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Learn React by building a web app — Week 1

TLDR: I've made some good progress in my first week, I need to tidy a few bits up and would like to start looking at applying some patterns, but I'm happy with what I have achieved so far. Take a look at the end of week one section to see the final code.

I've just completed my first week learning React by building an application and it has gone better than expected 👐

This post is a run-through of what I spent my week working on rather than a guide to how I did it.

What features will be in the first iteration?

Last week I decided on some of the features of the app. This week, for the first iteration, I wanted to

  • Get a basic app up and running on CodeSandbox.
  • Break sections of the app into components.
  • Display a list of coins.
  • Provide a way for the user to mark a coin as "owned".
  • Have filters for coin denomination and showing "owned" & "needed" coins.
  • Retrieve data from a database (stretch goal).
  • Require users to log in so that their data can be saved to a database (stretch goal).
  • Set up continuous integration (stretch goal).
  • Look into unit testing best practices in React (stretch goal).

Choosing a name

Before I could start creating the project I had to come up with a name. I needed a name which isn't in use in any of the app stores already and something which is easy to remember. I came up with Coinsly, which is nice and short and, I think, sounds "app like".

Starting out

I started by using the Create-React-App sandbox on CodeSandbox, this provided a great starting point — no need to add npm dependencies, configure webpack config, or set up linting tasks — it's all done for you.

My thinking was that If Create-React-App becomes too restrictive then I'll run the eject task so I can tweak the config in any way I see fit. Hopefully I won't have to do that for a while yet as I'd like to stay focused on building the app rather than having to dig into webpack configuration.

Component details

There are a few conventions which I picked up and used throughout my components.

Arrow functions on class properties

I started using arrow functions on class properties for event handler functions quite early on as it means that you do not have to bind the context for the function in the constructor.

So rather than this:

class App extends Component {
  constructor() {


  handleFilterSubmit () {

You can do this instead:

class App extends Component {
  handleFilterSubmit = e => {

Much clearer and less code to write.

Types of components

There are a few styles you can use to write components in React, I used two of the most common; class and stateless functional components. I'll describe where I used each in the component details below.


PropTypes were added to each component to ensure that while developing the app the correct required & optional props were being passed. This saved me a lot of headaches as DevTools shows a warning when you forget to pass a prop or pass the wrong type.


Create-React-App supports async/await out of the box so I went ahead and used the feature in place of the promise syntax across the entire codebase.

The components

Right, on with the component details!

Based on my plan above I figured that I'd need five components in total. I'll describe each one in detail below starting with the lowermost child and working up.

Coin Component

This is a stateless functional component which displays details for an individual coin as well as providing a way for the user to mark the coin as "owned".


  • coin - an object which holds the details of a single coin.
  • owned - an array of all of the coin ID's the user has marked as "owned". Has a default value of [].
  • handleOwnedChange - this function is called when a coin is marked as "owned". It updates the owned property in state.

Not much to say about this component other than it sets the checked property of the owned checkbox by looking up the coin ID in the owned state:

  checked={owned.find(o => o.coinId === !== undefined}

Filters component

This is a stateless functional component which contains the filter elements allowing the user to filter which coins are visible.


  • filter - a string which is set to the current filter. Has a default value of 'all'.
  • handleSubmit - a function which is called when submitting the filters form. It updates the filter property in state.
  • handleChange - this function is called when selecting a filter. It updates the filter property in state.

CoinList component

This is a stateless functional component which handles filtering and displaying the list of coins. Each coin in the filtered list is then passed to a Coin component.

If there are no coins to display then a message is shown to the user.


  • coins - an array of all the coins before any filters have been applied. Has a default value of [].
  • owned - an array of all of the coins the user has marked as "owned". Has a default value of [].
  • filter - a string which is set to the current filter.
  • handleOwnedChange - a function which is passed down and used in the Coin component.
  • handleSubmit - a function which is called when submitting the coin list form.

There are three functions which live in the same module as this component; filterCoins, filterNeeded, and filterOwned.

