Cross-origin resource sharing (CORS) post request works from plain javascript, but why not with jQuery

Why Cross-Origin Resource Sharing (CORS) POST Request Works from Plain JavaScript, but Not with jQuery? 🤔

If you've ever encountered a situation where a Cross-Origin Resource Sharing (CORS) POST request worked perfectly fine when made from plain JavaScript, but failed when made using jQuery, you're not alone. This can be quite a frustrating problem to troubleshoot, but fear not! In this blog post, we'll dive deep into this issue, explore the possible causes, and provide easy solutions to help you overcome this roadblock. So, let's get started! 🚀

Understanding the Problem 🕵️‍♀️

The context for this question involves a local machine (machineA) with two web servers. One is an in-built XBMC server on port 8080, which displays a library, while the other is a CherryPy python script on port 8081, used to trigger file conversions on demand. The issue arises when attempting to make an AJAX POST request from the page served by the XBMC server to the CherryPy server using jQuery.

The Request and Response Flow 🔄

To better understand why the same request works from one site but not the other, let's examine the request and response flow in both scenarios:

Scenario 1: Plain JavaScript Request

When making the POST request from the XBMC server using plain JavaScript, the following flow occurs:

1. The XBMC server issues an HTTP OPTIONS request to the CherryPy server with the appropriate headers, including Access-Control-Request-Method: POST and Access-Control-Request-Headers: x-requested-with.

2. The CherryPy server responds to the OPTIONS request with the necessary CORS headers, including Access-Control-Allow-Origin: *, Access-Control-Allow-Methods: POST, GET, OPTIONS, and Access-Control-Allow-Headers: *.

3. The browser, upon receiving the correct CORS headers, proceeds to issue the POST request, and the conversation continues successfully.

Scenario 2: jQuery Request

When making the same POST request from the XBMC server using jQuery, the following flow occurs:

1. The XBMC server issues an HTTP OPTIONS request to the CherryPy server with the appropriate headers.

2. The CherryPy server responds to the OPTIONS request with the necessary CORS headers, just like in scenario 1.

3. Here's where things diverge: instead of proceeding to issue the POST request, the conversation stops abruptly. The POST request is not sent, leading to the failure of the request.

Identifying the Issue 🎯

So, what's causing the POST request to fail when using jQuery? The most common issue is related to the presence of cahced CORS preflight requests. These cached requests occur when the browser has already made a previous OPTIONS request for the same URL. As a result, subsequent OPTIONS requests are skipped, and the browser never proceeds to issue the POST request.

Resolving the Issue 💡

To resolve the issue, you have a few options:

1. Clear the Browser Cache: Manually clear the browser cache and try making the request again using jQuery. This will ensure that the OPTIONS request is sent and the subsequent POST request is issued.

2. Disable CORS Preflight Caching: Add the Access-Control-Max-Age: 0 header to the OPTIONS response from the CherryPy server. This tells the browser to treat each request as a new request and not use a cached response. However, keep in mind that this might impact performance, as the browser will have to make an OPTIONS request for every subsequent POST request.

3. Force an OPTIONS Request: Before making the POST request using jQuery, explicitly send an OPTIONS request to the CherryPy server. You can achieve this by using the $.ajax function or the lower-level XMLHttpRequest. This ensures that the browser sends the OPTIONS request every time, even if a cached response exists.

Your Turn! 💪

Now that you have a better understanding of why CORS POST requests work with plain JavaScript but not with jQuery, it's time to put this knowledge into action! If you encountered this issue, try implementing one of the proposed solutions and see if it resolves the problem. Don't forget to share your success story in the comments below or on social media using the hashtag #CORSPostRequests!

Remember, understanding these technical nuances and finding creative solutions is what makes us all masterful developers. Until next time, happy coding! ✨👩‍💻✨

Note: Remember to update the URLs and other details in the provided code examples to match your specific scenario.