HelloWorld.webapp-hello.js

// Refer to UI elements
var button = document.getElementById('MyButton');
var txtInput = document.getElementById('MyInputText');

// Bind click event of the button with an event listerner
button.addEventListener('click', function () {
    'use strict';
    var text = txtInput.value;

    // Show alert box with this text
    window.alert(text);
})();