How to Avoid Setting Custom Headers in the 'Access-Control-Request-Headers' Header in jQuery AJAX POST Requests?

Handling Complex Headers in AJAX POST Requests with jQuery

Background:

In AJAX POST requests using jQuery, custom headers can be added to enhance communication with the server. The challenge lies in observing that custom headers are being added to the less commonly observed "Access-Control-Request-Headers" header instead of the intended headers.

Explanation:

When a browser sends an AJAX request to a different domain than the one it originated from, the browser first sends a "preflight" request (OPTIONS method) to check if the server allows the custom headers. This is done to prevent cross-site scripting (XSS) attacks.

As part of this preflight request, the browser adds the "Access-Control-Request-Headers" header, which lists the custom headers that will be sent in the actual request. The server responds with a "204 No Content" status code and an "Access-Control-Allow-Headers" header specifying which custom headers are allowed.

Solution:

To avoid this behavior and set custom headers directly in the POST request, you can explicitly set the headers in the "beforeSend" function of the jQuery Ajax call. This ensures that the headers are added to the request before it is sent, bypassing the preflight request process.

Here's an example of how to set a header in a jQuery Ajax call:

$.ajax({
  type: "POST",
  beforeSend: function(request) {
    request.setRequestHeader("Authority", authorizationToken);
  },
  url: "entities",
  data: "json=" + escape(JSON.stringify(createRequestObject)),
  processData: false,
  success: function(msg) {
    $("#results").append("The result =" + StringifyPretty(msg));
  }
});

By setting the header in the "beforeSend" function, jQuery will add the "Authority" header to the actual POST request, enabling you to pass custom headers and receive the expected server response.

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