achieve more in a shorter amount of time.

Example of using jQuery to change the text content of an element:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv">Hello, world!</div>

  <script>
    // Using jQuery to change the content of the element with id "myDiv"
    $(document).ready(function() {
      $("#myDiv").text("Hello, jQuery!");
    });
  </script>
</body>
</html>

2. What are the benefits of using jQuery compared to JavaScript?

jQueryJavaScript
Simplifies DOM manipulationNative DOM manipulation can be verbose
Cross-browser compatibilityInconsistencies across browsers
Easy event handlingTedious event registration
AJAX abstractionMore boilerplate for AJAX requests
Animated effects and transitionsComplex animations require more code

3. How can you include jQuery in your project?

You can include jQuery in your project by adding the script tag pointing to the jQuery library in the head or body section of your HTML file.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Include Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <!-- Your HTML content here -->

  <!-- jQuery code can be added here -->
</body>
</html>

4. How do you select elements using jQuery?

You can select elements using jQuery with CSS-style selectors.

Example of selecting all paragraphs and changing their text color:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Element Selection</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <p>Paragraph 1</p>
  <p>Paragraph 2</p>
  <p>Paragraph 3</p>

  <script>
    // Select all paragraphs and change their text color
    $(document).ready(function() {
      $("p").css("color", "blue");
    });
  </script>
</body>
</html>

5. What is a jQuery selector? Can you give some examples?

A jQuery selector is a function that allows you to select and manipulate DOM elements based on CSS-style selectors.

Example of using different jQuery selectors:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Selector Examples</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <p class="highlight">Paragraph 1</p>
  <p>Paragraph 2</p>
  <p id="special">Paragraph 3</p>

  <script>
    // Selecting elements using different selectors
    $(document).ready(function() {
      // Select by tag name
      $("p").css("font-weight", "bold");

      // Select by class
      $(".highlight").css("color", "red");

      // Select by ID
      $("#special").css("background-color", "yellow");
    });
  </script>
</body>
</html>

6. What is the difference between $(this) and ‘this’ in jQuery?

$(this)‘this’
jQuery objectRegular JavaScript DOM element
Used inEvent handlers, callbacks
Access jQuery methods and propertiesAccess DOM properties and methods

7. How can you hide or show elements in jQuery?

You can use the hide() and show() functions in jQuery to hide or show elements.

Example of hiding/showing an element on a button click:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Hide/Show Elements with jQuery</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <button id="toggleBtn">Toggle Element</button>
  <p>This is a paragraph.</p>

  <script>
    $(document).ready(function() {
      // Hide or show the paragraph on button click
      $("#toggleBtn").click(function() {
        $("p").toggle(); // You can also use hide() or show() individually
      });
    });
  </script>
</body>
</html>

8. What are jQuery Events? Can you list some common events?

jQuery events are actions that occur on web elements, such as clicks, keypresses, or mouse movements. They allow you to perform actions in response to user interactions.

Common jQuery events:

  • click
  • mouseenter
  • mouseleave
  • keypress
  • submit

Example of handling a click event:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Event Handling</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <button id="myBtn">Click Me</button>

  <script>
    $(document).ready(function() {
      // Click event handler for the button
      $("#myBtn").click(function() {
        alert("Button clicked!");
      });
    });
  </script>
</body>
</html>

9. What are the differences between $(document).ready() and $(window).load()?

$(document).ready()$(window).load()
Fires when DOM is readyFires when all assets are loaded
Preferred for DOM manipulationUseful for working with images,
Multiple events can be registeredOnly one event can be registered

10. How can you set or get data from HTML elements in jQuery?

You can use the data() method to set or get data from HTML elements in jQuery. The data() method allows you to store arbitrary data associated with the selected elements.

Example of setting and getting data using jQuery:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Data Example</title>
  <script src="https://code.jquery.com/jquery-3

.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv">This is a div.</div>

  <script>
    $(document).ready(function() {
      // Set data attribute
      $("#myDiv").data("customData", "Hello, jQuery!");

      // Get data attribute
      var customDataValue = $("#myDiv").data("customData");
      console.log(customDataValue); // Output: Hello, jQuery!
    });
  </script>
</body>
</html>

11. What are jQuery methods? Can you list some common methods?

jQuery methods are functions provided by the jQuery library that allow you to perform various operations on selected elements.

Common jQuery methods:

  • addClass(): Add one or more classes to elements.
  • removeClass(): Remove one or more classes from elements.
  • attr(): Get or set attributes of elements.
  • val(): Get or set the value of form elements.
  • html(): Get or set the HTML content of elements.

Example of using the addClass() method:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Methods Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <p>This is a paragraph.</p>

  <script>
    $(document).ready(function() {
      // Add a class to the paragraph
      $("p").addClass("highlight");
    });
  </script>
</body>
</html>

12. What are jQuery effects? Can you list some common effects?

jQuery effects are animations and visual changes that can be applied to elements, such as fading, sliding, or toggling visibility.

Common jQuery effects:

  • fadeIn(): Fade in an element.
  • fadeOut(): Fade out an element.
  • slideUp(): Slide up an element to hide it.
  • slideDown(): Slide down an element to show it.
  • toggle(): Toggle the visibility of an element.

