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![]() ![]() ![]() With preflight requests in place, browsers can check this and adjust accordingly.įinally, CORS is backward compatible. In addition to this, s ervers may change what kind of requests and headers they allow as they’re developed. If there is a side effect to a particular request which a server doesn’t allow from another origin the preflight request helps to protect the unconsenting servers by checking first and blocking the request if the server responds with headers that indicate its refusal. With preflight requests, servers can examine requests before they’re executed and get a chance to indicate if they allow them. Since CORS was proposed after the SOP had been in place for some time, the idea of using preflight requests presented some advantages. ![]() We can see a slightly different error in the browser console because the API did not respond to the preflight request with the needed headers: 3.3. Here we have an example in the browser where I am trying to access the Google Book API with a non-simple request that contains a custom header. If the server had responded without the necessary headers, the browser would have prevented the request from going out. Let’s take a look at how this interaction will look like if the server allows this kind of request:Īs we can see, the server responded with the correct headers and the browser continued to make the actual request. The browser will identify this request as non-simple and will initiate a preflight request to the server to check if it allows it. Xhr.setRequestHeader(‘custom-header', ’test') Let’s see this in action by modifying our previous request slightly, we can make it non-simple if we add a custom header: const xhr = new XMLHttpRequest() Similar to simple requests, if the server doesn’t include any of the CORS headers, the browser will assume that this server doesn’t allow this request and won’t continue with the actual request.
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