Why Does My Browser Send an OPTIONS Request Before a POST Request?

When developing web applications, you might notice that before a POST request is sent, your browser first sends an OPTIONS request. This behavior is often confusing, especially when working with CORS (Cross-Origin Resource Sharing) and different types of requests. So, why does your client always send an OPTIONS request before a POST, and when does it happen?

Understanding Preflight Requests

When you send a POST request from your client-side JavaScript code, the browser may first send an OPTIONS request. This is known as a preflight request. Preflight requests are automatically triggered by the browser before it sends a real request in certain situations to check with the server whether the actual request is safe to send.

A preflight request occurs when the browser detects that the POST request doesn’t qualify as a “simple request”. So, the browser will first send an OPTIONS request to ensure that the server allows the actual request from the origin (domain) of the client.

What is a “Simple Request”?

A simple request in CORS refers to an HTTP request that meets the following criteria:

1. Method: The request method is either GET, POST, or HEAD.

2. Headers: The request only uses certain allowed headers, such as Content-Type, Accept, and a few others.

3. Content-Type: The Content-Type must be one of the following:

• application/x-www-form-urlencoded

• multipart/form-data

• text/plain

If your request meets these conditions, the browser considers it “simple” and won’t send an OPTIONS preflight. However, if the request contains custom headers (like Authorization or X-Custom-Header) or uses a content type like application/json, the browser will send a preflight OPTIONS request first.

Why Does My Browser Send OPTIONS Before POST?

In your case, the browser is likely sending an OPTIONS request before the POST because your POST request doesn’t qualify as a “simple request.” For example:

• Custom Headers: If your POST request includes headers like Authorization or custom headers, the browser will trigger a preflight.

• Content Type: If you’re sending data as application/json (instead of application/x-www-form-urlencoded or multipart/form-data), the browser will also trigger a preflight request.

These preflight requests exist to ensure that your server allows the cross-origin request before the client actually sends the POST request. If the server doesn’t respond correctly with the allowed methods, origins, or headers, the actual request will be blocked by the browser.

What is a HEAD Request?

Another method you might see in HTTP is the HEAD request. This is similar to a GET request but with one key difference: the server only returns the headers of the response and no body. It’s often used to check if a resource exists, to see metadata (like content length or type), or to test a URL without downloading the actual data.

Conclusion: Why Does My Browser Send OPTIONS Requests?

Your browser sends an OPTIONS request to perform a preflight check when the POST request doesn’t meet the criteria for a simple request. This is a security feature built into CORS to prevent unauthorized cross-origin requests. The server responds to the OPTIONS request, allowing the browser to proceed with the actual POST if the request is allowed.

Understanding how preflight requests work helps you debug issues related to CORS and cross-origin requests more effectively, ensuring your web application communicates securely with external servers.