jest-skill

Name: jest-skill
Author: LambdaTest

335stars67forks

How to install jest-skill?

lambdatest/agent-skills/jest-skill

$npx -y skills add LambdaTest/agent-skills --skill jest-skill

Installs into the current project.

Summary

Generates Jest unit and integration tests in JavaScript or TypeScript. Covers mocking, snapshots, async testing, and React component testing. Use when user mentions "Jest", "describe/it/expect", "jest.mock", "toMatchSnapshot". Triggers on: "Jest", "expect().toBe()", "jest.mock",

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SKILL.md

1	# Jest Testing Skill
2
3	## Core Patterns
4
5	### Basic Test
6
7	```javascript
8	describe('Calculator', () => {
9	let calc;
10	beforeEach(() => { calc = new Calculator(); });
11
12	test('adds two numbers', () => {
13	expect(calc.add(2, 3)).toBe(5);
14	});
15
16	test('throws on division by zero', () => {
17	expect(() => calc.divide(10, 0)).toThrow('Division by zero');
18	});
19	});
20	```
21
22	### Matchers
23
24	```javascript
25	expect(value).toBe(exact); // === strict
26	expect(value).toEqual(object); // deep equality
27	expect(value).toBeTruthy();
28	expect(value).toBeNull();
29	expect(value).toBeGreaterThan(3);
30	expect(value).toBeCloseTo(0.3, 5);
31	expect(str).toMatch(/regex/);
32	expect(arr).toContain(item);
33	expect(arr).toHaveLength(3);
34	expect(obj).toHaveProperty('name');
35	expect(obj).toMatchObject({ name: 'Alice' });
36	expect(() => fn()).toThrow(CustomError);
37	```
38
39	### Mocking
40
41	```javascript
42	// Mock function
43	const mockFn = jest.fn();
44	mockFn.mockReturnValue(42);
45	mockFn.mockResolvedValue({ data: 'test' });
46	expect(mockFn).toHaveBeenCalledWith('arg1');
47	expect(mockFn).toHaveBeenCalledTimes(1);
48
49	// Mock module
50	jest.mock('./database');
51	const db = require('./database');
52	db.getUser.mockResolvedValue({ name: 'Alice' });
53
54	// Mock with implementation
55	jest.mock('./api', () => ({
56	fetchUsers: jest.fn().mockResolvedValue([{ name: 'Alice' }]),
57	}));
58
59	// Spy
60	const spy = jest.spyOn(console, 'log').mockImplementation();
61	expect(spy).toHaveBeenCalledWith('expected');
62	spy.mockRestore();
63
64	// Fake timers
65	jest.useFakeTimers();
66	jest.advanceTimersByTime(1000);
67	jest.useRealTimers();
68	```
69
70	### Async Testing
71
72	```javascript
73	test('fetches users', async () => {
74	const users = await fetchUsers();
75	expect(users).toHaveLength(3);
76	});
77
78	test('resolves with data', () => {
79	return expect(fetchData()).resolves.toEqual({ data: 'value' });
80	});
81
82	test('rejects with error', () => {
83	return expect(fetchBadData()).rejects.toThrow('not found');
84	});
85	```
86
87	### React Component Testing (Testing Library)
88