Example of using the fadeIn() effect:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Effects Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv" style="display: none;">This is a hidden div.</div>
  <button id="showBtn">Show Div</button>

  <script>
    $(document).ready(function() {
      // Show the hidden div with fadeIn effect on button click
      $("#showBtn").click(function() {
        $("#myDiv").fadeIn();
      });
    });
  </script>
</body>
</html>

13. What does the jQuery .ajax() function do?

The jQuery .ajax() function is used for making asynchronous HTTP requests to a server. It allows you to perform various types of requests (GET, POST, etc.) and handle the server’s response.

Example of using the .ajax() function for a GET request:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Ajax Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <button id="loadDataBtn">Load Data</button>
  <div id="dataContainer"></div>

  <script>
    $(document).ready(function() {
      // Load data from a server on button click
      $("#loadDataBtn").click(function() {
        $.ajax({
          url: "https://jsonplaceholder.typicode.com/posts/1",
          method: "GET",
          success: function(data) {
            $("#dataContainer").text(data.title);
          },
          error: function() {
            alert("Failed to load data.");
          }
        });
      });
    });
  </script>
</body>
</html>

14. What is the difference between .get() and .post() methods in jQuery?

.get().post()
Used for making GET requestsUsed for making POST requests
Parameters sent in the URLParameters sent in the request body
Limited data sizeCan handle larger data size

15. What are the differences between .bind() and .live() methods in jQuery?

.bind().live()
Attach event handlers to existing elementsAttach event handlers to current and future elements that match the selector
Replaced by .on() in jQuery version 1.7+Deprecated in jQuery version 1.7+

16. What is the use of the jQuery filter function?

The jQuery filter() function is used to narrow down the set of matched elements based on a specific condition.

Example of using the filter() function to select even-numbered list items:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Filter Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
    <li>Item 5</li>
  </ul>

  <script>
    $(document).ready(function() {
      // Select even-numbered list items using filter()
      $("li").filter(function(index) {
        return index % 2 === 1;
      }).css("color", "blue");
    });
  </script>
</body>
</html>

17. What are the different ways to deal with multiple AJAX requests?

There are a few ways to handle multiple AJAX requests:

  1. Using $.when(): This method allows you to synchronize multiple AJAX requests and execute a callback once all requests are completed.

Example:

JavaScript
$.when($.ajax("/api/data1"), $.ajax("/api/data2"))
  .done(function(response1, response2) {
    // Both requests are done, and you can process the responses here
    var data1 = response1[0];
    var data2 = response2[0];
  })
  .fail(function() {
    // Handle errors if any request fails
  });
  1. Chaining AJAX requests: You can chain AJAX requests using the .then() method to make sure they are executed sequentially.

Example:

JavaScript
$.ajax("/api/data1")
  .then(function(response1) {
    // Process response1
    return $.ajax("/api/data2");
  })
  .then(function(response2) {
    // Process response2
  })
  .catch(function(error) {
    // Handle errors if any request fails
  });
  1. Using Promise.all(): If you are using Promises, you can use Promise.all() to handle multiple AJAX requests concurrently.

Example:

JavaScript
const request1 = fetch("/api/data1");
const request2 = fetch("/api/data2");

Promise.all([request1, request2])
  .then(function(responses) {
    // Both requests are resolved, and you can process the responses here
    return Promise.all(responses.map((response) => response.json()));
  })
  .then(function([data1, data2]) {


 // Process data1 and data2
  })
  .catch(function(error) {
    // Handle errors if any request fails
  });

18. What is the purpose of the jQuery .each() function?

The jQuery .each() function is used to iterate over a collection of elements and perform a function on each element.

Example of using .each() to loop through list items and apply a class:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery each() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
    <li>Item 5</li>
  </ul>

  <script>
    $(document).ready(function() {
      // Add a class to each list item using each()
      $("li").each(function() {
        $(this).addClass("highlight");
      });
    });
  </script>
</body>
</html>

19. How can you create a custom jQuery function?

You can create a custom jQuery function by extending the $.fn object. This allows you to add your custom function to the jQuery namespace, making it accessible on jQuery objects.

Example of creating a custom highlight() function:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Custom jQuery Function Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <p>This is a paragraph.</p>

  <script>
    // Custom jQuery function to highlight text
    $.fn.highlight = function() {
      this.css("background-color", "yellow");
    };

    $(document).ready(function() {
      // Using the custom highlight() function on the paragraph
      $("p").highlight();
    });
  </script>
</body>
</html>

20. What are jQuery plugins and how are they used?

jQuery plugins are additional libraries or modules that extend the functionality of jQuery. They are created to be reusable and can be easily integrated into jQuery projects to perform specific tasks or provide new features.

Plugins are used by including the plugin script after the main jQuery library. Once included, you can use the plugin’s methods just like any other jQuery method.

