React Advanced

🙋‍♂️

Frontend engineer

9 years FE experience
AngularJS, Angular, Ember, React
currently Staff engineer at Productboard

❤️ 📸

❤️ 🛞

Slides + code

https://slides.com/nuc

https://github.com/MartinNuc

Schedule

9-12 morning
- recap of the previous day
- breaks: 10:00, 11:00
12-13 lunch
13-17 afternoon
- breaks: 14:00, 15:00, 16:00

Course

exercises build on top of each other!
discussions welcomed
on Wednesday if we have time let's do a project
- multipage
- tested
- optimized
- correct project structure

Your experience?

Setup IDE

Install

Google Chrome
nodejs
- https://nodejs.org/en/
Visual Studio Code
- https://code.visualstudio.com
React dev tools

🧠 React basics

components, component tree
props, events
styling the app
children props
controlled input
useState, useEffect
making HTTP requests
custom hooks
React Context

➡️ Let's make http request

open API request in browser to see structure of response
- https://api.chucknorris.io
display joke in the component
create a button to load another joke
disable button when loading the joke

GET https://api.chucknorris.io/jokes/random

// create a new project
npx create-react-app react-playground --template typescript

// install axios
npm i axios

npm start

Let's do more React! 💪

React hooks

✅ useState
✅ useEffect
✅ useContext
useRef
useImperativeHandle
useCallback
useMemo
useReducer

useRef

manipulate with DOM elements
object with mutable current property

import { useRef } from 'react';

function Component() {
  const inputRef = useRef<HTMLInputElement>(null);
  
  function handleClick() {
    inputRef.current?.focus();
  }
  
  return <div>
    <input ref={inputRef} />
    <button onClick={handleClick}>Focus the input</button>
  </div>
}

Child to parent?

Parent

Child

do something

Control child component?

Parent

Child

do something

Expose element from
a component

ref cannot be used as prop
need to wrap component with forwardRef

function Parent() {
  const inputRef = useRef<HTMLInputElement>(null);

  const handleClick = () => inputRef.current?.focus();

  return <>
    <Child ref={inputRef} />
    <button onClick={handleClick}>
      Focus
    </button>
  </>
}

const Child = forwardRef<HTMLInputElement>((props, ref) => {
  return <div>
    <input ref={ref} />
  </div>
})

Expose imperative API

used to expose API of Child component to the Parent

function Parent() {
  const childApiRef = useRef<{ focus: () => void }>(null);
  const handleClick = () => childApiRef.current?.focus();

  return <>
    <Child ref={childApiRef} />
    <button onClick={handleClick}>
      Focus
    </button>
  </>
}

const Child = forwardRef<{ focus: () => void }>((props, ref) => {
  const inputRef = useRef<HTMLInputElement>(null);
  useImperativeHandle(ref, () => ({
    focus: () => inputRef.current?.focus()
  }), [inputRef])

  return <div>
    <input ref={inputRef} />
  </div>
})

React hooks

✅ useState
✅ useEffect
✅ useContext
✅ useRef
✅ useImperativeHandle
useCallback
useMemo
useReducer

Performance

Problem

lot of rerenders
every render creates new function, object etc
DOM operations are expensive

React Virtual DOM

virtual representation of DOM = big object
React updates the virtual DOM
then creates a diff agains the real DOM
and applies only necessary changes to the real DOM

Profiler in dev tools

import { useState } from "react";
import { AlbumItem } from "./AlbumItem";

export type Album = {
  id: number;
  title: string;
  rating: number;
}

export const AlbumVoting = () => {
  const [albums, setAlbums] = useState(new Array(15).fill({
    id: 0,
    title: '',
    rating: 0
  }).map((album, index) => ({ ...album, id: index })));

  function handleChange (updatedAlbum: Album) {
    setAlbums(albums => albums.map(album => album.id === updatedAlbum.id ? updatedAlbum : album));
  };

  return <div>
    {albums.map(album => <AlbumItem key={album.id} album={album} onChange={handleChange} />)}
  </div>
}

import { Album } from "./AlbumVoting";

type Props = {
  album: Album,
  onChange: (updated: Album) => void
}

export const AlbumItem = ({ album, onChange }: Props) => {

  function handleChange(change: Partial<Album>) {
    onChange({
      ...album,
      ...change
    });
  }

  return <p>
    Title:
    <input value={album.title}
      onChange={(e) => handleChange({ title: e.target.value })}
    />

    Rating
    <input type='number'
      value={album.rating}
      onChange={(e) => handleChange({ rating: Number(e.target.value) })}
    />
  </p>
}

AlbumVoting.tsx

AlbumItem.tsx

shows rerenders
shows why

React.memo

rerenders component only on prop change

const ExpensiveComponentMemoized = React.memo(function ExpensiveComponent() {
  ...
});
  
<ExpensiveComponentMemoized />

useMemo

precompute value
for computation-expensive values
avoids main thread lock

const useFibonacci = (n) => {
  const result = useMemo(() => fibonacci(n), [n]);
  
  return result;
}
                         
function fibonacci(n) {
  return n < 1 ? 0
       : n <= 2 ? 1
       : fibonacci(n - 1) + fibonacci(n - 2)
}

useCallback

used to retain a single function reference
avoids problem with recreating handler every render

type Props = {
  me: string;
}

function Component({me}: Props) {  
  const handleClick = useCallback(
    (name) => console.log(`Hello ${name} and ${me}`)
  , [me]);
  
  return <ExpensiveComponent onClick={handleClick} />;
}

➡️ Optimize album voting

only row which is updated should rerender

import { useState } from "react";
import { AlbumItem } from "./AlbumItem";

