Testing React Hooks with Confidence Radoslav Stankov 29/01/2020

Radoslav Stankov @rstankov blog.rstankov.com
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https://rstankov.com/appearances

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Automated Testing Test Driven Development

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import * as React from 'react'; import Memory from './Memory'; import Grid from './Grid'; import Button from './Button'; const OPERATIONS = { '+': (a, b) => a + b, '-': (a, b) => a - b, '*': (a, b) => a * b, }; export default function Calculator() { const [memory, setMemory] = React.useState('0'); const actions = React.useMemo( () => ({ add(input) { if (!input.match(/\d/)) { if (!memory.match(/\d$/)) { return; } if (!OPERATIONS[input]) { return; } setMemory(`${memory} ${input} `); return;

const actions = React.useMemo( () => ({ add(input) { if (!input.match(/\d/)) { if (!memory.match(/\d$/)) { return; } if (!OPERATIONS[input]) { return; } setMemory(`${memory} ${input} `); return; } if (memory.match(/(^| )0$/)) { setMemory(`${memory.slice(0, -1)}${input}`); return; } setMemory(`${memory}${input}`); }, remove() { const newMemory = memory.match(/\d$/) ? memory.slice(0, -1) : memory.slice(0, -3); setMemory(newMemory || '0');

if (memory.match(/(^| )0$/)) { setMemory(`${memory.slice(0, -1)}${input}`); return; } setMemory(`${memory}${input}`); }, remove() { const newMemory = memory.match(/\d$/) ? memory.slice(0, -1) : memory.slice(0, -3); setMemory(newMemory || '0'); }, reset() { setMemory('0'); }, evaluate() { const chunks = memory.split(' '); let value = parseInt(chunks[0], 10); for (let i = 1; i < chunks.length; i += 2) { const other = chunks[i + 1]; if (typeof other === 'undefined' || other.length === 0) { continue; } const operation = OPERATIONS[chunks[i]];

reset() { setMemory('0'); }, evaluate() { const chunks = memory.split(' '); let value = parseInt(chunks[0], 10); for (let i = 1; i < chunks.length; i += 2) { const other = chunks[i + 1]; if (typeof other === 'undefined' || other.length === 0) { continue; } const operation = OPERATIONS[chunks[i]]; if (!operation) { throw new Error(`Invalid "${chunks[i]}" operand`); } value = operation(value, parseInt(other, 10)); } setMemory(value.toString()); }, }), [memory, setMemory], ); return (

); return ( {memory.trim()} actions.add('1')}>1 actions.add('2')}>2 actions.add('3')}>3 actions.add('4')}>4 actions.add('5')}>5 actions.add('6')}>6 actions.add('7')}>7 actions.add('8')}>8 actions.add('9')}>9 C actions.add('0')}>0 ← = actions.add('+')}>+ actions.add('-')}>- actions.add('*')}>* ); }

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! Smoke Test

return ( {memory.trim()} actions.add('1')}>1 actions.add('2')}>2 actions.add('3')}>3 actions.add('4')}>4 actions.add('5')}>5 actions.add('6')}>6 actions.add('7')}>7 actions.add('8')}>8 actions.add('9')}>9 C actions.add('0')}>0 ← = actions.add('+')}>+ actions.add('-')}>- actions.add('*')}>* ); }

return ( {memory.trim()} actions.add('1')}>1 actions.add('2')}>2 actions.add('3')}>3 actions.add('4')}>4 actions.add('5')}>5 actions.add('6')}>6 actions.add('7')}>7 actions.add('8')}>8 actions.add('9')}>9 C actions.add('0')}>0 ← = actions.add('+')}>+ actions.add('-')}>- actions.add('*')}>* ); }

yarn add --dev @testing-library/react

import { render, fireEvent, screen } from '@testing-library/react'; const click = (dt) => fireEvent.click(screen.getByTestId(dt)); const expectComponent = (dt) => expect(screen.getByTestId(dt)); describe(Component.name, () => { it('works', () => { render(); expectComponent('result').toHaveTextContent('0'); click('1'); click('+'); click('1'); expectComponent('result').toHaveTextContent('1 + 1'); click('remove'); expectComponent('result').toHaveTextContent('1 +'); click('2'); expectComponent('result').toHaveTextContent('1 + 2'); click('evaluate'); expectComponent('result').toHaveTextContent('3'); click('reset'); expectComponent('result').toHaveTextContent('0'); }); });