Example of using a hypothetical “lightbox” jQuery plugin to display images:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Plugin Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script src="lightbox-plugin.js"></script> <!-- The plugin script -->
</head>
<body>
  <img src="image1.jpg" class="lightbox-image" alt="Image 1">
  <img src="image2.jpg" class="lightbox-image" alt="Image 2">
  <!-- More images here -->

  <script>
    $(document).ready(function() {
      // Activate the lightbox plugin on the images with the "lightbox-image" class
      $(".lightbox-image").lightbox();
    });
  </script>
</body>
</html>

21. What is chaining in jQuery?

Chaining in jQuery is the technique of connecting multiple jQuery methods together in a single line of code. This is possible because most jQuery methods return the jQuery object itself, allowing you to call another method on the same set of elements without reselecting them.

Example of chaining in jQuery:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Chaining in jQuery Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv">This is a div.</div>

  <script>
    $(document).ready(function() {
      // Chaining multiple jQuery methods
      $("#myDiv").addClass("highlight").css("color", "blue").text("Updated content");
    });
  </script>
</body>
</html>

22. What does the ‘jQuery.noConflict’ function do?

The jQuery.noConflict() function is used to release control of the $ symbol from jQuery, allowing other JavaScript libraries that use $ to work without conflicts.

Example of using jQuery.noConflict() to release the $ symbol:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery noConflict Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script src="other-library.js"></script> <!-- Some other library using $ -->
</head>
<body>
  <!-- Your HTML content here -->

  <script>
    // Using noConflict() to release the $ symbol for other libraries
    $.noConflict();

    // Now, you can use jQuery with the 'jQuery' alias instead of '$'
    jQuery(document).ready(function() {
      jQuery("p").css("color", "red");
    });
  </script>
</body>
</html>

23. What are the differences between jQuery’s detach() and remove() methods?

.detach().remove()
Removes elements from the DOM but keeps their data and event handlers attachedRemoves elements from the DOM along with their data and event handlers
Allows reinserting the elements laterElements cannot be reinserted after removal
More suitable for temporary removalSuitable for permanent removal

24. What is the difference between prop() and attr() in jQuery?

.prop().attr()
Used to get or set properties of elementsUsed to get or set attributes of elements
Works with boolean attributes (e.g., “checked”)Works with any attribute of elements
Returns the current property valueReturns the initial HTML attribute value

25. What are jQuery animations?

jQuery animations are effects applied to elements to create smooth transitions or movements. They are achieved using methods like fadeIn(), fadeOut(), slideUp(), slideDown(), etc.

Example of using the slideDown() animation:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Animation Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <style>
    #myDiv {
      display: none;
    }
  </style>
</head>
<body>
  <button id="showBtn">Show Div</button>
  <div id="myDiv">This is a hidden div.</div>

  <script>
    $(document).ready(function() {
      // Slide down the hidden div on button click
      $("#showBtn").click(function() {
        $("#myDiv").slideDown();
      });
    });
  </script>
</body>
</html>

26. How can you stop an animation in jQuery?

You can use the .stop() method in jQuery to stop an ongoing animation on an element.

Example of stopping a sliding animation:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Stop Animation in jQuery</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <style>
    #myDiv {
      width: 200px;
      height: 200px;
      background-color: red;
    }
  </style>
</head>
<body>
  <button id="animateBtn">Animate</button>
  <div id="myDiv"></div>

  <script>
    $(document).ready(function() {
      var animating = false;

      $("#animateBtn").click(function() {
        if (!animating) {
          animating = true;
          $("#myDiv").animate({
            width: "400px",
            height: "400px"
          }, 2000, function() {
            animating = false;
          });
        } else {
          // Stop the ongoing animation
          $("#myDiv").stop();
          animating = false;
        }
      });
    });
  </script>
</body>
</html>

27. What does the jQuery .serialize() function do?

The jQuery .serialize() function is used to create a URL-encoded string representation of form data. It is typically used when submitting form data via AJAX or in URL parameters.

Example of using .serialize() on a form:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery serialize() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <form id="myForm">
    <input type="text" name="username" value="John">
    <input type="email" name="email" value="[email protected]">
  </form>
  <button id="submitButton">Submit</button>

  <script>
    $(document).ready(function() {
      $("#submitButton").click(function() {
        // Serialize the form data
        var formData = $("#myForm").serialize();
        alert(formData);
      });
    });
  </script>
</body>
</html>

28. How do you check if an element is empty via jQuery?

You can use the .isEmptyObject() method to check if an object is empty, or use the .length property to check if a collection of elements is empty.

Example of checking if a div is empty:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Check if Element is Empty in jQuery</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv">This is a div.</div>

  <script>
    $(document).ready(function() {
      var divIsEmpty = $("#myDiv").isEmptyObject();
      console.log(divIsEmpty); // Output: false
    });
  </script>
</body>
</html>

29 . What does the .end() function do in jQuery?

The .end() function is used to return to the previous set of elements in the chain of jQuery methods after using filtering methods like .find() or .filter().

Example of using .end() to return to the previous set:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery end() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="container">
    <p>Paragraph 1</p>
    <p>Paragraph 2</p>
  </div>

  <script>
    $(document).ready(function() {
      // Find all paragraphs and add a class to them, then return to the #container div
      $("#container").find("p").addClass("highlight").end().addClass("background");
    });
  </script>
</body>
</html>

30. How can you add elements to the DOM using jQuery?

You can use various methods in jQuery to add elements to the DOM, such as .append(), .prepend(), .before(), and .after().

