Cookieの設定

// Cookieを使うときはOriginは直接指定する
w.Header().Set("Access-Control-Allow-Origin", "http://localhost:3000")
w.Header().Set("Access-Control-Allow-Credentials", "true")

c := new(http.Cookie)
c.Name = "key"
c.Value = "value"
// cookieのSame-SiteをStrictに指定
c.SameSite = http.SameSiteStrictMode
http.SetCookie(w, c)

const res = await fetch(`http://localhost:8000`, {
  // credentialsをincludeに設定
  credentials: "include"
})