Top jQuery Interview Questions for Experienced

Question: What is jQuery’s $.ajaxSetup() method?

Answer:

The $.ajaxSetup() method in jQuery is used to set default values for all AJAX requests that are made on a page. It allows you to define global settings for AJAX requests, such as the default HTTP method, data type, timeout, or headers, so you don’t need to specify them each time you make an AJAX request using methods like $.ajax(), $.get(), or $.post().

Using $.ajaxSetup(), you can configure the default settings for all AJAX calls, which can help reduce redundancy and ensure consistency across your AJAX requests.

Syntax:

$.ajaxSetup({
    url: 'default-url',
    type: 'GET',
    dataType: 'json',
    timeout: 5000,
    headers: {
        'Authorization': 'Bearer token',
        'Custom-Header': 'value'
    },
    beforeSend: function(xhr, settings) {
        // Code to execute before sending the request
    },
    complete: function(xhr, status) {
        // Code to execute after the request is complete
    },
    success: function(data, textStatus, xhr) {
        // Code to execute on success
    },
    error: function(xhr, textStatus, errorThrown) {
        // Code to handle errors
    }
});

Parameters:

The $.ajaxSetup() method accepts a single object as a parameter, containing key-value pairs of AJAX settings that will apply to all future AJAX requests.

Some common settings you can configure include:

• url: The default URL for all AJAX requests.
• type: The default HTTP request method (e.g., 'GET', 'POST', etc.).
• dataType: The default data type to expect in the response (e.g., 'json', 'xml', 'text', etc.).
• timeout: The default timeout (in milliseconds) for AJAX requests.
• headers: Set default headers to include with all AJAX requests.
• beforeSend: A callback function that will be executed before sending the request. You can use this to modify the request or cancel it by returning false.
• success: A callback function that will be executed if the request succeeds.
• error: A callback function that will be executed if the request fails.
• complete: A callback function that will be executed when the request finishes (regardless of success or failure).

Example:

// Set up default AJAX settings for all requests
$.ajaxSetup({
    url: 'https://api.example.com/',
    type: 'GET',
    dataType: 'json',
    timeout: 5000,
    beforeSend: function(xhr) {
        console.log('Request is about to be sent...');
    },
    complete: function(xhr, status) {
        console.log('Request is complete.');
    },
    error: function(xhr, textStatus, errorThrown) {
        console.log('An error occurred: ' + textStatus);
    }
});

// Now any AJAX request you make will automatically use these defaults
$.ajax({
    url: 'getData', // This URL will automatically use 'https://api.example.com/getData'
    success: function(response) {
        console.log(response);
    }
});

Use Cases:

1. Consistent Settings: If you want all AJAX requests in your application to share the same base URL, data type, or other settings (like authentication headers), $.ajaxSetup() is an easy way to define those defaults.

2. Authentication: You can use it to add a default authentication header (e.g., JWT token or API key) to all AJAX requests.

3. Error Handling: Define a global error handler that will be used for all AJAX requests in the application.

Important Notes:

• $.ajaxSetup() affects all subsequent AJAX requests, so be careful when modifying it globally, especially if you want different AJAX calls to have different settings.
• If you only need to customize settings for a particular AJAX request, it’s better to specify the settings directly in the individual $.ajax() call instead of modifying them globally with $.ajaxSetup().

Example without using $.ajaxSetup():

If you don’t want to use $.ajaxSetup() for all requests but just want to set some settings for one specific request, you can specify those settings directly:

$.ajax({
    url: 'https://api.example.com/getData',
    type: 'GET',
    dataType: 'json',
    success: function(response) {
        console.log(response);
    }
});

Summary:

• $.ajaxSetup() is a jQuery method used to set global default settings for all AJAX requests.
• It allows you to define default values for parameters like url, type, dataType, timeout, headers, and callback functions (such as beforeSend, success, error).
• This helps you avoid repeating the same settings for every AJAX request, ensuring consistency and reducing code duplication.

Question: How do you optimize performance in large-scale jQuery applications?

Answer:

Optimizing performance in large-scale jQuery applications is crucial to ensuring a smooth user experience, especially when dealing with a large number of DOM elements and complex interactions. Here are some strategies to optimize performance in such applications:

1. Minimize DOM Manipulations:

   • Batch DOM Updates: Instead of updating the DOM multiple times in a loop, collect all changes and apply them in a single operation using append(), html(), css(), etc.
   • Avoid Reflows and Repaints: Excessive DOM manipulation can cause unnecessary reflows and repaints, which can slow down performance. Try to minimize changes to the layout (such as CSS styles or element positioning).

2. Use Event Delegation:

   • Instead of binding events to each individual element, use event delegation. This involves binding a single event listener to a parent element and letting events propagate to the child elements. For example:

     $(document).on('click', '.child-element', function() {
         // Handle click event
     });

   This reduces the number of event listeners and makes it more efficient, especially when dynamically adding new elements.

3. Cache Selectors:

   • Repeatedly querying the DOM with $() can be inefficient. Cache your jQuery selectors by storing the result in a variable to avoid repeated DOM traversal:

     var $element = $('#element-id');
     $element.addClass('new-class');
     $element.css('color', 'red');

4. Limit the Use of jQuery:

   • jQuery is powerful, but it can be slow for certain tasks. For simple tasks such as selecting and manipulating a single element, consider using native JavaScript methods like document.getElementById() or querySelector() for better performance.
   • Also, avoid chaining too many methods, as it can be inefficient for complex operations.

5. Defer Non-Critical JavaScript:

   • For performance-critical applications, defer non-essential JavaScript files using the defer or async attribute in the <script> tag to ensure that scripts don’t block the page rendering:

     <script src="script.js" defer></script>

6. Lazy Load Images and Content:

   • Implement lazy loading for images and other content to prevent loading everything upfront, which can cause performance bottlenecks. You can use the loading="lazy" attribute for images or use a JavaScript solution to load content as the user scrolls.

7. Optimize CSS and JavaScript Files:

   • Minimize and compress your CSS and JavaScript files to reduce their size and improve load times. Tools like UglifyJS, Terser, and CSSNano can help achieve this.

8. Use jQuery Plugins Wisely:

   • Avoid using heavy or unnecessary plugins. Select lightweight alternatives or implement custom solutions that address only the specific functionality needed for your application.

9. Debounce and Throttle Events:

   • For events that trigger frequently, such as scroll, resize, or keyup, use debouncing or throttling to reduce the frequency of the event handler calls:
     • Debouncing ensures that the event handler is called only after a certain delay after the last event.
     • Throttling ensures that the event handler is executed at regular intervals.

   Example of debouncing a search input:

   var debounceSearch = _.debounce(function() {
       // Perform search
   }, 300); // 300ms delay
   $('#search-input').on('input', debounceSearch);

10. Avoid Memory Leaks:

    • Ensure proper cleanup of event listeners and intervals to avoid memory leaks. Use .off() to remove event listeners when they are no longer needed:

      $(element).off('click');

By implementing these best practices, you can significantly improve the performance and scalability of your jQuery-based applications, especially as they grow in size and complexity.

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