Send request using fetch from HTML file served through golang

I am using the following code to serve a HTML file.

func main() {
	http.HandleFunc("/", func(rw http.ResponseWriter, r *http.Request) {
		path := r.URL.Path
		if path == "/" {
			path = "index.html"
		}

		http.ServeFile(rw, r, "./"+path)
	})

	http.ListenAndServe(":5555", nil)
}

This HTML file includes a JavaScript file that uses fetch to retrieve some data. This works fine when serving through apache, but not when served through the Go-server.

This is the fetch-request:

const fetchSettings = {
        method: "POST",
        body: JSON.stringify(requestBody),
        headers: {
            "Content-Type": "application/json",
        }
    };
const response = await fetch("https://some.url", fetchSettings);

And here is the error I'm getting:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://some.url. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://some.url. (Reason: CORS request did not succeed).

You need to include an Access-Control-Allow-Origin header:

rw.Header().Set("Access-Control-Allow-Origin", "*")

That one allows all origins, you can read more here: https://perennialsky.medium.com/handle-cors-in-golang-7c5c3902dc08

Here is how it would fit in your code:

func main() {
    http.HandleFunc("/", func(rw http.ResponseWriter, r *http.Request) {
        path := r.URL.Path
        if path == "/" {
            path = "index.html"
        }
        rw.Header().Set("Access-Control-Allow-Origin", "*")
        http.ServeFile(rw, r, "./"+path)
    })
    
    http.ListenAndServe(":5555", nil)
}