export type Album = {
  id: number;
  title: string;
  rating: number;
}

export const AlbumVoting = () => {
  const [albums, setAlbums] = useState(new Array(15).fill({
    id: 0,
    title: '',
    rating: 0
  }).map((album, index) => ({ ...album, id: index })));

  function handleChange (updatedAlbum: Album) {
    setAlbums(albums => albums.map(album => album.id === updatedAlbum.id ? updatedAlbum : album));
  };

  return <div>
    {albums.map(album => <AlbumItem key={album.id} album={album} onChange={handleChange} />)}
  </div>
}

import { Album } from "./AlbumVoting";

type Props = {
  album: Album,
  onChange: (updated: Album) => void
}

export const AlbumItem = ({ album, onChange }: Props) => {

  function handleChange(change: Partial<Album>) {
    onChange({
      ...album,
      ...change
    });
  }

  return <p>
    Title:
    <input value={album.title}
      onChange={(e) => handleChange({ title: e.target.value })}
    />

    Rating
    <input type='number'
      value={album.rating}
      onChange={(e) => handleChange({ rating: Number(e.target.value) })}
    />
  </p>
}

AlbumVoting.tsx

AlbumItem.tsx

React hooks

✅ useState
✅ useEffect
✅ useContext
✅ useRef
✅ useImperativeHandle
✅ useCallback
✅ useMemo
useReducer

React Context

Context

"global" state for subtree of components
avoids passing props deep
Provider + Consumer

type ContextValue = boolean;

const MyContext = React.createContext<ContextValue>(false);

function App() {
  return <MyContext.Provider value={true}>
    <Component />
  </MyContext.Provider>;
}

function Component() {
  const value = useContext(MyContext);
  
  return <div>{value}</div>;
}

Context

pass object with value + setter

type ContextValue = {
  value: number;
  setValue: (value: ContextValue['value']) => void
};

const MyContext = React.createContext<ContextValue>({} as unknown as ContextValue);

function App() {
  const [value, setValue) = useState(0);
  
  return <MyContext.Provider value={{ value, setValue}}>
    <Component />
  </MyContext.Provider>;
}

function Component() {
  const {value, setValue} = useContext(MyContext);
  
  return <>
    <div>{value}</div>
    <button onClick={() => setValue(value + 1)}>Click</button>
  </>;
}

Encapsulate context

Provider component instead of App component

type ContextValue = {
  value: number;
  setValue: (value: ContextValue['value']) => void
};

// not exported - private
const MyContext = React.createContext<ContextValue>({} as unknown as ContextValue);

export function MyContextProvider({children}: PropsWithChildren) {
  const [value, setValue) = useState(0);
  
  return <MyContext.Provider value={{ value, setValue}}>
    {children}
  </MyContext.Provider>;
}
// only way to read the value:
export const useMyContext = () => useContext(MyContext);

// in another file
import {useMyContext} from './my-context';

function Component() {
  const { value, setValue } = useMyContext();
  
  return <>
    <div>{value}</div>
    <button onClick={() => setValue(value + 1)}>Click</button>
  </>;
}

<MyContextProvider>
   <Component />
</MyContextProvider>

Context initial state

pass via props

type ContextValue = {
  value: number;
  changeValue: (newValue: ContextValue['value']) => void;
}

const MyContext = React.createContext<ContextValue>({} as unknown as ContextValue);

type Props = {
  initialState: ContextValue['value'];
  children: React.ReactNode
}

export function MyContextProvider({initialState, children}: Props) {
  const [value, setValue] = useState(initialState);
  
  return <MyContext.Provider value={{value, setValue}}>
    {children}
  </MyContext.Provider>;
}

<MyContextProvider initialState={5}>
   ...
</MyContextProvider>

➡️ UserContext

create a context for session with values:
- user
  - username
  - email
- login = sets the user
- logout = sets user to null
create login component with login
create component CurrentUserInfo showing currently logged user & logout button

export function UserInfoPane() {
  const { user } = useUser();

  return <div>
    {user ? <CurrentUserInfo /> : <LoginForm />}
  </div>
}

export function LoginForm() {
  const { login } = useUser();
  const [username, setUsername] = useState('');
  const [email, setEmail] = useState('');

  return <div>
    <input placeholder="username" value={username} onChange={(e) => setUsername(e.target.value)} />
    <input placeholder="email" value={email} onChange={(e) => setEmail(e.target.value)} />

    <button onClick={() => login({username, email})}>Login</button>
  </div>
}

type UserContext = {
  user: User | null,
  login: (user: User) => void;
  logout: () => void;
}

const useUser = () => useContext(UserContext);

➡️ UserContext

export function UserInfoPane() {
  const { user } = useUser();

  return <div>
    {user ? <CurrentUserInfo /> : <LoginForm />}
  </div>
}

export function LoginForm() {
  const { login } = useUser();
  const [username, setUsername] = useState('');
  const [email, setEmail] = useState('');

  return <div>
    <input placeholder="username" value={username} onChange={(e) => setUsername(e.target.value)} />
    <input placeholder="email" value={email} onChange={(e) => setEmail(e.target.value)} />

    <button onClick={() => login({username, email})}>Login</button>
  </div>
}

type UserContext = {
  user: User | null,
  login: (user: User) => void;
  logout: () => void;
}

UserContextProvider

UserInfoPane

CurrentUserInfo

LoginForm

= missing

const useUser = () => useContext(UserContext);

❗️Impact on performance

Change of the context triggers rerender

Testing

unit/component testing
integration testing
e2e testing

Jest

testing framework
https://jestjs.io
provides assertions, mocks, test runner

import {add} from './add';

it('adds numbers', () => {
  expect(add(1, 3)).toBe(4);
})

import {add} from './add';

test('adding two numbers', () => {
  expect(add(1, 3)).toBe(4);
})

How to run tests?

