Very odd error, go api works with curl and bruno but not javascript fetch

Getting Help
1

I’m new to go. I’m liking go but I’m loosing my mind over this part.

Here is the relevant go code. All it does is send a tcp string command.

type CasparRequest struct {
		Command string
	}
	mux.HandleFunc("/casparconnecting", func(w http.ResponseWriter, req *http.Request) {
		enableCors(&w)

		connection, err := amcp.Dial("127.0.0.1:5250")
		if err != nil {

			// log.Fatal(err)

			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusBadRequest)

		}

		if connection == nil {

			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusBadRequest)
			w.Write([]byte("Connection has not been established"))
			return
		}
		defer connection.Close()
		var command CasparRequest

		err1 := json.NewDecoder(req.Body).Decode(&command)

		if err1 != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
			return
		}

		fmt.Println("the command sent", command.Command)

		code, replyData, err := connection.Do(command.Command)

		if err != nil {
			log.Fatal(err)

			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusBadRequest)
		}
		fmt.Println("replyCode", code)
		fmt.Println("replyCode", replyData)

		replyString := fmt.Sprintf("%v", replyData)

		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusOK) // Set your code here, after setting your headers
		w.Write([]byte(replyString))
		// defer c.Close()
	})

The following curl request works:
curl --header "Content-Type: application/json" --request POST --data '{"command":"PLAY 1-1 myfile000.mp4"}' http://localhost:8080/casparconnecting

However as soon as I do a request from the browser the server panicks


            const fetched = await fetch("http://localhost:8080/casparconnecting", {
                        headers: {
                            "Content-Type": "application/json",
                        },
                        method: "POST",
                        body: JSON.stringify({ command: "PLAY 1-1 myfile000.mp4" }),
                    });

This is the details of javascript POST request

{
  "args": {}, 
  "data": "{\"command\":\"PLAY 1-1 myfile000.mp4\"}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, deflate, br, zstd", 
    "Accept-Language": "en-GB,en-US;q=0.9,en;q=0.8", 
    "Connection": "keep-alive", 
    "Content-Length": "36", 
    "Content-Type": "application/json", 
    "Host": "localhost:9090", 
    "Origin": "http://localhost:5173", 
    "Referer": "http://localhost:5173/", 
    "Sec-Ch-Ua": "\"Chromium\";v=\"128\", \"Not;A=Brand\";v=\"24\", \"Google Chrome\";v=\"128\"", 
    "Sec-Ch-Ua-Mobile": "?0", 
    "Sec-Ch-Ua-Platform": "\"Linux\"", 
    "Sec-Fetch-Dest": "empty", 
    "Sec-Fetch-Mode": "cors", 
    "Sec-Fetch-Site": "same-site", 
    "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36"
  }, 
  "json": {
    "command": "PLAY 1-1 myfile000.mp4"
  }, 
  "method": "POST", 
  "origin": "172.17.0.1", 
  "url": "http://localhost:9090/anything"
}

This is the details of the curl request:

{
  "args": {}, 
  "data": "{\"command\":\"play 1-1 myfile000.mp4\"}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "*/*", 
    "Content-Length": "36", 
    "Content-Type": "application/json", 
    "Host": "localhost:9090", 
    "User-Agent": "curl/7.81.0"
  }, 
  "json": {
    "command": "play 1-1 myfile000.mp4"
  }, 
  "method": "POST", 
  "origin": "172.17.0.1", 
  "url": "http://localhost:9090/anything"
}

why is javascript request causing an issue here. Even if I copy the request as curl from the browser it works. But then I don’t understand how is this causing an error on the backend.
This is how my go code fails

2024/09/04 12:58:22 http: panic serving [::1]:34408: runtime error: invalid memory address or nil pointer dereference
goroutine 6 [running]:
net/http.(*conn).serve.func1()
/usr/local/go/src/net/http/server.go:1898 +0xbe
panic({0x6b2360?, 0x93e4f0?})
/usr/local/go/src/runtime/panic.go:770 +0x132
main.main.func3({0x785120, 0xc0000e6620}, 0xc0000ca360)
/home/john_doe/repos/basketball-backend-go/main.go:139 +0x7f5
net/http.HandlerFunc.ServeHTTP(0xc0000bab60?, {0x785120?, 0xc0000e6620?}, 0x651dda?)
/usr/local/go/src/net/http/server.go:2166 +0x29
net/http.(*ServeMux).ServeHTTP(0x467db9?, {0x785120, 0xc0000e6620}, 0xc0000ca360)
/usr/local/go/src/net/http/server.go:2683 +0x1ad
net/http.serverHandler.ServeHTTP({0xc0000b8cf0?}, {0x785120?, 0xc0000e6620?}, 0x6?)
/usr/local/go/src/net/http/server.go:3137 +0x8e
net/http.(*conn).serve(0xc0000fe000, {0x7856b0, 0xc0000b8c00})
/usr/local/go/src/net/http/server.go:2039 +0x5e8
created by net/http.(*Server).Serve in goroutine 1
/usr/local/go/src/net/http/server.go:3285 +0x4b4

Please help. I don’t understand where it could be going wrong.

2

ANSWER:

It was a cors error from the browser sending preflight request with a method of options. The browser does not show that in the network tab.
Using debugger in vscode helped

I was able to fix it with following snippet:

		if req.Method == "OPTIONS" {
			w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE")
			w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Origin, Accept, token")
			w.WriteHeader(http.StatusOK)
			return
		}
3

You might want to use a middleware library for handling CORS; it’s more complicated than meets the eye.

For instance, req.Method == "OPTIONS" is too restrictive, because it “traps” all OPTIONS requests, and not all OPTIONS requests are CORS-preflight requests; more on this topic elsewhere.

Shameless plug: GitHub - jub0bs/cors: perhaps the best CORS middleware library for Go

4

Wow thank you so much that is such a useful blog. I never knew! Quick look at MDN and stack overflow and I thought the options method catch was enough.

I was just trying to use only standard libraries to learn more since that’s what everyone recommends.
Thanks!

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