main.js

let outputHtmlObject; //HTML object that will be used to display tools
let unitConvertButton, mortgageButton, currencyButton, mainPageButton; //HTML button objects
let mainPageContent; //Object used to store content for the main page

//Function called when app loaded for the first time, to initialize all variables and add event listeners to all html objects
const initialize = () => {
    //Initialize all html objects that will be used in the application    
    outputHtmlObject = document.getElementById("output");
    mainPageButton = document.getElementById("mainPageButton");
    unitConvertButton = document.getElementById("unitConvertButton");
    mortgageButton = document.getElementById("mortgageButton");
    currencyButton = document.getElementById("currencyButton");

    //Add event listeners to all html objects
    mainPageButton.addEventListener("click", renderMainPage, false);
    unitConvertButton.addEventListener("click", unitConversion, false);
    unitConvertButton.addEventListener("click", unitConversion, false);
    mortgageButton.addEventListener("click", mortgageCalculator, false);
    currencyButton.addEventListener("click", currencyConverter, false);

    //Render main page when app is loaded for the first time
    renderMainPage();
};

//Function rendering main page content
const renderMainPage =  () => {
    if (mainPageContent === undefined) {
        mainPageContent = "<div class='container'><p class='description'>" +  
        "The app consists of three tools: <i>Unit Converter</i>, <i>Morgtgage Calculator</i> and <i>Currency converter</i>. <br /> Please note that you will need an access to the Internet to use" +
        " the Currency converter, because the exchange rates are fetched from a publicly available API. This app is created with vanilla JavaScript and JQuery</p></div>";
    }
    
    outputHtmlObject.innerHTML =  mainPageContent;
};

//Add load event listener to window object 
window.addEventListener("load", initialize, false);