Origin null is not allowed by Access-Control-Allow-Origin

Origin null is the local file system, so that suggests that you’re loading the HTML page that does the load call via a file:/// URL (e.g., just double-clicking it in a local file browser or similar). Most browsers apply the Same Origin Policy to local files by disallowing even loading files from the same directory as the document. (It used to be that … Read more

Trying to use fetch and pass in mode: no-cors

I can hit this endpoint, http://catfacts-api.appspot.com/api/facts?number=99 via Postman and it returns JSON Additionally I am using create-react-app and would like to avoid setting up any server config. In my client code I am trying to use fetch to do the same thing, but I get the error: No ‘Access-Control-Allow-Origin’ header is present on the requested … Read more

What’s the point of the X-Requested-With header?

A good reason is for security – this can prevent CSRF attacks because this header cannot be added to the AJAX request cross domain without the consent of the server via CORS. Only the following headers are allowed across origins: Accept Accept-Language Content-Language Last-Event-ID Content-Type any others cause a “pre-flight” request to be issued in CORS supported browsers. … Read more

XMLHttpRequest cannot load XXX No ‘Access-Control-Allow-Origin’ header

tl;dr — There’s a summary at the end and headings in the answer to make it easier to find the relevant parts. Reading everything is recommended though as it provides useful background for understanding the why that makes seeing how the how applies in different circumstances easier. About the Same Origin Policy This is the Same Origin Policy. It is a … Read more

How to resolve ‘preflight is invalid (redirect)’ or ‘redirect is not allowed for a preflight request’

Short answer: Make the request URL in your code isn’t missing a trailing slash. A missing-trailing-slash problem is the most-common cause of the error cited in the question. But that’s not the only cause — just the most common. Read on for more details. When you see this error, it means your code is triggering your browser … Read more