just execute jest
- to run them repeatedly: --watch
looks for files named *.spec.js / *.test.js
for Typescript it needs further configuration
- in create-react-app just run ➡️ npm test script
- looks for files named *.spec.ts / *.test.ts

Arrange / Act / Assert

import {add} from './add';

it('adds numbers', () => {
  expect(add(1, 3)).toBe(4);
})

import {add} from './add';

it('adds numbers', () => {
  // Arrange
  const a = 1;
  const b = 3;
  // Act
  const result = add(a, b);
  // Assert
  expect(result).toBe(4);
})

arrange = prepare for tests (inputs, mocks...)
act = run the tested unit
assert = check the results (output, mocks)

Structuring tests

describe
- beforeEach
- test
- test
- test
describe
- beforeEach
- afterEach
- test
- describe
  - test
  - test

expect(add(1, 3)).toBe(4);

subject

matcher

Matchers

.toBe() = exact match (like ===)
.toEqual() = deep equal
.toBeNull(), toBeUndefined()
.toContain() = is the item in an array?
.toContainEqual()

import {add} from './add';

it('adds numbers', () => {
  expect(add(1, 3)).toBe(4);
})

Negation

.not.toBe()
.not.toEqual()
.not.toBeNull()
...

import {add} from './add';

it('adds numbers', () => {
  expect(add(1, 3)).not.toBe(5);
})

Match substring

toEqual(expect.stringContaining(...))

expect('How are you?').toEqual(expect.stringContaining('How'));

Match objects

.toEqual({...})
.toEqual(expect.objectContaining({ key: value }))
.toHaveProperty(key, value)

expect({a: 1, b: 2}).toBe({a: 1, b: 2}); // ❌ DON'T
expect({a: 1, b: 2}).toEqual({a: 1, b: 2}); // ✅ DO

expect({a: 1, b: 2}).toEqual(expect.objectContaining({a: 1}));

expect({a: 1, b: 2}).toHaveProperty('a', 1);
expect({
  one: 1,
  two: {
    nested: 2
  }
}).toHaveProperty('two.nested', 2);

Match arrays

.toEqual([...])
.toEqual(expect.arrayContaining([...]))

expect([1,2,3]).toBe([1,2,3]); // ❌
expect([1,2,3]).toEqual([1,2,3]); // ✅

expect([1,2,3]).toEqual(expect.arrayContaining([1,2]));

Match exceptions

expect( () => {...} ).toThrow()
expect( () => {...} ).toThrow('Error message')
expect( () => {...} ).toThrow(MyError)

expect(testedFn()).toThrow(); // ❌ DON'T

expect(() => testedFn()).toThrow(); // ✅ DO

Total number of asserts

expect.assertions(2)
used usually for async code

function doNTimes(fn, n) {
  for(let i = 0 ; i < n; i++) {
    fn();
  }
}

// ❌ DON'T - better to use mocks (later)
it('calls the function n times', () => {
  expect.assertions(3);
  function fn() {
    expect(true).toBe(true);
  }

  doNTimes(fn, 3)
})

➡️ Update package.json

{
  "name": "...",
  "jest": {
    "transformIgnorePatterns": [
      "/node_modules/(?!(axios))"
    ]
  },
  ...
}

➡️ Update App.test.tsx

test.skip('renders learn react link', () => {
  render(<App />);
  const linkElement = screen.getByText(/learn react/i);
  expect(linkElement).toBeInTheDocument();
});

➡️ Fix yellow warning

npm i -D @babel/plugin-proposal-private-property-in-object

➡️ Run tests

npm test

➡️ Test the division

each test should test only one thing
test the happy path
test edge cases

export function divideWithRemainder(a: number, b: number) {
  if (b === 0) {
    throw new Error('Cannot divide by zero.');
  }
  
  const result = Math.ceil(a / b);
  const remainder = a % b;
  
  return {
    result,
    remainder
  };
}

Mocks

also called spies
lets you spy on the behavior
can also replace existing behavior
- you should mock the boundary of your system

Creating a mock

const fn = jest.fn(); // empty function

// pass implementation
const fn = jest.fn(() => {
  return 5;
});

// override implementation
fn.mockImplementation(() => {
  return 6;
});
fn.mockReturnValue(5);
fn.mockResolvedValue(5); // returns promise which resolves in 5

fn.mockReturnValueOnce(1);
fn.mockReturnValueOnce(2);
fn.mockReturnValue(3);
fn() // 1
fn() // 2
fn() // 3
fn() // 3

create mock
define its behavior

Mock matchers

const fn = jest.fn(() => 42); // empty function
expect(fn).toHaveBeenCalled(); // fails
fn();
expect(fn).toHaveBeenCalled(); // passes

expect(fn).toHaveBeenCalledTimes(1);

fn(1,2,3)
expect(fn).toHaveBeenLastCalledWith(1,2,3);

expect(fn).toHaveReturnedWith(42); // test output

evaluates if mock has been called

➡️ Test doNTimes

use mocks

function doNTimes(fn, n) {
  for(let i = 0 ; i < n; i++) {
    fn();
  }
}

Spy on existing

jest.spyOn(Math, 'random');
Math.random();
expect(Math.random).toHaveBeenCalled();

// change behavior
const randomMock = jest.spyOn(Math, 'random');
randomMock.mockReturnValue(1);
expect(Math.random()).toBe(1);

possible only on instances of classes or objects
spyOn(object, methodName)

Mocking modules

used to mock 3rd party library
jest.mock hoists up before all imports

import { fetchUser } from './fetch-user';
import axios from 'axios';

jest.mock('axios');
const mockedAxios = axios as jest.Mocked<typeof axios>;

it('fetches user data', async () => {
  const response = { data: { id: 1, name: 'Martin' } };
  mockedAxios.get.mockResolvedValue(response);

  expect(await fetchUser()).toEqual({ id: 1, name: 'Martin' });
});

import axios from 'axios';

function fetchUser() {
  return axios.get('/url').then((response) => response.data);
}

