examples/dom/add_event_listener.html
<button id="btn">Click me</button>

<div id="display"></div>

<script src="add_event_listener.js"></script>

examples/dom/add_event_listener.js
"use strict";

function clicked() {
    document.getElementById('display').innerHTML = 'Hello World';
}

document.getElementById('btn').addEventListener('click', clicked);