89	```javascript
90	import { render, screen, fireEvent, waitFor } from '@testing-library/react';
91	import '@testing-library/jest-dom';
92	import LoginForm from './LoginForm';
93
94	test('submits login form', async () => {
95	const onSubmit = jest.fn();
96	render(<LoginForm onSubmit={onSubmit} />);
97
98	fireEvent.change(screen.getByLabelText('Email'), {
99	target: { value: 'user@test.com' },
100	});
101	fireEvent.change(screen.getByLabelText('Password'), {
102	target: { value: 'password123' },
103	});
104	fireEvent.click(screen.getByRole('button', { name: /login/i }));
105
106	await waitFor(() => {
107	expect(onSubmit).toHaveBeenCalledWith({
108	email: 'user@test.com', password: 'password123',
109	});
110	});
111	});
112	```
113
114	### Snapshot Testing
115
116	```javascript
117	test('renders correctly', () => {
118	const tree = renderer.create(<Button label="Click" />).toJSON();
119	expect(tree).toMatchSnapshot();
120	});
121	// Update: jest --updateSnapshot
122	```
123
124	### Anti-Patterns
125
126	\| Bad \| Good \| Why \|
127	\|-----\|------\|-----\|
128	\| `expect(x === y).toBe(true)` \| `expect(x).toBe(y)` \| Better errors \|
129	\| No `await` on async \| Always `await` \| Swallows failures \|
130	\| Snapshot everything \| Snapshot UI, assert logic \| Snapshot fatigue \|
131
132	## Quick Reference
133
134	\| Task \| Command \|
135	\|------\|---------\|
136	\| Run all \| `npx jest` \|
137	\| Watch \| `npx jest --watch` \|
138	\| Coverage \| `npx jest --coverage` \|
139	\| Update snapshots \| `npx jest --updateSnapshot` \|
140	\| Run file \| `npx jest tests/calc.test.js` \|
141	\| Single test \| `test.only('name', () => {})` \|
142
143	## Deep Patterns
144
145	For production-grade patterns, see `reference/playbook.md`:
146
147	\| Section \| What's Inside \|
148	\|---------\|--------------\|
149	\| §1 Production Config \| Node + React configs, path aliases, coverage thresholds \|
150	\| §2 Mocking Deep Dive \| Module/partial/manual mocks, spies, timers, env vars \|
151	\| §3 Async Patterns \| Promises, rejections, event emitters, streams \|
152	\| §4 test.each \| Array, tagged template, describe.each for table-driven tests \|
153	\| §5 Custom Matchers \| toBeWithinRange, toBeValidEmail, TypeScript declarations \|
154	\| §6 React Testing Library \| userEvent, hooks, context providers \|
155	\| §7 Snapshot Testing \| Component, inline, property matchers \|
156	\| §8 API Service Testing \| Mocked axios, CRUD patterns, error handling \|
157	\| §9 Global Setup \| Multi-project config, DB setup/teardown \|
158	\| §10 CI/CD \| GitHub Actions with coverage gates \|
159	\| §11 Debugging Table \| 10 common problems with fixes \|
160	\| §12 Best Practices \| 15-item production checklist \|

