![]() 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. #Cors preflight request fullA full list of possible CORS response headers are listed in the MDN Web Docs.access-control-max-age – tells the browser how long (in seconds) to cache the response to the preflight request.access-control-allow-headers – a comma-separated list of headers that the server will allow.access-control-allow-methods – a comma-separated list of methods that the server will allow.access-control-allow-origin – the origin that the server will allow.This could lead to console errors, for example, the image below shows the type of errors the Chrome browser generates if we try to make a GET request to from our website: 3.2. If the server doesn’t respond with this header or if the header value is a domain that doesn’t match the origin of the request then the browser will prevent the response from being passed back to the script. The value of this header can either be a single origin to tell the browser to allow access to that specific origin, or it can be * which tells the browser to allow any origin. The access-control-allow-origin header is one of the main CORS headers a server can use to show what requests it will allow. We can summarise this exchange in the image below: The browser will enforce this by allowing the requestHandler method to access the response data.The server responds with the requested data and also includes an access-control-allow-origin header set to which indicates to the browser that the server allows requests from this origin.First, the browser sends the request with an origin header identifying the origin to the server. ![]()
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