Example of appending a new paragraph to a div:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Add Element to DOM with jQuery</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
</head>
<body>
  <div id="myDiv">
    <p>Existing paragraph.</p>
  </div>
  <button id="addBtn">Add Paragraph</button>

  <script>
    $(document).ready(function() {
      $("#addBtn").click(function() {
        // Append a new paragraph to the div
        $("#myDiv").append("<p>New paragraph.</p>");
      });
    });
  </script>
</body>
</html>

Advanced Questions

1. How does jQuery chaining work under the hood?

jQuery chaining allows you to chain multiple methods together on a selected set of elements, reducing the number of times you need to re-select elements. It works by having most jQuery methods return the jQuery object itself after performing an operation.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Chaining Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      // Chaining example
      $("#myList li")
        .css("color", "blue")
        .fadeOut(1000)
        .fadeIn(1000);
    });
  </script>
</head>
<body>
  <ul id="myList">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
  </ul>
</body>
</html>

In this example, we select the list items with the ID “myList”, change their text color to blue, then apply a fadeOut effect followed by a fadeIn effect, all in a single chain.

2. How do you create a custom pseudo-class selector in jQuery?

To create a custom pseudo-class selector in jQuery, you can use the $.expr[':'] syntax. This allows you to define a custom filter function for your pseudo-class selector.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Custom Pseudo-class Selector Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    // Custom pseudo-class selector
    $.expr[':'].startsWithA = function(elem) {
      return elem.textContent.charAt(0) === 'A';
    };

    $(document).ready(function() {
      // Using the custom pseudo-class selector
      $("li:startsWithA").css("font-weight", "bold");
    });
  </script>
</head>
<body>
  <ul>
    <li>Apple</li>
    <li>Orange</li>
    <li>Banana</li>
  </ul>
</body>
</html>

In this example, we create a custom pseudo-class selector :startsWithA, which selects elements whose text content starts with the letter ‘A’. The list item “Apple” will have its font-weight set to bold.

3. How would you add and remove classes on elements in jQuery?

You can use the addClass() and removeClass() methods in jQuery to add and remove classes on elements.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Add/Remove Class Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      // Add class to element
      $("#myElement").addClass("highlight");

      // Remove class from element after a delay
      setTimeout(function() {
        $("#myElement").removeClass("highlight");
      }, 2000);
    });
  </script>
  <style>
    .highlight {
      background-color: yellow;
    }
  </style>
</head>
<body>
  <div id="myElement">This is a highlighted element.</div>
</body>
</html>

In this example, we add the class “highlight” to the element with ID “myElement” on page load. After a 2-second delay, we remove the class, resulting in a brief highlight effect.

4. What is jQuery’s event handling model? How does it differ from the traditional DOM event handling model?

jQuery’s event handling model simplifies and normalizes event handling across different browsers. It uses the on() method to attach event handlers and provides various helper methods for common event tasks.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery Event Handling Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    // jQuery event handling
    $(document).ready(function() {
      $("#myButton").on("click", function() {
        alert("Button clicked!");
      });
    });

    // Traditional DOM event handling
    document.addEventListener("DOMContentLoaded", function() {
      var button = document.getElementById("myButton");
      button.addEventListener("click", function() {
        alert("Button clicked!");
      });
    });
  </script>
</head>
<body>
  <button id="myButton">Click me</button>
</body>
</html>

In the jQuery example, we use the on() method to attach a “click” event handler to the button with ID “myButton”. The traditional DOM event handling example achieves the same functionality but with more verbose syntax using addEventListener().

5. What is the use of jQuery’s .data() method? Can you provide a usage example?

jQuery’s .data() method allows you to associate arbitrary data with selected elements without modifying the DOM. It is commonly used to store and retrieve metadata related to the elements.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .data() Method Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      // Storing data using .data()
      $("#myElement").data("price", 50.99);

      // Retrieving data using .data()
      var price = $("#myElement").data("price");
      console.log("Price: $" + price.toFixed(2));
    });
  </script>
</head>
<body>
  <div id="myElement">Product Name</div>
</body>
</html>

In this example, we use the .data() method to store and retrieve the product price associated with the element with ID “myElement”.

6. What is event delegation in jQuery? How can it be implemented?

Event delegation is a technique in jQuery where you attach an event handler to a parent element, which will then handle events triggered by its child elements. This is particularly useful when you have a large number of dynamically added elements or elements that are added and removed frequently, as it reduces the number of event handlers needed.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Event Delegation Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      // Event delegation
      $("#parentList").on("click", "li", function() {
        alert($(this).text() + " clicked!");
      });

      // Add new list item dynamically
      $("#addButton").click(function() {
        $("<li>New Item</li>").appendTo("#parentList");
      });
    });
  </script>
</head>
<body>
  <ul id="parentList">
    <li>Item 1</li>
    <li>Item 2</li>
  </ul>
  <button id="addButton">Add Item</button>
</body>
</html>

In this example, we use event delegation to attach a “click” event handler to the parent list (#parentList). When you click on any of its child list items (li), the event will be handled correctly, even for dynamically added items.

