[Golang] Variadic Function Example - addEventListener

GopherJS [2] is a compiler from Go to JavaScript, which helps you write front-end Go code running in browsers.

To register event listeners, we can use the following code to call addEventListener method:

foo := js.Global.Get("document").Call("getElementById", "foo")
// register onclick event
foo.Call("addEventListener", "click", func(event *js.Object) {
  // do something
}, false)
// or
foo.Call("addEventListener", "click", func(event *js.Object) {
  // do something
})

You can pass two or three arguments to addEventListener method. But the above code is really ugly. We want to wrtie the code in idiomatic Go way, so I implement the addEventListener as a Go variadic function, which accepts two or more arguments.

The implementation of addEventListener in Go via GopherJS is as follows:

func (o *Object) AddEventListener(t string, listener func(Event), args ...interface{}) {
      if len(args) == 1 {
              o.Call("addEventListener", t, listener, args[0])
      } else {
              o.Call("addEventListener", t, listener)
      }
}

where the type Object and Event is as follows:

type Object struct {
      *js.Object
}

type Event struct {
      *js.Object
}

JavaScript object is *js.Object in GopherJS, so the type Object and Event are in fact the wrapper for *js.Object.

Now we can register the event listeners in the following idiomatic Go way:

foo.AddEventListener("click", func(e Event) {
  // do something
})
// or
foo.AddEventListener("click", func(e Event) {
  // do something
}, false)

You can pass more than three arguments, but the additional arguments after third argument will not be used in my implementation.

Tested on: Ubuntu Linux 16.10, Go 1.8

References:

[2]	GopherJS - A compiler from Go to JavaScript (GitHub, GopherJS Playground, )