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Type

jest-skill

335stars67forks

View on GitHub

How to install jest-skill?

lambdatest/agent-skills/jest-skill

$npx -y skills add LambdaTest/agent-skills --skill jest-skill

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Jest Testing Skill
2
3	## Core Patterns
4
5	### Basic Test
6
7	```javascript
8	describe('Calculator', () => {
9	let calc;
10	beforeEach(() => { calc = new Calculator(); });
11
12	test('adds two numbers', () => {
13	expect(calc.add(2, 3)).toBe(5);
14	});
15
16	test('throws on division by zero', () => {
17	expect(() => calc.divide(10, 0)).toThrow('Division by zero');
18	});
19	});
20	```
21
22	### Matchers
23
24	```javascript
25	expect(value).toBe(exact); // === strict
26	expect(value).toEqual(object); // deep equality
27	expect(value).toBeTruthy();
28	expect(value).toBeNull();
29	expect(value).toBeGreaterThan(3);
30	expect(value).toBeCloseTo(0.3, 5);
31	expect(str).toMatch(/regex/);
32	expect(arr).toContain(item);
33	expect(arr).toHaveLength(3);
34	expect(obj).toHaveProperty('name');
35	expect(obj).toMatchObject({ name: 'Alice' });
36	expect(() => fn()).toThrow(CustomError);
37	```
38
39	### Mocking
40
41	```javascript
42	// Mock function
43	const mockFn = jest.fn();
44	mockFn.mockReturnValue(42);
45	mockFn.mockResolvedValue({ data: 'test' });
46	expect(mockFn).toHaveBeenCalledWith('arg1');
47	expect(mockFn).toHaveBeenCalledTimes(1);
48
49	// Mock module
50	jest.mock('./database');
51	const db = require('./database');
52	db.getUser.mockResolvedValue({ name: 'Alice' });
53
54	// Mock with implementation
55	jest.mock('./api', () => ({
56	fetchUsers: jest.fn().mockResolvedValue([{ name: 'Alice' }]),
57	}));
58
59	// Spy
60	const spy = jest.spyOn(console, 'log').mockImplementation();
61	expect(spy).toHaveBeenCalledWith('expected');
62	spy.mockRestore();
63
64	// Fake timers
65	jest.useFakeTimers();
66	jest.advanceTimersByTime(1000);
67	jest.useRealTimers();
68	```
69
70	### Async Testing
71
72	```javascript
73	test('fetches users', async () => {
74	const users = await fetchUsers();
75	expect(users).toHaveLength(3);
76	});
77
78	test('resolves with data', () => {
79	return expect(fetchData()).resolves.toEqual({ data: 'value' });
80	});
81
82	test('rejects with error', () => {
83	return expect(fetchBadData()).rejects.toThrow('not found');
84	});
85	```
86
87	### React Component Testing (Testing Library)
88
89	```javascript
90	import { render, screen, fireEvent, waitFor } from '@testing-library/react';
91	import '@testing-library/jest-dom';
92	import LoginForm from './LoginForm';
93
94	test('submits login form', async () => {
95	const onSubmit = jest.fn();
96	render(<LoginForm onSubmit={onSubmit} />);
97
98	fireEvent.change(screen.getByLabelText('Email'), {
99	target: { value: 'user@test.com' },
100	});
101	fireEvent.change(screen.getByLabelText('Password'), {
102	target: { value: 'password123' },
103	});
104	fireEvent.click(screen.getByRole('button', { name: /login/i }));
105
106	await waitFor(() => {
107	expect(onSubmit).toHaveBeenCalledWith({
108	email: 'user@test.com', password: 'password123',
109	});
110	});
111	});
112	```
113
114	### Snapshot Testing
115
116	```javascript
117	test('renders correctly', () => {
118	const tree = renderer.create(<Button label="Click" />).toJSON();
119	expect(tree).toMatchSnapshot();
120	});
121	// Update: jest --updateSnapshot
122	```
123
124	### Anti-Patterns
125
126	\| Bad \| Good \| Why \|
127	\|-----\|------\|-----\|
128	\| `expect(x === y).toBe(true)` \| `expect(x).toBe(y)` \| Better errors \|
129	\| No `await` on async \| Always `await` \| Swallows failures \|
130	\| Snapshot everything \| Snapshot UI, assert logic \| Snapshot fatigue \|
131
132	## Quick Reference
133
134	\| Task \| Command \|
135	\|------\|---------\|
136	\| Run all \| `npx jest` \|
137	\| Watch \| `npx jest --watch` \|
138	\| Coverage \| `npx jest --coverage` \|
139	\| Update snapshots \| `npx jest --updateSnapshot` \|
140	\| Run file \| `npx jest tests/calc.test.js` \|
141	\| Single test \| `test.only('name', () => {})` \|
142
143	## Deep Patterns
144
145	For production-grade patterns, see `reference/playbook.md`:
146
147	\| Section \| What's Inside \|
148	\|---------\|--------------\|
149	\| §1 Production Config \| Node + React configs, path aliases, coverage thresholds \|
150	\| §2 Mocking Deep Dive \| Module/partial/manual mocks, spies, timers, env vars \|
151	\| §3 Async Patterns \| Promises, rejections, event emitters, streams \|
152	\| §4 test.each \| Array, tagged template, describe.each for table-driven tests \|
153	\| §5 Custom Matchers \| toBeWithinRange, toBeValidEmail, TypeScript declarations \|
154	\| §6 React Testing Library \| userEvent, hooks, context providers \|
155	\| §7 Snapshot Testing \| Component, inline, property matchers \|
156	\| §8 API Service Testing \| Mocked axios, CRUD patterns, error handling \|
157	\| §9 Global Setup \| Multi-project config, DB setup/teardown \|
158	\| §10 CI/CD \| GitHub Actions with coverage gates \|
159	\| §11 Debugging Table \| 10 common problems with fixes \|
160	\| §12 Best Practices \| 15-item production checklist \|

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6 picks

Type