7. How would you debug jQuery code? What tools would you use?

To debug jQuery code, you can use browser developer tools, which typically include a console for logging messages and inspecting variables. Popular choices are Chrome DevTools or Firefox Developer Tools.

Example:

JavaScript
$(document).ready(function() {
  var num1 = 10;
  var num2 = 20;

  console.log("Debugging jQuery code...");
  console.log("Adding numbers: " + (num1 + num2));
});

In this example, we use console.log() to output messages and variable values to the browser console. You can then open the browser developer tools, navigate to the console tab, and see the logged messages.

8. What are Deferred and Promise objects? How can they be used in jQuery?

Deferred and Promise objects in jQuery are used for handling asynchronous operations, such as AJAX requests. Deferred represents a computation that may not have completed yet, while Promise represents a value that may be available in the future.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Deferred and Promise Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    function simulateAsyncRequest() {
      var deferred = $.Deferred();
      setTimeout(function() {
        deferred.resolve("Data from server");
      }, 2000);
      return deferred.promise();
    }

    $(document).ready(function() {
      $("#fetchButton").click(function() {
        var promise = simulateAsyncRequest();
        promise.done(function(data) {
          console.log("Received data: " + data);
        });
        promise.fail(function() {
          console.log("Error occurred during the request.");
        });
      });
    });
  </script>
</head>
<body>
  <button id="fetchButton">Fetch Data</button>
</body>
</html>

In this example, simulateAsyncRequest() returns a Deferred object. We use the promise() method to return a Promise object from the Deferred. When the “Fetch Data” button is clicked, the promise’s done() method is called with the successful response, and the fail() method is called in case of an error.

9. What is the difference between jQuery’s .on() and .click() methods? When would you use each?

MethodUsageEvent DelegationMultiple Events
.on()Preferred method for event binding.Supports delegation.Can handle multiple events using a space-separated list, e.g., .on('click mouseenter', handler).
.click()Shortcut for .on('click', handler).Does not support delegation.Only handles the “click” event.

Use .on() when you want to attach event handlers, especially when using event delegation or handling multiple events on the same element. Use .click() for simple cases where you only need to handle the “click” event on an element.

10. How does jQuery handle cross-browser compatibility? Can you provide examples of cross-browser issues that jQuery solves?

jQuery abstracts away many cross-browser inconsistencies, providing a consistent API across different browsers. For example, differences in DOM manipulation, event handling, and AJAX requests are handled by jQuery.

Example of cross-browser issue:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Cross-Browser Compatibility Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      // Cross-browser event handling
      $("#myButton").on("click", function() {
        alert("Button clicked!");
      });

      // Cross-browser AJAX request
      $.ajax({
        url: "https://api.example.com/data",
        method: "GET",
        success: function(data) {
          console.log("Received data: ", data);
        },
        error: function(xhr, status, error) {
          console.log("Error occurred: ", error);
        }
      });
    });
  </script>
</head>
<body>
  <button id="myButton">Click me</button>
</body>
</html>

In this example, jQuery takes care of the differences in event handling and AJAX requests across various browsers.

11. What is the difference between jQuery’s .bind(), .live(), and .delegate() methods?

MethodUsageEvent DelegationWorks with Dynamically Added Elements
.bind()Attach event handlers directly to elements.NoNo
.live()Attach event handlers to current and future elements matching the selector.YesYes
.delegate()Attach event handlers to a parent element and delegate the event to child elements matching the selector.YesYes

12. How can you handle exceptions or errors in jQuery .ajax() calls?

You can handle exceptions and errors in jQuery .ajax() calls using the error callback or the fail() method of the returned Promise object.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery AJAX Error Handling Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $.ajax({
        url: "https://api.example.com/data",
        method: "GET",
        success: function(data) {
          console.log("Received data: ", data);
        },
        error: function(xhr, status, error) {
          console.log("Error occurred: ", error);
        }
      });
    });
  </script>
</head>
<body>
</body>
</html>

In this example, if there’s an error in the AJAX request, the error callback will be executed, and the error information will be logged to the console.

13. What is a JSONP request, and how can it be made with jQuery?

JSONP (JSON with Padding) is a technique to overcome the same-origin policy limitation in web browsers for making cross-domain AJAX requests. It involves creating a dynamic <script> element to load the data as a script and wrapping the JSON response within a callback function.

Example:

Suppose we have a server that provides JSON data in a callback function named jsonpCallback:

JavaScript
jsonpCallback({ "name": "John", "age": 30 });

To make a JSONP request with jQuery, you can use the $.ajax() method and set the dataType to "jsonp".

HTML
<!DOCTYPE html>
<html>
<head>
  <title>JSONP Request Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $.ajax({
        url: "https://api.example.com/data",
        dataType: "jsonp",
        jsonpCallback: "jsonpCallback", // Name of the callback function
        success: function(data) {
          console.log("Received data: ", data);
        },
        error: function(xhr, status, error) {
          console.log("Error occurred: ", error);
        }
      });
    });

    function jsonpCallback(data) {
      console.log("Callback function executed with data: ", data);
    }
  </script>
</head>
<body>
</body>
</html>

In this example, the JSONP request is made to the server, and the response is processed through the jsonpCallback() function.