The filterCoins function calls one of the other two methods based on the filter string passed to it. If the filter is 'all' then it returns the coin list unfiltered. If the filter is onlyNeeded then the filterNeeded function is called returning only the coins which the user has marked as "needed". Finally, if the filter is onlyOwned the then filterOwned function is called returning only the coins which the user has marked as "owned".

I'm not completely happy with these functions living in the same module as the CoinList component so I'll most likely move them out into a helper module in the future, that way I can keep the modules focused and can unit test the logic a little easier.

Header component

The header component is a stateless functional component which displays the app title at the top of the page. I didn't really have to break this out into a component but I was thinking that more could be added in the future such as the logged in users name.


  • title - the name of the app.

App component

The top-level component which, at the moment, holds all of the state for the app. It also passes down event handlers to child components.

App is a class component as it holds state and makes use of the componentDidMount lifecycle method to load data and handle authentication events.

Initial State

These properties have mostly been covered already in the component props details above but I'll add descriptions again here for reference.

  • user - an object containing the logged-in users details.
  • isLoading - a boolean which handles displaying a loading indicator to the user.
  • coins - an array of all the coins in the app, before any filters have been applied.
  • owned - an array of all of the coin ID's the user has marked as "owned".
  • filter - a string which is set to the current filter. Values can be 'all', 'onlyNeeded', or 'onlyOwned'.


Once the component has mounted an event listener is added which checks for firebase authentication changes (more on this in the next section). Once it detects that a user has logged in it'll load in the coin and owned data from the database and add it to state.

login() & logout()

These two functions call firebase to handle the authentication steps. login will add the user details into state, and logout will reset state back to its initial ..state?!

Render() method

This is where we tell React how we want all of the imported components to be displayed and which props we'd like to pass to each.

I made use of conditional logic a fair bit here, for example the loading indicator is displayed if state.isLoading is true:

  this.state.isLoading && <p>Loading</p>

React will only render the HTML if this.state.isLoading has a truthy value.

index module

Not a component, but included here for completeness, this is the entry point for the app where we tell React where and what it should render.

Firebase — Stretch goal!

I've looked into hosted database options for web apps in the past and have always felt like there was a mountain of things to learn, loads of config to set up, and the API's tend to be quite complicated when it comes to interacting with the database itself.

Then, whilst looking at the source code for Kent C. Dodds' Repeat TODO app, I noticed he was using Firebase so I decided to check it out.

I found a great tutorial by Simon Bloom on CSS Tricks called Intro to Firebase and React. Following the steps in the article I was able to set up an app with basic authentication and the ability to talk to a database within 30 minutes.

Because I had such a good experience setting up Firebase I looked at their database offerings and chose to use a Cloud Firestore database. Firestore is in beta but it's heavily documented and, so far, I haven't had any issues with it.

Firebase Authentication

Firebase provides a few different ways to implement authentication into your application. I went for a Google auth provider which allows users to register or log in with their Google account. The code to get this working was very simple:

import firebase from 'firebase'

export const GoogleAuthProvider = new firebase.auth.GoogleAuthProvider()
export const auth = firebase.auth()

export const logIn = async () => {
  try {
    const result = await auth.signInWithPopup(GoogleAuthProvider)
    return result.user
  } catch (ex) {
    return null

export const logOut = async () => {
  await auth.signOut()

In the componentDidMount lifecycle method of the App component I added a small bit of code which was able to detect when a user has signed in:

async componentDidMount() {
  auth.onAuthStateChanged(async user => {

That's all it took to implement some basic authentication into the app!

Adding data

I wanted to add some coin data to the database so I created a firebaseService.js module which exposed the Firestore database object:

import firebase from 'firebase'

export const db = firebase.firestore()

Then I created a coinService.js module which contained a function that could add one coin to the store at a time using the database object:

import { db } from './firebaseService'

export const addCoin = async ({ denomination, name, year, order }) =>
  await db.collection('coins').add({ denomination, name, year, order })

I passed my coin data to this function one coin at a time until it was all added, which took less than a second!