React testing

Testing React

testing components, hooks
test should be as close as possible to how user will use the component
- test what he sees
- test how he interacts (mouse, keyboard etc)
mock boundary of the system
- mock HTTP, local storage etc.
- Jest uses JSDOM to emulate browser

🔨 React testing library

provides API for:
1. querying the DOM
2. doing user actions
3. matchers
https://testing-library.com/docs/

1. Queries

Query DOM

get = 1 match immediately
query = 0 or 1 match immediately
find = 1 match, waiting

Type of Query	0 Matches	1 Match	>1 Matches	Retry (Async/Await)
Single Element
getBy...	Throw error	Return element	Throw error	No
queryBy...	Return null	Return element	Throw error	No
findBy...	Throw error	Return element	Throw error	Yes
Multiple Elements
getAllBy...	Throw error	Return array	Return array	No
queryAllBy...	Return [ ]	Return array	Return array	No
findAllBy...	Throw error	Return array	Return array	Yes

Query By

Accessibility
- getByRole - by ARIA role
- getByLabelText = good for forms
- getByPlaceholderText = good for forms without labels
- getByText = what user sees, good outside of forms
- getByDisplayValue = value of an input
HTML semantic
- getByAltText
- getByTitle
Test IDs
- getByTestId = for invisible elements or dynamic text

import { screen } from '@testing-library/react';

screen.getByRole('button', {name: /submit/i});

Query by role

check all ARIA roles
default roles (W3 ARIA spec)
- button - "button"
- a - "link"
- h1...h6 - "heading"
accessible name
- usually what you want :-)
- label for form, alt for image...

getByRole('button', { name: 'Submit' });

// // checked input type checkbox
getByRole('checkbox', { checked: true });

 // usually h2
getByRole('heading', { level: 2 });

Query by text

screen.getByText('Hello World'); // full string match
screen.getByText('llo Worl', {exact: false}); // substring match
screen.getByText('hello world', {exact: false}); // ignore case

screen.getByText(/World/); // regexp

// custom function
screen.getByText((content, element) => content.startsWith('Hello'));

what user sees

Query by test-id

<div class="background red" data-testid="background">
  ...
</div>

const backgroundEl = screen.getByTestId('background');

test by HTML attribute data-testid
used when the text is dynamic
used when element has no content

Query within element

useful when we want to scope query in specific area

import { screen, within } from '@testing-library/react';

const container = screen.getByTestId('container');
const helloMessage = within(container).getByText('hello');

waitFor

waits for element to appear
- or disappear (waitForElementToBeRemoved)
prefer findBy*

import { screen, waitFor } from '@testing-library/react';

await waitFor(() => {
  expect(screen.getByText('Some text')).toBeInTheDocument();
});

2. User actions

fireEvent

fires DOM event on an element
fires only a single event
- fireEvent.click doesn't trigger mouseDown, mouseUp

// click on an element
fireEvent.click(screen.getByText('Login'));

// change input value
fireEvent.change(getByLabelText(/username/i), {target: {value: 'martin'}});

3. Matchers

Matchers

from jest-dom library
makes tests easy to read

// check text content
const element = screen.getByTestId('title');
expect(element).toHaveTextContent('This is title');

// check if is in the document
const element = screen.queryByText('Submit');
expect(element).toBeInTheDocument();

// test focus
const input = screen.getByTestId('password');
expect(input).toHaveFocus();

// test checkbox state
const rememberPass = screen.getByRole('checkbox', {name: 'Remember password'});
expect(rememberPass).toBeChecked();

Render component

import { render } from '@testing-library/react';

it('renders', () => {
  render(<App />);
  const element = screen.queryByText('Hello');
  expect(element).toBeInTheDocument();
});

Debugging tests

screen.debug()
screen.logTestingPlaygroundURL()

import { screen } from '@testing-library/react';

➡️ Test UserInfoPane

1. render -> check LoginForm is visible

test:
- current user
- login
- logout

➡️ Test Joke component

mock the server response
test:
- loading state
- the fetched joke is displayed
- button to load the next joke
- button should be disabled when fetching

{
  "name": "...",
  "jest": {
    "transformIgnorePatterns": [
      "/node_modules/(?!(axios))"
    ]
  },
  ...
}

package.json

Testing hooks

renderHook

simulates component lifecycle
prefer testing hook when testing the component
act() simulates work of React
- wrap when you expect something to change

import { act, renderHook } from "@testing-library/react";
import { useCounter } from "./use-counter";

it('increments', () => {
  const {result} = renderHook(() => useCounter());
  expect(result.current.count).toBe(0);
  act(() => result.current.increment());
  expect(result.current.count).toBe(1);
})

export function useCounter() {
  const [count, setCount] = useState(0);
  const increment =() => setCount(count + 1);
  return { count, increment };
}

➡️ Test useCounter hook

export function useCounter(initialValue: number) {
  const [count, setCount] = useState(initialValue);

  const increment = () => setCount(count + 1);
  const randomize = () => setCount(Math.random());

  return { count, increment, randomize };
}

Render props

Children

</WrapperComponent>

Render props

wrapping component renders something
composition pattern

function Component() {
  return <MousePosition render={
    (x, y) => <p>Position is x={x}, y={y}</p>
  } />
}

type Props = {
  render: (x: number, y: number) => React.ReactNode;
}

function MousePosition({ render }: Props) {
  ...
  