import { render, fireEvent, screen } from '@testing-library/react'; const click = (dt) => fireEvent.click(screen.getByTestId(dt)); const expectComponent = (dt) => expect(screen.getByTestId(dt)); describe(Component.name, () => { it('works', () => { render(); expectComponent('result').toHaveTextContent('0'); click('1'); click('+'); click('1'); expectComponent('result').toHaveTextContent('1 + 1'); click('remove'); expectComponent('result').toHaveTextContent('1 +'); click('2'); expectComponent('result').toHaveTextContent('1 + 2'); click('evaluate'); expectComponent('result').toHaveTextContent('3'); click('reset'); expectComponent('result').toHaveTextContent('0'); }); });

import { render, fireEvent, screen } from '@testing-library/react'; const click = (dt) => fireEvent.click(screen.getByTestId(dt)); const expectComponent = (dt) => expect(screen.getByTestId(dt)); describe(Component.name, () => { it('works', () => { render(); expectComponent('result').toHaveTextContent('0'); click('1'); click('+'); click('1'); expectComponent('result').toHaveTextContent('1 + 1'); click('remove'); expectComponent('result').toHaveTextContent('1 +'); click('2'); expectComponent('result').toHaveTextContent('1 + 2'); click('evaluate'); expectComponent('result').toHaveTextContent('3'); click('reset'); expectComponent('result').toHaveTextContent('0'); }); });

import { render, fireEvent, screen } from '@testing-library/react'; const click = (dt) => fireEvent.click(screen.getByTestId(dt)); const expectComponent = (dt) => expect(screen.getByTestId(dt)); describe(Component.name, () => { it('works', () => { render(); expectComponent('result').toHaveTextContent('0'); click('1'); click('+'); click('1'); expectComponent('result').toHaveTextContent('1 + 1'); click('remove'); expectComponent('result').toHaveTextContent('1 +'); click('2'); expectComponent('result').toHaveTextContent('1 + 2'); click('evaluate'); expectComponent('result').toHaveTextContent('3'); click('reset'); expectComponent('result').toHaveTextContent('0'); }); });

import { render, fireEvent, screen } from '@testing-library/react'; const click = (dt) => fireEvent.click(screen.getByTestId(dt)); const expectComponent = (dt) => expect(screen.getByTestId(dt)); describe(Component.name, () => { it('works', () => { render(); expectComponent('result').toHaveTextContent('0'); click('1'); click('+'); click('1'); expectComponent('result').toHaveTextContent('1 + 1'); click('remove'); expectComponent('result').toHaveTextContent('1 +'); click('2'); expectComponent('result').toHaveTextContent('1 + 2'); click('evaluate'); expectComponent('result').toHaveTextContent('3'); click('reset'); expectComponent('result').toHaveTextContent('0'); }); });

" This is placeholder " Not a "real" unit test. It doesn't follow the 4 phases of good test: setup ➡ action ➡ assert ➡ teardown

import * as React from 'react'; import Memory from './Memory'; import Grid from './Grid'; import Button from './Button'; const OPERATIONS = { '+': (a, b) => a + b, '-': (a, b) => a - b, '*': (a, b) => a * b, }; export default function Calculator() { const [memory, setMemory] = React.useState('0'); const actions = React.useMemo( () => ({ add(input) { if (!input.match(/\d/)) { if (!memory.match(/\d$/)) { return; } if (!OPERATIONS[input]) { return; } setMemory(`${memory} ${input} `); return; $

% Extract Custom Hook

import * as React from 'react'; import Memory from './Memory'; import Grid from './Grid'; import Button from './Button'; import useCalculatorState from './useCalculatorState'; export default function Calculator() { const [memory, actions] = useCalculatorState(); return ( {memory.trim()} actions.add('1')}>1 actions.add('2')}>2 actions.add('3')}>3 actions.add('4')}>4 actions.add('5')}>5 actions.add('6')}>6 actions.add('7')}>7 actions.add('8')}>8 actions.add('9')}>9 C actions.add('0')}>0 ←

import * as React from 'react'; import Memory from './Memory'; import Grid from './Grid'; import Button from './Button'; import useCalculatorState from './useCalculatorState'; export default function Calculator() { const [memory, actions] = useCalculatorState(); return ( {memory.trim()} actions.add('1')}>1 actions.add('2')}>2 actions.add('3')}>3 actions.add('4')}>4 actions.add('5')}>5 actions.add('6')}>6 actions.add('7')}>7 actions.add('8')}>8 actions.add('9')}>9 C actions.add('0')}>0 ←

import * as React from 'react'; const OPERATIONS = { '+': (a, b) => a + b, '-': (a, b) => a - b, '*': (a, b) => a * b, }; export default function useCalculatorState(initalState = '0') { const [memory, setMemory] = React.useState(initalState); const actions = React.useMemo( () => ({ add(input) { if (!input.match(/\d/)) { if (!memory.match(/\d$/)) { return; } if (!OPERATIONS[input]) { return; } setMemory(`${memory} ${input} `); return; } if (memory.match(/(^| )0$/)) {

