index.go

package main

import "syscall/js"

func main() {
  document := js.Global().Get("document")
  button := document.Call("getElementById", "btn")
  cb1 := js.NewCallback(func(args []js.Value) {
    println("Clicked");
  })
  button.Call("addEventListener", "click", cb1)
  <-make(chan struct{}, 0)
}