  return <>{render(position.x, position.y)}</>;
}

Render props using children

export function MyComponent() {
  function setWallet(n: number) { /* ... */ }
  
  return (
    <DropdownButton label="Menu">
      <button onClick={() => setWallet(10)}>+10</button>
      <button onClick={() => setWallet(30)}>+30</button>
      <button onClick={() => setWallet(50)}>+50</button>
    </DropdownButton>
  );
}

export function DropdownButton({ label, children }: PropsWithChildren<Props>) {
  const [isOpen, setIsOpen] = useState(false);

  return (
    <div style={{ position: "relative" }}>
      <button onClick={() => setIsOpen(!isOpen)}>{label}</button>
      {isOpen && (
        <div
          style={{
            position: "absolute",
            background: "white",
            padding: "10px",
            border: "1px solid black",
          }}
        >
          {children}
        </div>
      )}
    </div>
  );
}

Render props using children

export function MyComponent() {
  function setWallet(n: number) { /* ... */ }
  
  return (
    <DropdownButton label="Menu">
      {({close}) => <>
        <button onClick={() => setWallet(10)}>+10</button>
        <button onClick={() => setWallet(30)}>+30</button>
        <button onClick={() => setWallet(50)}>+50</button>
        <button onClick={close}>X</button>
      </>}
    </DropdownButton>
  );
}

Render props using children

type DropdownApi = {
  close: () => void;
};
type Props = {
  label: string;
  children: (api: DropdownApi) => React.ReactNode;
};

export function DropdownButton({ label, children }: Props) {
  const [isOpen, setIsOpen] = useState(false);

  function close() {
    setIsOpen(false);
  }

  return (
    <div style={{ position: "relative" }}>
      <button onClick={() => setIsOpen(!isOpen)}>{label}</button>
      {isOpen && (
        <div
          style={{
            position: "absolute",
            background: "white",
            padding: "10px",
            border: "1px solid black",
          }}
        >
          {children({ close })}
        </div>
      )}
    </div>
  );
}

Error Boundary

Error boundary

catches errors from render/component life cycle
catches errors in Error Boundary children
create-able only using the class component
wrap any part of the app with error boundary

<ErrorBoundary>
  <Joke />
</ErrorBoundary>

Error Boundary

import React, { PropsWithChildren } from "react";

type Props = PropsWithChildren;

type State = {
  hasError: boolean;
  error?: unknown;
}

export class JokeErrorBoundary extends React.Component<Props, State> {
  constructor(props: Props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error: unknown) {
    return { hasError: true, error };
  }

  // optional
  componentDidCatch(error: unknown, errorInfo: ErrorInfo) {
    console.error(error, errorInfo);
  }

  render() {
    const {error, hasError} = this.state;
    if (hasError) {
      // fallback
      return <div>{String(error)}</div>;
    }

    return this.props.children; 
  }
}

➡️ Use ErrorBoundary

create "error" component (code below)
create a global error boundary
when something bad happens it shows
- "Upss, something went wrong"
- + retry button

export function ThrowComponent() {
  
  if (Math.random() < 0.5) {
    throw 'Something went wrong.'
  };

  return <p>Didn't throw</p>
}

Routing

React router

used to create multiple pages
install react-router-dom + type definitions
- npm i react-router-dom @types/react-router-dom
docs

Building blocks

Layout page
App.tsx
router

Routable pages

Navbar

Layout.tsx

import { Outlet } from "react-router";

export function Layout() {
  return <div>
    <NavBar />

    <Outlet />
  </div>
}

template for the root component

Here other routes render

Routable pages

App.tsx

import { RouterProvider, createBrowserRouter } from 'react-router-dom';

const router = createBrowserRouter([
  ... // (later)
]);

function App() {
  return (
    <RouterProvider router={router} />
  );
}

Router

const router = createBrowserRouter([
  {
    path: '/',
    element: <Layout />,
    children: [
      {
        index: true,
        element: <Home />
      },
      {
        path: 'about',
        element: <About />
      },
      {
        path: 'articles',
        element: <ArticlesContainer />,
        children: [
          {
            path: ':articleId',
            element: <Article />
          }
          {
            path: ':articleId/comments',
            element: <ArticleComments />
          }
        ]
      }
    ]
  }
]);

Must have <Outlet /> inside

Layout component

Nested routes

components can define subroutes

Routable pages

Navbar

Routable pages

Navigation using links

<Link to="/">Home</Link>
<Link to="/about">About</Link>
<Link to="/articles">Articles</Link>
<Link to="/articles/{articleId}">Specific article</Link>

Imperative navigation

const navigate = useNavigate();

navigate('/articles');

using useNavigate() hook

Reading url parameters

import { useParams } from "react-router-dom";

function Joke() {
  const params = useParams();
  
  return (
    {params.category}
  );
}

/path-segment/:pathParam/something?query1=one&query2=two

useSearchParams

useParams

➡️ Create multiple pages

NavBar
- should be always visible
- links to home & categories routes
UserContextPane should be always visible
Create routes
- /-> what we had before
- /categories -> categories list
- /categories/:category -> joke from a category

Sources

// joke.tsx
import axios from "axios";
import { useEffect, useState } from "react";
import { useParams } from "react-router-dom";

type JokeResponse = {
  value: string;
};

export function Joke() {
  const [joke, setJoke] = useState<string | null>(null);
  const { category } = useParams();

  useEffect(() => {
    const queryParams = new URLSearchParams();
    if (category) {
      queryParams.set('category', category);
    }
    axios<JokeResponse>(
      `https://api.chucknorris.io/jokes/random?${queryParams.toString()}`
    ).then((response) => setJoke(response.data.value));
  }, [category]);

  return <p>{joke}</p>;
}

// joke-categories.tsx
import axios from "axios";
import { useEffect, useState } from "react";

export function JokeCategories() {
  const [categories, setCategories] = useState<string[]>([]);

  useEffect(() => {
    axios<string[]>("https://api.chucknorris.io/jokes/categories").then(
      (response) => setCategories(response.data)
    );
  }, []);

  return <>
    {/* TODO */}
  </>;
}

// navbar.tsx
import { Link } from "react-router-dom"

export const NavBar = () => {
  return <ul>
    <Link to="/">Home</Link>
    <Link to="/categories">Joke categories</Link>
  </ul>
}