& Test Custom hook

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

yarn add --dev @testing-library/react-hooks

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

import useCalculatorState from './useCalculatorState'; import { renderHook, act } from '@testing-library/react-hooks'; describe(useCalculatorState.name, () => { function initHook(initalState) { return renderHook(() => useCalculatorState(initalState)).result; } describe('add', () => { function call(initalState, value) { const hook = initHook(initalState); act(() => { hook.current[1].add(value); }); return hook.current[0]; } it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1');

it('can add digits', () => { expect(call('1', '2')).toEqual('12'); }); it('ensures numbers cannot start with 0', () => { expect(call('0', '1')).toEqual('1'); expect(call('1 + 0', '0')).toEqual('1 + 0'); expect(call('1 + 0', '1')).toEqual('1 + 1'); expect(call('10', '0')).toEqual('100'); }); it('can add operand', () => { expect(call('1', '+')).toEqual('1 + '); }); it('guards against add operand before digit', () => { expect(call('', '+')).toEqual(''); }); it('guards against two operands one after the other', () => { expect(call('1 + ', '+')).toEqual('1 + '); }); it('guards against add invalid operator', () => { expect(call('1', '~')).toEqual('1'); }); });

describe('remove', () => { function call(initalState) { const hook = initHook(initalState); act(() => { hook.current[1].remove(); }); return hook.current[0]; } it('removes one digit', () => { expect(call('1234')).toEqual('123'); }); it('removes one operand', () => { expect(call('1 + ')).toEqual('1'); }); it('returns at least 0 when fully cleared', () => { expect(call('1')).toEqual('0'); }); }); describe('evaluate', () => { function call(initalState) { const hook = initHook(initalState);

describe('evaluate', () => { function call(initalState) { const hook = initHook(initalState); act(() => { hook.current[1].evaluate(); }); return hook.current[0]; } describe('operations', () => { it('handles noop', () => { expect(call('12')).toEqual('12'); }); it('handles "1 + 1" input', () => { expect(call('1 + 1')).toEqual('2'); }); it('handles "1 - 1" input', () => { expect(call('2 - 1')).toEqual('1'); }); it('handles "2 * 3" input', () => { expect(call('2 * 3')).toEqual('6'); }); });

expect(call('2 * 3')).toEqual('6'); }); }); describe('edge cases', () => { it('handles zero in input', () => { expect(call('1 + 0')).toEqual('1'); }); it('handles larger number in input', () => { expect(call('123 + 456')).toEqual((123 + 456).toString()); }); it('handles multiple operations input', () => { expect(call('1 + 1 + 1')).toEqual('3'); }); it('handles incomplete operation input', () => { expect(call('1 + ')).toEqual('1'); }); it('handles negative values', () => { expect(call('-1 + 2')).toEqual('1'); }); }); }); });

' Refactor Custom Hook

import * as React from 'react'; export default function useCalculatorState(initalState = '0') { const [memory, dispatch] = React.useReducer(calculatorReducer, initalState); const actions = React.useMemo( () => ({ add(value) { dispatch({ type: 'add', payload: value }); }, remove() { dispatch({ type: 'remove' }); }, reset() { dispatch({ type: 'reset' }); }, evaluate() { dispatch({ type: 'evaluate' }); }, }), [dispatch], ); return [memory, actions]; } const OPERATIONS = {

function calculatorReducer(memory, action) { switch (action.type) { case 'add': const input = action.payload; if (!input.match(/\d/)) { if (!memory.match(/\d$/)) { return memory; } if (!OPERATIONS[input]) { return memory; } return `${memory} ${input} `; } if (memory.match(/(^| )0$/)) { return `${memory.slice(0, -1)}${input}`; } return `${memory}${input}`; case 'remove': const newMemory = memory.match(/\d$/) ? memory.slice(0, -1) : memory.slice(0, -3);

case 'reset': return '0'; case 'evaluate': const chunks = memory.split(' '); let value = parseInt(chunks[0], 10); for (let i = 1; i < chunks.length; i += 2) { const other = chunks[i + 1]; if (typeof other === 'undefined' || other.length === 0) { continue; } const operation = OPERATIONS[chunks[i]]; if (!operation) { throw new Error(`Invalid "${chunks[i]}" operand`); } value = operation(value, parseInt(other, 10)); } return value.toString(); default: throw new Error(`Invalid action - ${action.type}`); } }

Recap

( started with messy code ) added "smoke test" * extracted custom hook + added test for the custom hook , refactored the hook - removed "smoke test"

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