14. How would you manipulate an iframe’s content from its parent page with jQuery?

To manipulate an iframe’s content from its parent page using jQuery, you can use the contents() method to access the iframe’s document and then perform operations as if it were a regular DOM.

Example:

Parent Page (index.html):

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Parent Page</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $("#changeButton").click(function() {
        var iframeDocument = $("#myIframe").contents();
        var iframeContent = iframeDocument.find("#iframeContent");
        iframeContent.text("Content changed from parent page!");
      });
    });
  </script>
</head>
<body>
  <iframe id="myIframe" src="child.html"></iframe>
  <button id="changeButton">Change Content</button>
</body>
</html>

Child Page (child.html):

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Child Page</title>
</head>
<body>
  <div id="iframeContent">Initial Content</div>
</body>
</html>

In this example, when the “Change Content” button is clicked on the parent page, the content of the iframe is changed to “Content changed from parent page!”.

15. How would you organize jQuery code for a large-scale, complex web application?

For a large-scale, complex web application, it is essential to follow a structured and modular approach to organize jQuery code. Some common practices include:

  • Use a module pattern or an MVC (Model-View-Controller) architecture to separate concerns and promote code reusability.
  • Divide the code into logical modules based on functionalities and features.
  • Keep the code DRY (Don’t Repeat Yourself) by using functions and utilities.
  • Use a build tool like Webpack or RequireJS to bundle and manage dependencies.
  • Separate HTML, CSS, and JavaScript into different files to improve maintainability.
  • Use jQuery event delegation for efficient handling of dynamically added elements.
  • Minimize direct manipulation of the DOM; prefer templates and data-binding libraries.

16. What is the .animate() function in jQuery, and how can you use it?

The .animate() function in jQuery is used to create animations by gradually changing CSS properties of elements over a specified duration.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .animate() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $("#animateButton").click(function() {
        $("#myElement").animate({
          opacity: 0.25,
          left: "250px",
          fontSize: "20px"
        }, 1000);
      });
    });
  </script>
  <style>
    #myElement {
      width: 100px;
      height: 100px;
      background-color: blue;
      position: relative;
    }
  </style>
</head>
<body>
  <div id="myElement"></div>
  <button id="animateButton">Animate</button>
</body>
</html>

In this example, when you click the “Animate” button, the #myElement div will gradually change its opacity to 0.25, move 250 pixels to the right, and increase its font size to 20 pixels over a duration of 1000 milliseconds.

17. What is the difference between the jQuery :first and :first-child selectors?

SelectorDescription
:firstSelects the first matched element.
:first-childSelects elements that are the first child of their parent.

The main difference is that :first selects the first element in the matched set, regardless of its position relative to its siblings, while :first-child selects elements that are the first child of their parent.

18. What is the use of the jQuery `load()` method, and how does it work?

The jQuery load() method is used to load data from the server and place it into selected elements. It simplifies AJAX requests and allows you to directly inject the loaded content into elements.

Example:

index.html:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .load() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $("#loadButton").click(function() {
        $("#result").load("data.html");
      });
    });
  </script>
</head>
<body>
  <button id="loadButton">Load Data</button>
  <div id="result"></div>
</body>
</html>

data.html:

HTML
<h2>Data Loaded via jQuery .load()</h2>
<p>This content is loaded dynamically from data.html.</p>

In this example, when you click the “Load Data” button, the content from data.html is fetched and inserted into the #result div on the index.html page.

19. Can you use jQuery to make a cross-domain request? How?

Yes, you can use jQuery to make a cross-domain request using JSONP (JSON with Padding) or CORS (Cross-Origin Resource Sharing). JSONP is typically used for older browsers that do not support CORS.

Example using JSONP:

Suppose the server provides data in a callback function named jsonpCallback.

HTML
<!DOCTYPE html>
<html>
<head>
  <title>JSONP Cross-Domain Request Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $.ajax({
        url: "https://api.example.com/data",
        dataType: "jsonp",
        jsonpCallback: "jsonpCallback",
        success: function(data) {
          console.log("Received data: ", data);
        },
        error: function(xhr, status, error) {
          console.log("Error occurred: ", error);
        }
      });
    });

    function jsonpCallback(data) {
      console.log("Callback function executed with data: ", data);
    }
  </script>
</head>
<body>
</body>
</html>

In this example, the JSONP request is made to the server, and the response is processed through the jsonpCallback() function.

20. What is the use of the .end() method in jQuery?

The .end() method in jQuery is used to revert the selection back to its previous state in a chain of methods. It is commonly used after using a filtering method like .find() or .filter() to return to the original set of elements.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .end() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $("ul")
        .find("li")
        .css("color", "blue")
        .end() // Revert to the original set of "ul" elements
        .css("background-color", "yellow");
    });
  </script>
  <style>
    ul {
      background-color: lightgray;
    }
  </style>
</head>
<body>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
  </ul>
  <ul>
    <li>Item 3</li>
    <li>Item 4</li>
  </ul>
</body>
</html>

In this example, the text color of the list items within the ul elements is changed to blue, but the background color of the ul elements is reverted to the original state using .end().

