How do I send values from golang to vue js (server to client)?

super newbie developer here. So say I have a variable (key) (i.e. type string) in golang that I want to send client-side (Vue js). I've tried sending it locally but Vue js isn't able to read it. So I'm 100% sure I'm doing it wrong within golang.

Would I need to POST it to a local server (ex: localhost:3001) and GET it from vue js?
How should I send this POST request in go? Are there better options?

Snippet of current vue code :

<template>
  <div class="about">
    <h1>This is an about page</h1>
    <p>Lorem</p>
  </div>

      <div v-if="key !== null">
    <p>{{key}}</p>
    </div>
    <div v-else>
      <p>Waiting for key....</p>
    </div>
    
</template>

<script>
export default {
  props: ['id'],
  data() {
    return {
      job: null,
      key: null,
    }
  },
  mounted() {

    fetch('http://localhost:3001', {
                    method: 'GET'
                })
      .then(res => res.json())
      .then(data => this.key = data.message)
      .catch(err => console.log(err.message))
  }
}
</script>

Thank you!

EDIT: My current golang code

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"
)

type ResponseData struct {
	Message string `json:"message"`
}

func getData(w http.ResponseWriter, r *http.Request) {
	json.NewEncoder(w).Encode(ResponseData{Message: "Hello, World"})
}

func main() {
	http.HandleFunc("/", getData)

	fmt.Println("Server listening on port 3001")
	log.Panic(
		http.ListenAndServe(":3001", nil),
	)
}

When I go http://localhost:3001/, I am able to see the message {"message":"Hello, World"}. However, I am still not receiving anything when I run a GET command on vue. What am I doing wrong in golang? Also, how do I program it so that I can send my own variable to the getData() function and display it on port 3001? Thank you.

Without seeing exactly how your Go code is written it's difficult to fully understand what you're doing on that side, especially since you stated:

> So I'm 100% sure I'm doing it wrong within golang

However, as an example to fill in the blank, I'm going to make the assumption you're doing something like so:

type ResponseData struct {
    Message string `json:"message"`
}

func getData(w http.ResponseWriter, r *http.Request) {
    // CORS
    w.Header().Set("Access-Control-Allow-Origin", "*")
    w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS, PUT, DELETE")
    w.Header().Set("Access-Control-Allow-Headers", "Accept, Content-Type, Content-Length, Authorization")


    json.NewEncoder(w).Encode(ResponseData{ Message : "Hello, World" }
}

From within your Vue application you'll need to extract that "message" data from the returned data response as using just "data" will include other information that isn't relevant to the actual "message" such as the status code.

fetch('http://localhost:3001')
  .then(res => res.json())
  .then(data => this.key = data.message)
  .catch(err => console.log(err.message))

I'm also assuming that your Go server is listening on port 3001.