Pyrolistical

// BAD
const LocaleContext = React.createContext();
const OverrideLocaleContext = React.createContext()

const App = () => {
  const [locale, setLocale] = useState('english');
  return <LocaleContext.Provider value={locale}>
    <Navigation />
  </LocaleContext.Provider>;
};

Pyrolistical

// React.memo caches the functional component if all the props are triple equals ===
const Increment = React.memo(({ caller, onClick }) => {
  console.log(`${caller} button rendered`);
  return <button onClick={onClick}>Increment</button>;
});

const BadComponent = () => {
  const [count, setCount] = useState(0);
  // declared functions never triple equals ===, even if its the same code
  // ie. (() => {}) === (() => {}) is false,

Pyrolistical

import {createContext, useContext} from 'react';

const LocaleContext = createContext();

export const ProvideLocale = Locale.Provider;

export const useLocale = () => {
  const locale = useContext(LocaleContext);
  return locale;
};

Pyrolistical

export default (db) => {
  return {
    async checkIn(teamID) {
      const disqualifiedPlayers = await db.collection('playerDisqualifications').find({
        teamID
      })
        .toArray();
      if (disqualifiedPlayers.length > 0) {
        throw new Error('cannot check-in with disqualified players');
      }

Pyrolistical

export default (db) => {
  return {
    async findDisqualifiedPlayers(teamID) {
      return db.collection('playerDisqualifications').find({
        teamID
      })
        .toArray();
    },
    async checkInTeam(teamID) {
      return db.collection('checkIns').insert({

Pyrolistical

import Service from './service';

test('teams with disqualified players cannot check-in', async () => {
  const db = {
    collection(collection) {
      switch (collection) {
        case 'playerDisqualifications': {
          return {
            find({teamID}) {
              expect(teamID).toBe('some team id');

Pyrolistical

import {jest} from '@jest/globals'

import Service from './service';

test('teams with disqualified players cannot check-in', async () => {
  const playerDisqualificationsFindToArray = jest.fn()
    .mockResolvedValueOnce([
      {
        teamID: 'some team id',
        reason: 'was a big meanie'

Pyrolistical

import {jest} from '@jest/globals'

import Service from './service';

test('teams with disqualified players cannot check-in', async () => {
  const repository = {
    findDisqualifiedPlayers: jest.fn()
      .mockResolvedValueOnce([
        {
          teamID: 'some team id',

Pyrolistical

async checkInTeam(teamID) {
  return db.collection('teams').updateOne({
    _id: teamID
  }, {
    $set: {
      checkedInAt: new Date()
    }
  });
}

Pyrolistical

import _ from 'lodash';

export class InsufficientStock extends Error {
  constructor(message) {
    super(message);
    this.name = 'InsufficientStock';
  }
}

export default (repository) => {