Browser not saving cookie sent by Golang backend

I know this question has been asked a bunch of times, but I tried most of the answers and still can't get it to work.

I have a Golang API with net/http package and a JS frontend. I have a function

func SetCookie(w *http.ResponseWriter, email string) string {
	val := uuid.NewString()
	http.SetCookie(*w, &http.Cookie{
		Name:     "goCookie",
		Value:    val,
		Path:     "/",
	})
	return val
}

This function is called when the user logs in, and I expect it to be sent to all the other endpoints. This works as expected with Postman. However, when it comes to the browser, I can't seem to get it to remember the cookie or even send it to other endpoints.

An example of JS using an endpoint

async function getDataWithQuery(query, schema){
    
    let raw = `{"query":"${query}", "schema":"${schema}"}`;
    let requestOptions = {
        method: 'POST',
        body: raw,
        redirect: 'follow',
    };
    try{
        let dataJson = await fetch("http://localhost:8080/query/", requestOptions)
        data = await dataJson.json();
    }catch(error){
        console.log(error);
    }

    return data;
}

I tried answers like setting SameSite attribute in Golang, or using credential: "include" in JS with no luck.

Thanks to the discussion in the comments, I found some hints about the problem.

Saving cookies (both API and frontend on the same host)

I used document.cookie to save the cookie. I set the options by hand since calling res.cookie on the response of the API fetch only returned the value. An example is document.cookie = `goCookie=${res.cookie}; path=/; domain=localhost;.

Sending cookies

This has been answered before in previous questions and answered again in the comments. The problem was that I used credential:'include' instead of the correct credentials:'include' (plural).

CORS and cookies

In case the API and the frontend are not on the same host you will have to modify both the API and the frontend.

frontend

The cookie has to have the domain of the API since it's the API that requires it, not the frontend. So, for security reasons, you can't set a cookie for a domain (API) from another domain (frontend). A solution would be redirect the user to an API endpoint that returns Set-Cookie header in the response header. This solution signals the browser to register that cookie with the domain attached to it (the API's domain, since the API sent it).

Also, you still need to include credentials:'include' in the frontend.

API

You will need to set a few headers. The ones I set are

	w.Header().Set("Access-Control-Allow-Origin", frontendOrigin)
	w.Header().Set("Access-Control-Allow-Credentials", "true")
	w.Header().Set("Access-Control-Allow-Headers", "Content-Type, withCredentials")
	w.Header().Set("Access-Control-Allow-Methods", method) // use the endpoint's method: POST, GET, OPTIONS

You need to expose the endpoint where the frontend will redirect the user and set the cookie in the response. Instead of setting the domain of the API by hand, you can omit it, the browser will fill it with the domain automatically.

To handle the CORS and let JS send the cookie successfully, you will have to set the SameSite=None and Secure attributes in the cookie and serve the API over https (I used ngrok to make it simple).

Like so

func SetCookie(w *http.ResponseWriter, email string) string {
	val := uuid.NewString()
	http.SetCookie(*w, &http.Cookie{
		Name:     "goCookie",
		Value:    val,
		SameSite: http.SameSiteNoneMode,
		Secure:   true,
		Path:     "/",
	})
   // rest of the code
}

I recommend you also read the difference between using localStorage and document.cookie, it was one of the problems I had.

Hope this helps.