AJAX - HTTP status codes

Categories of HTTP Status Codes

HTTP status codes are grouped by their first digit:

  1. 1xx (Informational) – Request received, still processing.

  2. 2xx (Success) – Request was successfully received and processed.

  3. 3xx (Redirection) – Client must take additional steps (like following a new URL).

  4. 4xx (Client Error) – Something is wrong with the client’s request.

  5. 5xx (Server Error) – The server failed to fulfill a valid request.

Common HTTP Status Codes

2xx – Success

  • 200 OK – The request was successful, and the server returned the expected response.

    • Example: AJAX GET /users returns JSON with user data.

  • 201 Created – A resource was successfully created (common with POST).

    • Example: Submitting a form via AJAX to create a new account.

  • 204 No Content – Request succeeded, but there’s no response body.

    • Example: Deleting a record with AJAX.

3xx – Redirection

  • 301 Moved Permanently – Resource has a new permanent URL.

  • 302 Found – Temporary redirection; resource is elsewhere for now.

  • 304 Not Modified – Cached version is still valid (used in AJAX caching).

4xx – Client Errors

  • 400 Bad Request – Server cannot process request due to invalid syntax.

    • Example: Sending malformed JSON in an AJAX request.

  • 401 Unauthorized – Authentication required or failed.

    • Example: AJAX call without a valid token.

  • 403 Forbidden – Client is authenticated but not allowed access.

    • Example: User tries to fetch admin-only data.

  • 404 Not Found – The requested resource doesn’t exist.

    • Example: AJAX call to /api/user/999 when that user ID doesn’t exist.

  • 408 Request Timeout – The request took too long.

5xx – Server Errors

  • 500 Internal Server Error – Generic server-side failure.

    • Example: Database crash or unhandled exception in the backend.

  • 502 Bad Gateway – Server acting as a gateway got an invalid response.

  • 503 Service Unavailable – Server is overloaded or down for maintenance.

  • 504 Gateway Timeout – A server taking too long to respond.

Why This Matters in AJAX

In JavaScript, you can detect these codes and respond accordingly:

fetch('/api/data')
  .then(response => {
    if (response.ok) { // true for 200–299
      return response.json();
    } else {
      throw new Error(`Error ${response.status}: ${response.statusText}`);
    }
  })
  .then(data => console.log(data))
  .catch(error => console.error(error));