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Fundamentals of Web Apps

Traditional Web Applications

Most of the code below represents bad practice nowadays.

This is the code running on this website. It dynamically generates the HTML code based on the (changing) number of notes.

const getFrontPageHtml = (noteCount) => {
  return `
    <!DOCTYPE html>
        <div class='container'>
          <h1>Full stack example app</h1>
          <p>number of notes created ${noteCount}</p>
          <a href='/notes'>notes</a>
          <img src='kuva.png' width='200' />
app.get("/", (req, res) => {
  const page = getFrontPageHtml(notes.length);

Going to the notes page, we find out that the head-section of the HTML contains a script-tag, which makes your browser load a JavaScript file called main.js:

var xhttp = new XMLHttpRequest();
// this fetches the .json object from the server and puts its contents into list.
// this function is a callback function, i.e. a function executed by the browser
// at the appropriate time, i.e. when the event has occured.
xhttp.onreadystatechange = function () {
  // checks whether operation is complete and if the HTTP status is 200
  if (this.readyState == 4 && this.status == 200) {
    const data = JSON.parse(this.responseText);
    // cause your console to log
    // create an unordered list (ul)
    var ul = document.createElement("ul");
    ul.setAttribute("class", "notes");
    // witht the following list elements (li)
    data.forEach(function (note) {
      var li = document.createElement("li");
};"GET", "/data.json", true);

Document Object Model (DOM)

We can think of HTML-pages as tree structures. In the case of dynamic webpages, DOM is an Application Programming Interface (API) that acts as an interface between JavaScript and the object-oriented document (of which the browser renders a HTML representation) itself. Within a webpage, JavaScript can:

  • add, change, and remove any of the HTML elements and attributes
  • change any of the CSS styles
  • react to all the existing events
  • create new events

Manipulate the document-object from console

Turns out that we can manipulate the object from the browser console:

Transclude of 2020-06-05_09-36-20_screenshot.png"

The newly created note will render but disappear on a reload of the page since we have not pushed the change to the server yet.

Cascading Style Sheets (CSS)

CSS is a markup language that determines the style of a webpage. The main.css file that is loaded as a stylesheet in the HTML on our example page defines two class selectors. Class selectors match elements based on the contents of their class attributes. Here is a general example followed by the main.css file.

/* All elements with class="spacious" */
.spacious {
  margin: 2em;
/* All <li> elements with class="spacious" */
li.spacious {
  margin: 2em;
/* All <li> elements with a class list that includes both "spacious" and "elegant" */
/* For example, class="elegant retro spacious" */
li.spacious.elegant {
  margin: 2em;
.container {
  padding: 10px;
  border: 1px solid;
.notes {
  color: blue;

There are also other attributes than class. For example, there is the id attribute which is used by JavaScript to find the element.

Summary on how the browser loads a page containing JavaScript

Let’s revisit how the interplay between browser and server unfolds when the page is opened.

  1. Fetch the HTML code defining the content and basic structure of the page using a HTTP GET request
  2. Links in the HTML cause other stuff to be fetched, in this case:
    • the stylesheet main.css and
    • the JavaScript file main.js
  3. Now, the JavaScript code is executed. Another HTTP GET request is made to fetch the JSON Data from
  4. When the data is fetched, the browser executes the event handler, which renders the notes to the page using the DOM-API.

Forms and HTTP POST

Using the form on the notes page to add a new note causes five HTTP requests:

  1. The first is an HTTP POST request to the server address ending in new_note to which the server replies with HTTP status code 302. This is a URL redirect, with which the server is asked to do a
  2. new HTTP GET request to the address defined in the header’s Location, i.e. the address notes:
  3. Fetching main.css
  4. Fetching main.js
  5. Fetching notes data.json

In the HTML code the, form tag has attributes action and method, which stipulate that submitting the form is done as an HTTP POST request to the address new_note.

On the server, we now need some simple code to handle the POST request. Remember: This is code running on the server and not on the browser.

// data is sent as the body of the POST request which
// can be accessed by the server via the req.body field
// of the request object (req)"/new_note", (req, res) => {
  // create new note object and add it to array along
  // with date
    content: req.body.note,
    date: new Date(),
  return res.redirect("/notes");

The server does not save the array containing content and date to a database, so new notes vanish when Heroku restarts the service


The notes page from above uses a concept called Asynchronous JavaScript and XML (AJAX) which became popular in the early 2000s. It describes a revolutionary approach based on the advancements in browser technology that enabled the fetching of content using JavaScript included within the HTML without the need to rerender the entire page. On Wikipedia, it says that

With Ajax, web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows web pages and, by extension, web applications, to change content dynamically without the need to reload the entire page.

Prior to AJAX, everything that one could see in the browser was rendered HTML-code generated by the server that had to be rerendered once something changed on the server.