SWR

stale-while-revalidate

SWR

library for caching HTTP requests
cache shared across all components
npm i swr

import useSWR from 'swr'

const fetcher = (...args) =>
	fetch(...args).then(res => res.json());

function Joke() {
  const { data, isValidating } = useSWR(
    `https://api.chucknorris.io/jokes/random`,
    fetcher
  );
  
  return (
    <p>{isValidating ? 'Loading...' : data.value}</p>
  );  
}

Invalidate SWR cache

import useSWR, {useSWRConfig} from 'swr'

const fetcher = (...args) =>
	fetch(...args).then(res => res.json());

function Joke() {
  const { data, isValidating } = useSWR(
    `https://api.chucknorris.io/jokes/random`,
    fetcher
  );
  const { mutate } = useSWRConfig();
  
  return <div>
    <button onClick={() => mutate(`https://api.chucknorris.io/jokes/random`)}>
      Load next
    </button>
    <p>{isValidating ? 'Loading...' : data.value}</p>
  </div>;  
}

isValidating vs isLoading

isLoading = fetching & not loaded yet
isValidating = fetching (can be loaded from the last time)

Suspense

used to wait for something to finish
usually, we wait for the data to be loaded
or for JS chunk to load
Suspense wraps component which gets suspended

function Parent() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <ComponentWithLoading />
    </Suspense>
  )
}

How does it work?

suspendable component must be wrapped with <Suspense>
suspendable component throws an exception with a Promise in the render
until the promise is resolved, Suspense shows fallback
when the promise resolves, Suspense tries to render the component again

➡️ Suspend & SWR

fetch joke using useSWR
enable Suspense support for SWR
wrap Joke component with Suspense showing "Loading with Suspense"

const { data, isValidating } = useSWR(url, fetcher, { suspense: true });

const { data, isValidating } = useSWR(url, fetcher);

Split app into chunks

Chunks

by default we have only main chunk
- contains the whole app
better to usually split the app by routes
- user doesn't have to load code he doesn't need
vendor chunk = for 3rd party libraries

How to create chunk

combine React.lazy & import()

// default export
const ComponentLazy = React.lazy(
  () => import('./ComponentDefaultExport')
);

// rename named export as default
const ComponentLazy = React.lazy(() => 
  import('./ComponentNamedExport')
    .then(({ComponentNamedExport}) => ({default: ComponentNamedExport}))
);

function Parent() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <ComponentLazy prop="value" />
    </Suspense>
  );
}

Support in React Router

chunk per route
expects key Component

const router = createBrowserRouter([
  {
    path: '/',
    element: <Layout />,
    children: [      {
        path: 'about',
    	// key must be "Component"
        lazy: () => 
          import("./about").then(({ About }) => ({Component: About}))
      },
    ]
  }
]);

Must be under Component key

React Portal

Portal

used to render JSX anywhere in the DOM
everything works as if the JSX was rendered as normal
useful for modal dialog (z-index, positioning)

createPortal

first argument - JSX
second argument - DOM element

import {createPortal} from 'react-dom';

function Component() {
  return <div>
    {createPortal(<div>Hello</div>, document.body)}
  </div>
}

➡️ Create a modal dialog

create a component that renders children in a modal window
position it in the middle of the screen

<Modal>
  <h1>Hello</h1>
  <p>I am modal</p>
</Modal>

Strict mode

checks for common mistakes
only in dev mode
runs effects twice
renders components twice

➡️ What's wrong?

type Props = {
  children: React.ReactNode[];
}

type ExpandedState = Record<number, boolean>;

export function Accordeon({ children }: Props) {
  const [expandedState, setExpandedState] = useState<ExpandedState>({});

  const toggleExpansionHandler = useCallback(
    (index: number) => {
      setExpandedState(expandedState => {
        if (expandedState[index]) {
          delete expandedState[index];
        } else {
          expandedState[index] = true;
        }

        return {
          ...expandedState,
        };
      });
    },
    [],
  );

  return <>
    {React.Children.map(children, (child, index) => (
      <div>
        <button onClick={() => toggleExpansionHandler(index)}>
          {expandedState[index] ? '➖' : '➕'}
        </button>
        {child && expandedState[index] && child}
      </div>
    ))}
  </>;
}

React hooks

✅ useState
✅ useEffect
✅ useContext
✅ useRef
✅ useImperativeHandle
✅ useCallback
✅ useMemo
useReducer

useReducer

alternative to useState
better for complex logic
good when there are several useState depending on each other
"simple" redux built-in React

Example: counter

type State = number;

type Increment = { type: 'increment'; payload: number };
type Randomize = { type: 'randomize' };
type Action = Increment | Randomize;

function reducer(state: State, action: Action) {
  switch (action.type) {
    case "increment":
      return state + 1;
    case "randomize":
      return Math.random();
    default:
      return state;
  }
}

export function Counter() {
  const [ state, dispatch ] = useReducer(reducer, 0);

  return <div>
    Counter: {state}
    <button onClick={() => dispatch({ type: 'increment', payload: 1 })}>Increment</button>
    <button onClick={() => dispatch({ type: 'randomize' })}>Randomize</button>
  </div>
}

