How to realize action on button click with script?

You understand everything.

P. S. I don’t need to set script enabled on click, I need to know how to realize it ONLY with script and where I should to attach it. There are no any code examples in docs.
P. P. S. UnityScript (JavaScript) only, please.

import UnityEngine.UI;

var myButton : Button;

function Start() {
	myButton.onClick.AddListener( onClickFunction );

function onClickFunction() {
	Debug.Log("Hello World");

Attach your button in the inspector!