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Leveraging the 304 Not Modified Status for Efficient Caching - HTTP Error Code 304

Vishalini Paliwal
~ 6 min read | Published on Mar 28, 2024


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In today's web development landscape, optimizing performance is crucial for providing users with a seamless experience. One powerful tool in achieving this is leveraging the 304 Not Modified status code for efficient caching. In this comprehensive guide, we'll delve into what the 304 error is, its possible causes, how to handle it in JavaScript, best practices for its use, and methods for testing it in various environments.

What is a 304 Error?

The 304 status code, often referred to as "Not Modified," is an HTTP response status code indicating that a requested resource has not been modified since the last request. When a browser sends a conditional GET request for a resource, such as an image or CSS file, it includes headers like If-Modified-Since or If-None-Match. If the server determines that the resource has not been modified, it responds with a 304 status code instead of the resource itself, reducing unnecessary data transfer and improving performance.

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What are the Possible Causes for 304 Error?

Several factors can lead to the generation of a 304 error:

  1. Conditional Requests: When a client sends a conditional request for a resource using headers like If-Modified-Since or If-None-Match, the server checks if the resource has been modified since the specified date or entity tag.
  2. Caching Configuration: Improper caching configurations on the server or client-side can result in incorrect handling of conditional requests, leading to unexpected 304 responses.
  3. Resource Unchanged: If the requested resource has not been modified since the client's last request, the server responds with a 304 status code to indicate that the cached version can be used.

How to Handle 304 in JavaScript

In JavaScript, you can handle 304 responses when making XMLHttpRequests or fetch requests by checking the status code. Here's a basic example using fetch:

  .then(response => {
    if (response.status === 304) {
      // Resource not modified, use cached version
      // Implement logic here
    } else {
      // Handle other status codes
      // Implement logic here
  .catch(error => console.error('Error:', error));

Best Practices for Using 304 Status Code

To effectively leverage the 304 status code for efficient caching, consider the following best practices:

  1. Set Proper Cache-Control Headers: Ensure that your server sends appropriate Cache-Control headers to control caching behavior, including max-age, must-revalidate, and no-cache directives.
  2. Implement ETag or Last-Modified Headers: Use entity tags (ETags) or Last-Modified headers to enable conditional requests, allowing the server to validate whether a resource has been modified.
  3. Optimize Cache Validation: Minimize the overhead of cache validation by configuring servers to efficiently process conditional requests and respond with 304 status codes when appropriate.

How to Test 304 Status Code on Postman

Testing the 304 status code in Postman involves sending a conditional GET request and inspecting the response. Here's how to do it:

  1. Open Postman and create a new request.
  2. Set the request method to GET and enter the URL of the resource you want to test.
  3. In the Headers section, add an If-Modified-Since or If-None-Match header with a valid timestamp or entity tag.
  4. Send the request and observe the response status code. If the resource has not been modified, you should receive a 304 response.

How to Test 304 Status Code in DevTools Browser in Chrome

Testing the 304 status code in Chrome DevTools involves simulating a conditional request and examining the network tab. Follow these steps:

  1. Open Chrome DevTools by pressing F12 or right-clicking on the page and selecting "Inspect."
  2. Navigate to the Network tab.
  3. Reload the page while preserving the cache by holding down the Shift key and clicking the reload button or pressing Ctrl + F5.
  4. Find the request for the resource you want to test and examine the status code. A 304 status indicates that the resource has not been modified.

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Frequently Asked Questions

Q: How does the 304 status code differ from other HTTP status codes?

A: Unlike error status codes like 404 (Not Found) or 500 (Internal Server Error), the 304 status code is not an error but rather a signal that the requested resource has not been modified, allowing clients to use their cached copy.

Q: Can I force a server to return a 304 status code?

A: No, the decision to return a 304 status code is determined by the server based on the presence of conditional headers in the client's request and the current state of the requested resource.

Q: Does the use of the 304 status code eliminate the need for cache expiration settings?

A: No, cache expiration settings such as max-age and Expires headers are still necessary to control how long cached resources should be considered valid before revalidation is required.

Q: Are there any performance implications of using the 304 status code?

A: When used correctly, the 304 status code can improve performance by reducing unnecessary data transfer and server load. However, improper implementation or excessive use of conditional requests can negate these benefits.

Q: Can a client specify multiple conditions for a conditional request?

A: Yes, clients can include multiple conditional headers such as If-Modified-Since and If-None-Match to provide more precise criteria for cache validation.


Efficient caching plays a crucial role in optimizing web performance and reducing bandwidth usage. By leveraging the 304 Not Modified status code, developers can minimize unnecessary data transfer and improve the responsiveness of web applications. Remember to implement best practices for cache validation, test your implementations thoroughly, and monitor performance metrics to ensure optimal caching strategies. For comprehensive error monitoring and session replay capabilities, consider utilizing Zipy's tool to enhance your web development workflow.

Read more resources on 3xx error status codes

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