Initial state

➡️ useReducer in UserContext

write reducer with following actions
- user login
- user logout
- session timeout
  - 10 seconds after login, the session times out and logs the user out
  - show how much time is left

type State = {
  user: User | null;
  timeLeft: number
}

type Action = LOGIN | LOGOUT | SESSION_TICK;

Redux

state management library
one global store
- big object
actions to modify the state
browser extension: Redux Devtools
npm install @reduxjs/toolkit react-redux

Store

Actions
Reducers

Store example

{
  jokeSlice: {
    currentJoke: "Chuck Norris can speak Braille.",
    isLoading: false
  },
  counterSlice: {
    count: 5
  },
  uiSlice: {
    dropdownVisible: false
  }
}

View

Reducer

Store

dispatch an action

update the store

view reads data from the store

increment = (state) => {
  state.count += 1;
}

1. Create slices

// /src/store/counterSlice.ts
import { createSlice } from '@reduxjs/toolkit';
import type { PayloadAction } from '@reduxjs/toolkit';

export type CounterState = {
  count: number;
};

const initialState: CounterState = {
  count: 0,
};

export const counterSlice = createSlice({
  name: 'counter',
  initialState,
  reducers: {
    increment: (state, action) => {
      state.count += 1;
    }
  },
});

export const { increment } = counterSlice.actions;
export const counterReducer = counterSlice.reducer;

2. Create a store

// src/store/store.ts
import { configureStore } from '@reduxjs/toolkit';
import { jokeReducer } from './jokeSlice';
import { counterReducer } from './counterSlice';
import { uiReducer } from './uiSlice';

export const store = configureStore({
  reducer: {
    jokeSlice: jokeReducer,
    counterSlice: counterReducer,
    uiSlice: uiReducer
  },
});

export type RootState = ReturnType<typeof store.getState>;
export type AppDispatch = typeof store.dispatch;

3. Provide the store

// index.tsx
import { store } from './store/store'
import { Provider } from 'react-redux'

root.render(
  <React.StrictMode>
    <Provider store={store}>
      <App />
    </Provider>
  </React.StrictMode>
);

4. Created typed hooks

// src/store/hooks.ts
import { TypedUseSelectorHook, useDispatch, useSelector } from 'react-redux';
import type { RootState, AppDispatch } from './store';

export const useAppDispatch: () => AppDispatch = useDispatch;
export const useAppSelector: TypedUseSelectorHook<RootState> = useSelector;

avoid using useDispatch & useSelector
create copies typed by your store

5. Read the state in a component

import { useAppSelector } from '../store/hooks';

function Counter() {
  const count = useAppSelector(state => state.counterSlice.count);
  
  return <div>Current count: {count}</div>
}

a selector should transform data into a shape for the component

5. Dispatch actions

import { useAppDispatch } from '../store/hooks';
import { increment } from "../store/counterSlice";

function IncrementButton() {
  const dispatch = useAppDispatch();
  
  return (
    <button onClick={() => dispatch(increment())}>
      Increment
    </button>
  );
}

When to use Redux state?

global state
Is it used by multiple components?
Do you need the state after a component unmounts?
Do you want it to work with timetravel?
Do you want to cache it?

React antipatterns

Missing dependencies

always name ALL dependencies in useEffect/useMemo/useCallback
enable lint rule to check that

function Counter() {
  const [count, setCount] = useState(0);
 
  useEffect(() => {
    const id = setInterval(() => {
      setCount(count + 1);
    }, 1000);
    return () => clearInterval(id);
  }, []);
 
  return <h1>{count}</h1>;
}

Solution #1

useEffect(() => {
  const id = setInterval(() => {
    setCount(count + 1);
  }, 1000);
  return () => clearInterval(id);
}, [count]);

Solution #2 ❤️

useEffect(() => {
  const id = setInterval(() => {
    setCount(count => count + 1);
  }, 1000);
  
  return () => clearInterval(id);
}, []);

anytime state changes based on previous value, use the function

Syncing state using useEffect

function Form() {
  const [firstName, setFirstName] = useState('');
  const [lastName, setLastName] = useState('');
  const [fullName, setFullName] = useState('');
  
  useEffect(() => {
    setFullName(`${firstName} ${lastName}`);
  }, [firstName, lastName]);
  
  return <div>
    <input value={firstName} onChange={e => setFirstName(e.target.value)} />
    <input value={lastName} onChange={e => setLastName(e.target.value)} />
	Hello {fullName}
  </div>
}

it is derived state

Too many useState

use useReducer instead
less rerenders

Fat components

place logic into hooks
split into multiple components

Impure components

components & hooks should be pure
- don't touch anything outside
when reading from outside, use useSyncExternalStore
component should yield the same result for given
- props
- state
- context

Don't pass hooks

function ChatInput() {
  return <Button useData={useDataWithLogging} />
}

Using wrong matchers

expect(displayName === name).toBe(true);

Nesting components

const Parent = () => {
  const SecondChild = () => {
    return <div> <SecondChild/> </div>
  }
    
  const Child = () => {
    return <div> <SecondChild/> </div>
  }

  return (
    <div>
     <Child/>
    </div>
  )
}

unpredictable

Circular dependency

// album-item.tsx
import type { Album } from "./album-voting";
...

solution: extract the extra thing

// album-voting.tsx
import { AlbumItem } from "./album-item";
...

Using global CSS

components should be isolated

React 19

React history

React 16.8 (2019)
- hooks
React 17 (2020)
- multiple versions of react running simutaniously
React 18 (2022)
- concurrent React
React 19 (2024?)