While the notes page uses AJAX to display the notes, the application URLs (.../data.json, .../new_note) reflect bad practice nowadays as they don’t follow the coventions of RESTful APIs, which will be covered later.

Nowadays, everything uses AJAX so the term has become somewhat meaningless.

Single Page Applications (SPA)

While “traditional” web applications have all the logic running on the server with the browser continually rerendering the HTML it fetches, SPAs consist of a single HTML page, the contents of which are manipulated with JavaScript that executes in the browser.

The notes page is almost there. However, as we saw above, adding new notes still depends on the server dealing with a POST request and instructing the browser to reload the page with a 302 redirect.

In the SPA version of the notes page, we give the form an id tag such that we can write JavaScript that handles the note creation process. Now, upon filling out the form, the browser sends just one request with the content-type application/json:

Transclude of 2020-06-06_09-26-28_screenshot.png"

Let’s take a look at the part of JavaScript code in spa.js dealing with the form submission:

// fetch the form-element from the page and register
// event handler to handle the form submit
var form = document.getElementById("notes_form");
form.onsubmit = function (e) {
  // prevent default handling of form, i.e. GET request
  // create a new note
  var note = {
    date: new Date(),
  // add it to the notes list
  notes.push(note);[0].value = "";
  // rerender the note list
  // send note to server

Nonetheless, even the SPA version of the page does not adhere to current best practices.

JavaScript Libraries

The application above is mainly coded in pure or “vanilla” JavaScript as it only uses the DOM-API and built-in JavaScript features to manipulate the structure of the page. There is a range of libraries containing tools that make it easier to interact with the DOM-API. Some of the most popular include:

  • jQuery was used mainly back in the day for its cross-browser support but fell out of favour once VanillaJS and browsers can do most/all of the stuff it offered.
  • After BackboneJS, AngularJS became the de-facto standard of modern webdevelopment after its initial release by Google in 2012. Since Angular 2 was not designed to be backwards compatible with prior versions, it became less popular.
  • Today, the most popular tool for implementing the browser-side logic of web-applications is Facebook’s React library

Exercises 0.1.-0.6.

0.1 HTML

Review the basics of HTML by reading this tutorial from Mozilla.

0.2: CSS

Review the basics of CSS by reading this tutorial from Mozilla.

0.3: HTML forms

Learn about the basics of HTML forms by reading Mozilla’s tutorial “Your first form”.

0.4: new note

In chapter Loading a page containing JavaScript - revised the chain of events caused by opening the page is depicted as a sequence diagram

The diagram was made using websequencediagrams service as follows:

browser->server: HTTP GET
server-->browser: HTML-code
browser->server: HTTP GET
server-->browser: main.css
browser->server: HTTP GET
server-->browser: main.js
note over browser:
browser starts executing js-code
that requests JSON data from server
end note
browser->server: HTTP GET
server-->browser: [{ content: "HTML is easy", date: "2019-05-23" }, ...]
note over browser:
browser executes the event handler
that renders notes to display
end note

Create a similar diagram depicting the situation where the user creates a new note on page by writing something into the text field and clicking the submit button.

If necessary, show operations on the browser or on the server as comments on the diagram.

The diagram does not have to be a sequence diagram. Any sensible way of presenting the events is fine.

All necessary information for doing this, and the next two exercises, can be found from the text of this part. The idea of these exercises is to read the text through once more, and to think through what is going on where. Reading the application code is not necessary, but it is of course possible.

0.5: Single Page App

Create a diagram depicting the situation where the user goes to the single page app version of the notes app at

0.6: New Note

Create a diagram depicting the situation where user creates a new note using the single page version of the app.

This was the last exercise, and it’s time to push your answers to GitHub and mark the exercises as done in the submission application.

My solutions to the exercises are published here.

Intro to React

Debugging React Apps

Rules of Hooks

Never use useState or useEffect inside a loop. Only ever call hook from inside a function body defining a React component:

const App = (props) => {
  // these are ok
  const [age, setAge] = useState(0)
  const [name, setName] = useState('Juha Tauriainen')
  if ( age > 10 ) {
    // this does not work!
    const [foobar, setFoobar] = useState(null)
  for ( let i = 0; i < age; i++ ) {
    // also this is not good
    const [rightWay, setRightWay] = useState(false)
  const notGood = () => {
    // and this is also illegal
    const [x, setX] = useState(-1000)
  return (

Event Handlers

Assume we develop this application:

const App = (props) => {
  const [value, setValue] = useState(10);
  return (
      <button>reset to zero</button>
ReactDOM.render(<App />, document.getElementById("root"));

The clicking of a button should reset the state stored in the value variable.

