How to send an string to server using fetch API

I am using the fetch API to make a POST request to my server written in Go...

fetch('http://localhost:8080', {
	method:'POST',
	headers: {
		'Accept': 'application/json',
		'Content-Type': 'application/json'
	},
	body: JSON.stringify({
		image:"hello"
	})
})
.then((response) => response.json())
.then((responseJson) => {
	console.log("response");
})
.catch(function(error) {
	console.log(error);
})

On my Go server I am receiving the POST...

type test_struct struct {
    Test string
}

func GetImage(rw http.ResponseWriter, req *http.Request, _ httprouter.Params) {
    var t test_struct

    if req.Body == nil {
	    http.Error(rw, "Please send a request body", 400)
	    return
    }
    err := json.NewDecoder(req.Body).Decode(&t)
    fmt.Println(req.Body)
    if err != nil {
	    http.Error(rw, err.Error(), 400)
        return
    }
    fmt.Println(t.Test);
}

The POST request is made. I know this because fmt.Println(t.Test) is printing a blank line to the console.

fmt.Println(req.Body) gives me <nil> in the console.

The only error message I get is from the fetch API. It prints to the console the following error...

SyntaxError: Unexpected end of input

this is coming from the .catch declaration.

In short, how can I receive the string on the server?

When you decode req.Body, you can't read again the request body, because it's already read the buffer, for that reason you're getting the message SyntaxError: Unexpected end of input. If you want to use the value on req.Body, you must to save the content of req.Body in a variable, something like this.

buf, err := ioutil.ReadAll(r.Body)
// check err for errors when you read r.Body
reader := bytes.NewReader(buf)
err = json.NewDecoder(reader).Decode(&t)
fmt.Println(string(buf))
if err != nil {
    http.Error(rw, err.Error(), 400)
    return
}

Also, your JSON is like {"image":"hello"}, so your struct should be:

type test_struct struct {
    Test string `json:"image"`
}

If you want to map the image value into Test