Building up the services

Next, I created a few other services to handle retrieving and adding data for the "owned" coins list and user details.

This took a while as I had to reference the Firestore documentation a fair bit to get things working as I wanted them to.

All of the calls to the database use promises so I took the opportunity to use async/await everywhere.

Coin data structure

  denomination: string,
  name: string,
  year: number,
  order: number

Owned coins data structure

  coinId: string,
  userId: string

When a user is signed in I retrieve all of the records from this collection which have their user ID and then match those records against the coins displayed using coinId to show that they are "owned".

User data structure

  userId: string,
  displayName: string,
  email: string,
  photoURL: string,
  creationTime: string,
  lastSignInTime: string

The reason I added a userId property rather than use the generated document ID was because a user ID is passed back when the user logs in when using a sign-in provider which should be the same for a single user regardless of which provider they use to sign in.

Unit tests — Stretch goal!

I really wanted to start looking into unit testing best practices in React at this point, and coincidentally Kent C. Dodds had just sent out a newsletter which covered a new unit testing library which he just released which he described as "a very light-weight solution for testing React components". If you haven't realised already I am a huge fan of Kent and his work!

I followed the readme on the GitHub repository and wrote a couple of basic tests but left it at that point as I feel like I need to do some more reading up on writing React unit tests.

Continuous integration — Stretch goal!

I set up a TravisCI build which runs the yarn build and yarn test tasks against NodeJS versions 7, 8, and 9 whenever changes are pushed to the GitHub repository.

It also pushes the unit test coverage report up to Coveralls after a successful test run. My hope is that these reports will spur me on to write more tests and get the coverage up!


It wasn't all plain sailing this week, I did hit a few issues along the way.

State woes

I spent far too long trying to access state in child components which I'd set in the App component, completely forgetting that state is private to a component 🤦

Once I (finally) remembered that state in React is private to the component it is defined in I started passing the data down to the child components via props.

Filter logic

I fumbled with the format of the filter state for a while, originally I had each filter type as a separate property of a filter object in state:

filter: {
  all: true,
  onlyNeeded: false,
  onlyOwned: false

The filter elements were checkboxes which were a poor choice as it meant that when a filter was selected I had to call a function which set the active/inactive values for the state properties, with those states being applied to the checked attribute of each checkbox.

This clearly wasn't going to scale well if I wanted to add more filters in the future as I'd have to create a new property in state and then update the function to see which filter was currently active.

To get around this I created a single state property called filter which holds a string description of the current filter to be applied:

  filter: 'all'

Then I switched the checkboxes to radio buttons and grouped them together using the name attribute — this is standard HTML, nothing React specific here.

Each radio button then sets the checked attribute based on the filter state value

  checked={filter === 'all'}

  checked={filter === 'onlyNeeded'}

  checked={filter === 'onlyOwned'}

Snapshot testing in CodeSandox

One of the unit tests I wrote used the .toMatchSnapshot() assertion, this caused an issue in CodeSandbox as it isn't able to create snapshot files.

To get around this I had to clone the repository onto my laptop and run the unit tests with the -u flag (yarn run tests -u) to create the snapshot files, then check the updated files in. Once this was done the test passed in CodeSandbox.

Prop drilling

At the end of the week I ended up with most of the app logic and all of the handler functions defined in the App component which means that there is some prop drilling happening in order to pass state and event handlers down to child components.

Next week I plan to do some reading up on this subject to find out what the best practices are around this.

End of week one

You can check out the code on the GitHub repo, or view and run it in the CodeSandox window below.

I feel like I've made some good progress in my first week, I even managed to work on all of the stretch goals! The app is far from finished and I need to tidy a few bits up, but I'm happy with what I have achieved so far.

The app doesn't look great at the moment as I haven't put a design together yet and haven't written any styles. I'd like to figure out what this CSS in JS thing is all about before starting work on this.

Until next week ✌️

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