21. What are potential problems when using jQuery with other JavaScript libraries, and how can these be mitigated?

Potential problems when using jQuery with other JavaScript libraries include conflicts in naming conventions, mismatched versions causing unexpected behaviors, and performance overhead due to loading multiple libraries.

Mitigation strategies:

  • Use jQuery’s noConflict() method to release the control of the “$” variable and use another alias for jQuery, especially when integrating with other libraries using the “$” symbol.
  • Ensure that the versions of jQuery and other libraries are compatible. Avoid using multiple versions of jQuery on the same page.
  • Minimize the number of external libraries and scripts used to reduce the performance overhead.

22. How does jQuery handle CSS ‘box model’ inconsistencies between different browsers?

jQuery does not handle CSS ‘box model’ inconsistencies directly. Instead, it provides a consistent API to access and modify CSS properties across browsers. To deal with box model issues, developers need to use CSS resets or normalize.css to ensure consistent rendering across browsers.

23. What are some techniques for optimizing jQuery performance?

To optimize jQuery performance:

  • Use efficient selectors to target elements, avoiding expensive and over-specific selectors.
  • Cache frequently used selectors to avoid redundant DOM traversals.
  • Use event delegation to handle events on dynamically generated elements efficiently.
  • Minimize the use of animations and effects, especially on large-scale pages.
  • Use native JavaScript for simple tasks where jQuery is not necessary.
  • Concatenate and minify JavaScript files to reduce the number of HTTP requests.

24. What is the difference between jQuery’s .empty(), .remove(), and .detach() methods?

MethodDescriptionRemove DataRemove Event HandlersKeep in Memory
.empty()Remove all child elements and text content from the selected elements.NoYesNo
.remove()Remove the selected elements, including their data and event handlers.YesYesNo
.detach()Remove the selected elements, including their data and event handlers, but keep them in memory for potential reinsertion.YesYesYes

25. How do you use the .pushStack() method in jQuery?

The .pushStack() method in jQuery allows you to create a new jQuery object from a subset of elements and maintain the original set of elements as well.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .pushStack() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      var divElements = $("div");
      var pElements = divElements.find("p");

      // Create a new jQuery object with pElements and add divElements as a previous set
      var newElements = pElements.pushStack(divElements);

      // Modify the CSS properties of newElements
      newElements.css("border", "2px solid red");
    });
  </script>
  <style>
    div {
      width: 200px;
      height: 100px;
      background-color: lightgray;
      margin: 10px;
    }
    p {
      margin: 5px;
    }
  </style>
</head>
<body>
  <div>
    <p>Paragraph 1</p>
    <p>Paragraph 2</p>
  </div>
  <div>
    <p>Paragraph 3</p>
    <p>Paragraph 4</p>
  </div>
</body>
</html>

In this example, the .pushStack() method is used to create a new jQuery object containing the p elements and add the original div elements as the previous set. The new elements are then styled with a red border.

26. What are the uses and differences between the jQuery .map() and .grep() functions?

MethodDescriptionReturns
.map()Applies a function to each element in the set and returns an array of mapped values.Array of mapped values
.grep()Filters the set of elements using a function and returns a new array of matched elements.Array of matched elements

27. What is the difference between .closest() and .parents() methods in jQuery?

MethodDescriptionTraversalFilter
.closest()Returns the closest ancestor that matches the selector, traversing up the DOM hierarchy.YesYes
.parents()Returns all ancestors that match the selector, traversing up the DOM hierarchy.YesYes

28. How can you check if a particular version of jQuery is loaded on a webpage?

You can check if a specific version of jQuery is loaded using the $.fn.jquery property. It returns the version number as a string.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>Check jQuery Version Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      var jqueryVersion = $.fn.jquery;
      console.log("jQuery version: " + jqueryVersion);
    });
  </script>
</head>
<body>
</body>
</html>

In this example, the jQuery version number is logged to the console.

29. What is the use of jQuery’s .andSelf() function?

The .andSelf() function (deprecated in jQuery 1.8) is used to add the previous set of elements to the current set, allowing you to chain multiple selectors.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>jQuery .andSelf() Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      var listItems = $("li");
      var paragraph = $("p");

      var combinedElements = listItems.andSelf().add(paragraph);
      combinedElements.css("color", "blue");
    });
  </script>
  <style>
    li {
      color: green;
    }
    p {
      color: red;
    }
  </style>
</head>
<body>
  <ul>
    <li>List item 1</li>
    <li>List item 2</li>
  </ul>
  <p>Paragraph 1</p>
</body>
</html>

In this example, the .andSelf() method is used to add the previous set of li elements to the current set, and then the .add() method combines them with the p element. The combined elements are styled with a blue color.

30. How can you use jQuery to handle file uploads ?

To handle file uploads with jQuery, you can use the FormData object for AJAX-based file uploads.