React 19

☠️ forwardRef - ref as a normal prop
context provider
server components
server functions
use
useOptimistic
❓React Compiler (aka React Forget)
- no need for React.memo/useMemo/useCallback

☠️ forwardRef

function MyInput({ ref }: Props) {
  return <input ref={ref} />;
}

<MyInput ref={ref} />;

Context as a provider

const UserContext = React.createContext({});

function UserContextProvider({ children }) {
  return (
  	<UserContext value={...}>
      {children}
    </UserContext>
  );
}

Server components

rendered on server
- can access database
- cannot have state, effects
server components cannot be used within client component
supports hydration
"use client" for client code
"use server" for server functions
- components are by default server side

Server components

import {Expandable} from './Expandable';

async function Notes() {
  const notes = await db.notes.getAll();
  return (
    <div>
      {notes.map(note => (
        <ClientComponent key={note.id}>
          <p note={note} />
        </ClientComponent>
      ))}
    </div>
  )
}

Server functions

// Server component
function EmptyNote () {
  async function createNoteAction() {
    // Server Function
    'use server';
    
    await db.notes.create();
  }

  return <Button onClick={createNoteAction}/>;
}

Using server functions from client

"use server";

export async function createNoteAction() {
  await db.notes.create();
}

"use client";

import {createNote} from './actions';

function EmptyNote() {
  return <Button onClick={createNote}/>;
}

use

function Joke() {
  const response = use(axios('https://api.chucknorris.io/random'));
  const joke = response.data.value;
  
  return <p>{joke}</p>
}

function App() {
  return <Suspense fallback="Loading">
    <Joke />
  </Suspense>
}

for promises
can be used also for context

useOptimistic

function List() {
  const [state, setState] = useState([1, 2, 3]);
  const [optimisticState, addOptimistic] = useOptimistic(
    state,
    (currentState, optimisticValue) => {
      // merge and return new state
      // with optimistic value
      return [...currentState, optimisticValue]
    }
  );

  function handleAddNew(n: number) {
    addOptimistic(n);
    axios.post('/new-value').then((stateFromServer) => setState(stateFromServer));
  }

  return <ul>
  	{optimisticState.map(value => <li>{value}</li>)}
  	<button onClick={() => handleAddNew(Math.random())}>add new</button>
  </ul>
}

optimistic updates

React eco system

How to choose a library?

measurable metrics
- number of downloads, github stars, github issues, repository activity (opened PRs, merged PRs...)
- stackoverflow questions
documentation
- migration guide
- how often does it have breaking changes
Proof of concept
- does it work for us? How easy is to use? How easy is it to learn?

A typical project

router (react-router)
state management (redux, MobX, xstate...)
CSS library, design system framework
data fetching (axios, React Query)
form library (Formik,React Hook Form )
run & build (webpack, vite)
test (react-testing-library, jest)
check awesome-react for libraries

Webpack

module bundler
anything you import

New project

npm init -y

npm i react react-dom
npm i -D typescript \
    webpack webpack-cli webpack-dev-server \
    ts-loader @types/react @types/react-dom \
    html-webpack-plugin

npx tsc --init

// Update tsconfig: 
"jsx": "react"
"outDir": "./dist",

webpack.config.js

npx webpack serve // dev server
npx webpack // build

webpack.config.js

const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
    entry: './src/index.tsx',
    module: {
        rules: [
            {
                test: /\.tsx?$/,
                use: 'ts-loader',
                exclude: /node_modules/,
            },
        ],
    },
    resolve: {
        extensions: ['.tsx', '.ts', '.js', '.jsx'],
    },
    output: {
        filename: 'main.bundle.js',
        path: path.resolve(__dirname, 'dist'),
    },
    plugins: [
        new HtmlWebpackPlugin({template: './public/index.html'}),
    ],
};

Chunks example

const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');

module.exports = {
  entry: './src/index.tsx',
  module: {
    rules: [
      {
        test: /\.tsx?$/,
        use: 'ts-loader',
        exclude: /node_modules/,
      },
    ],
  },
  resolve: {
    extensions: ['.tsx', '.ts', '.js'],
  },
  output: {
    filename: '[name].bundle.js',
    path: path.resolve(__dirname, 'dist'),
  },
  plugins: [
    new webpack.optimize.SplitChunksPlugin({minSize: 1}),
    new HtmlWebpackPlugin({template: './public/index.html'}),
  ]
};

NX

tooling for monorepo
apps + libraries
define tasks for each app/library
collection of commands
- nx affected - runs task only for changed parts
plugins for adding storybook, generating e2e tests...

Dependency graph

npx nx graph
you can define boundaries of packages
- what can import what

React Advanced

🙋‍♂️

Frontend engineer

❤️ 📸

❤️ 🛞

Slides + code

Schedule

Course

Your experience?

Setup IDE

Install

🧠 React basics

➡️ Let's make http request

Let's do more React! 💪

React hooks

useRef

useRef

Child to parent?

Control child component?

Expose element from a component

Expose imperative API

React hooks

Performance

Problem

React Virtual DOM

Profiler in dev tools

React.memo

useMemo

useCallback

➡️ Optimize album voting

React hooks

React Context

Context

Context

Encapsulate context

Context initial state

➡️ UserContext

➡️ UserContext

❗️Impact on performance

Testing

Testing

Jest

How to run tests?

Arrange / Act / Assert

Structuring tests

Matchers

Matchers

Negation

Match substring

Match objects

Match arrays

Match exceptions

Total number of asserts

➡️ Update package.json

➡️ Update App.test.tsx

➡️ Fix yellow warning

➡️ Run tests

➡️ Test the division

Mocks

Mocks

Creating a mock

Mock matchers

➡️ Test doNTimes

Spy on existing

Mocking modules

React testing

Testing React

🔨 React testing library

1. Queries

Query DOM

Query By

Query by role

Query by text

Query by test-id

Query within element

waitFor

2. User actions

fireEvent

3. Matchers

Matchers

Expose element from
a component

`screen.debug()`

Actions
Reducers