fetch() does not send headers?

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The same-origin policy restricts the kinds of requests that a Web page can send to resources from another origin.

In the no-cors mode, the browser is limited to sending “simple” requests — those with safelisted methods and safelisted headers only.

To send a cross-origin request with headers like Authorization and X-My-Custom-Header, you have to drop the no-cors mode and support preflight requests (OPTIONS).

The distinction between “simple” and “non-simple” requests is for historical reasons. Web pages could always perform some cross-origin requests through various means (such as creating and submitting a form), so when Web browsers introduced a principled means of sending cross-origin requests (cross-origin resource sharing, or CORS), it was decided that such “simple” requests could be exempt from the preflight OPTIONS check.

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