Example:

HTML
<!DOCTYPE html>
<html>
<head>
  <title>File Upload with jQuery Example</title>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
  <script>
    $(document).ready(function() {
      $("#uploadForm").submit(function(event) {
        event.preventDefault();
        var formData = new FormData(this);

        $.ajax({
          url: "upload.php",
          method: "POST",
          data: formData,
          processData: false,
          contentType: false,
          success: function(response) {
            console.log("File uploaded successfully.", response);
          },
          error: function(xhr, status, error) {
            console.log("Error occurred during file upload.", error);
          }
        });
      });
    });
  </script>
</head>
<body>
  <form id="uploadForm" enctype="multipart/form-data">
    <input type="file" name="fileToUpload" id="fileToUpload">
    <input type="submit" value="Upload File" name="submit">
  </form>
</body>
</html>

In this example, the form is submitted using AJAX when the “Upload File” button is clicked. The file input’s selected file is sent to the server using the FormData object, and the server-side handling is done in the upload.php file.

MCQ Questions

1. What is jQuery?

A) A programming language
B) A JavaScript library
C) An operating system
D) A database management system
Answer: B) A JavaScript library

2. Which symbol is used as a shorthand for the jQuery function?

A) $
B) #
C) @
D) &
Answer: A) $

3. Which of the following selectors selects elements by their class name?

A) *
B) #
C) .
D) ^
Answer: C) .

4. Which method is used to insert content at the beginning of an element?

A) prepend()
B) before()
C) insertBefore()
D) prependTo()
Answer: A) prepend()

5. Which method is used to fade out an element?

A) hide()
B) fadeTo()
C) fadeOut()
D) fadeToggle()
Answer: C) fadeOut()

6. Which method is used to add a CSS class to an element?

A) addClass()
B) appendClass()
C) insertClass()
D) attachClass()
Answer: A) addClass()

7. Which method is used to retrieve the current value of an input element?

A) getValue()
B) retrieve()
C) val()
D) currentValue()
Answer: C) val()

8. Which method is used to make an AJAX request?

A) ajax()
B) get()
C) request()
D) fetch()
Answer: A) ajax()

9. Which method is used to stop the currently running animation?

A) stop()
B) cancel()
C) halt()
D) pause()
Answer: A) stop()

10. Which method is used to remove an element from the DOM?

A) detach()
B) remove()
C) erase()
D) destroy()
Answer: B) remove()

11. Which method is used to get the computed width of an element?

A) width()
B) getWidth()
C) computedWidth()
D) getComputedStyle()
Answer: A) width()

12. Which method is used to get the parent element of an element?

A) parent()
B) getParent()
C) ancestor()
D) findParent()
Answer: A) parent()

13. Which method is used to get the index of an element among its siblings?

A) getIndex()
B) position()
C) index()
D) siblingIndex()
Answer: C) index()

14. Which method is used to bind an event handler to an element?

A) bind()
B) on()
C) attach()
D) listen()
Answer: B) on()

15. Which method is used to animate an element’s properties?

A) animate()
B) move()
C) transition()
D) transform()
Answer: A) animate()

16. Which method is used to get the current position of the first matched element relative to the offset parent?

A) offsetTop()
B) position()
C) offset()
D) topPosition()
Answer: B) position()

17. Which method is used to find descendants of an element based on a selector?

A) find()
B) locate()
C) search()
D) discover()
Answer: A) find()

18. Which method is used to execute a function once the DOM is ready?

A) load()
B) ready()
C) init()
D) start()
Answer: B) ready()

19. Which method is used to get the current scroll position of an element?

A) scrollTop()
B) scroll()
C) getScrollTop()
D) currentScroll()
Answer: A) scrollTop()

20. Which method is used to filter elements based on a selector?

A) filter()
B) select()
C) sort()
D) match()
Answer: A) filter()

21. Which method is used to toggle the visibility of an element?

A) show()
B) toggle()
C) display()
D) visible()
Answer: B) toggle()

22. Which method is used to load JSON data from a server using an HTTP GET request?

A) getJSON()
B) fetchJSON()
C) loadJSON()
D) requestJSON()
Answer: A) getJSON()

23. Which method is used to set the HTML content of an element?

A) setHTML()
B) html()
C) setContent()
D) innerHTML()
Answer: B) html()

24. Which method is used to get the height of an element, including padding but excluding borders and margins?

A) outerHeight()
B) getHeight()
C) totalHeight()
D) offsetHeight()
Answer: A) outerHeight()

25. Which method is used to check if an element has a specific CSS class?

A) checkClass()
B) containsClass()
C) hasClass()
D) hasStyle()
Answer: C) hasClass()

26. Which method is used to delay the execution of subsequent items in the queue?

A) delay()
B) pause()
C) wait()
D) sleep()
Answer: A) delay()

27. Which method is used to retrieve the current position of the scrollable content area within an element?

A) scroll()
B) scrollLeft()
C) getScroll()
D) scrollPosition()
Answer: B) scrollLeft()

28. Which method is used to get the value of a CSS property for the first element in a matched set?

A) getStyle()
B) fetchStyle()
C) css()
D) style()
Answer: C) css()

29. Which method is used to perform a deep copy of elements?

A) copy()
B) clone()
C) duplicate()
D) replicate()
Answer: B) clone()

30. Which method is used to retrieve the width of an element, including content, padding, and borders?

A) totalWidth()
B) offsetWidth()
C) getWidth()
D) outerWidth()
Answer: D) outerWidth()

Also read from: https://www.fullstack.cafe/interview-questions/jquery

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