Thus, we need an even handler. Now, this is important: An event handler must always be a function or a reference to a function. Nothing else works. Don’t use a function call, i.e. setValue(0) either (there is one exception, see below). Only functions and references to functions such as:

<button onClick={() => setValue(0)}>button</button>


const App = (props) => {
  const [value, setValue] = useState(10);
  // defining the function
  const handleClick = () => {
    console.log("clicked the button");
  return (
      // referencing the function (not calling it)
      <button onClick={handleClick}>button</button>

You can also define an event handler to use a function that returns another function. Function inception. Then you can actually use a function call in the event handler because the returned object is a function. You can use this functionality if you need generic functionality (e.g. greetings via the console) with parameters (e.g. user names):

const App = (props) => {
  const [value, setValue] = useState(10);
  const hello = (who) => {
    const handler = () => {
      console.log("hello", who);
    return handler;
  return (
      <button onClick={hello("world")}>button</button>
      <button onClick={hello("react")}>button</button>
      <button onClick={hello("function")}>button</button>

A more compact way to write the hello function would be:

const hello = (who) => () => {
  console.log("hello", who);

We can also pass event handlers to child components. So let’s extract the button into its own component:

const Button = (props) => (
  // make sure the attributes correspond to the props passed to the component
  <button onClick={props.handleClick}>{props.text}</button>
const App = (props) => {
  const [value, setValue] = useState(10)
  const setToValue = (newValue) => {
  return (
      <button handleClick={() => setToValue(1000) text="thousand"}>button</button>
      <button handleClick={() => setToValue(0) text="reset"}>button</button>
      <button handleClick={() => setToValue(value +1) text="increment"}>button</button>

Another important rule is to never define components within components. It’s nasty.

Communicating with Server

Rendering a collection, modules

What’s the difference between an experienced JavaScript programmer and a rookie? The experienced one uses console.log 10-100 times more.

JavaScript const, let and var

  • was slightly confused and then read this article which cleared up my understanding. It boils down to the fact that var should be avoided.
  • Both let and const are block-scoped and hoisted to the top. While variables declared with let can be updated but not redeclared, const-declared variables can be neither. However, you can update the property of a const object as so:
const greeting = {
  message: "say Hi",
  times: 4,
greeting.message = "Say Hello instead";

Functional Programming in JavaScript

  • Watch this video series. It’s pretty good. Also watch this for a great explanation of map and reduce.
  • In every functional programming language, functions are values and you can exploit this by dividing your code into small and simple functions that can be composed together using higher order functions such filter, reject, map or reduce. See the following example for filter:
var animals = [
    { name: 'Tom', species: 'dog' },
    { name: 'Timmy', species: 'dog' }
    { name: 'Tony', species 'bird'}
// old-school for-loop to filter dogs
let oldSchoolDogs=[]
for (var i = 0; i < animals.length; i++) {
    if (animals[i].species === 'dog')
// much cooler way
let isDog = function(animal) {
    return animal.species === 'dog'}
let dogs = animals.filter(isDog)
let nonDogs = animals.reject(isDog)
  • map does not throw objects out of an array like filter or reject based on a Boolean value. Instead it transforms them:
// create new array only with the names and do some other transformation
let names = (animal) {
  return + " is a " + animal.species;
// Let's do the same as an arrow function
let namesArrow = => {
// Get rid of return and brackets as it is a single expression in the function body
let namesArrowShort = =>;
  • reduce is the swiss-army knife of list transformations. You can always fall back on it when the other built-in higher-order functions don’t solve your problem.
let orders = [
  { amount: 250 },
  { amount: 400 },
  { amount: 100 },
  { amount: 325 },
let totalAmount = orders.reduce(function (sum, order) {
  return sum + order.amount;
}, 0);

Anti-pattern: array indexes as keys

Avoid using the index of an array within the map function in React. See here for more infos. Possibly use shortid for ID-creation.

  {, i) => (
    <li key={i}>{note.content}</li>

Do it like this instead:

  { => (
    <li key={}>{note.content}</li>


  • See the example projects (esp. phonebook)

Getting data from server

  • Watch this video about event loops in JavaScript.
  • See the example projects in github and their final solutions (esp. countries)

Altering data in server

  • Routes are URLs + HTTP request types
  • Never mutate state directly. If a state is an array use a method like concat to create a new array (see notes app)
  • You need to understand promises. They are an integral part of modern JavaScript. The third Chapter of You Don’t Know JS and this article should be read and understood.

Adding styles to React app

Programming a Server with NodeJS and Express


  • see this article for a definition of the different levels of RESTful maturity. In this course, we are mostly operating at level 2.
    1. Resources
    3. Multimedia

Difference between SQL and NoSQL

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Testing Express Servers, User Administration

Testing React Apps

State Management with Redux

React Router, Custom Hooks, Styling App with CSS and Webpack