Test Patterns

# Test Patterns

Write, run, and debug tests across languages. Covers unit tests, integration tests, E2E tests, mocking, coverage, and TDD workflows.

When to Use

• Setting up a test suite for a new project
• Writing unit tests for functions or modules
• Writing integration tests for APIs or database interactions
• Setting up code coverage measurement
• Mocking external dependencies (APIs, databases, file system)
• Debugging flaky or failing tests
• Implementing test-driven development (TDD)

Node.js (Jest / Vitest)

Setup

```bash # Jest npm install -D jest # Add to package.json: "scripts": { "test": "jest" }

# Vitest (faster, ESM-native) npm install -D vitest # Add to package.json: "scripts": { "test": "vitest" } ```

Unit Tests

```javascript // math.js export function add(a, b) { return a + b; } export function divide(a, b) { if (b === 0) throw new Error('Division by zero'); return a / b; }

// math.test.js import { add, divide } from './math.js';

describe('add', () => { test('adds two positive numbers', () => { expect(add(2, 3)).toBe(5); });

test('handles negative numbers', () => { expect(add(-1, 1)).toBe(0); });

test('handles zero', () => { expect(add(0, 0)).toBe(0); }); });

describe('divide', () => { test('divides two numbers', () => { expect(divide(10, 2)).toBe(5); });

test('throws on division by zero', () => { expect(() => divide(10, 0)).toThrow('Division by zero'); });

test('handles floating point', () => { expect(divide(1, 3)).toBeCloseTo(0.333, 3); }); }); ```

Async Tests

```javascript // api.test.js import { fetchUser } from './api.js';

test('fetches user by id', async () => { const user = await fetchUser('123'); expect(user).toHaveProperty('id', '123'); expect(user).toHaveProperty('name'); expect(user.name).toBeTruthy(); });

test('throws on missing user', async () => { await expect(fetchUser('nonexistent')).rejects.toThrow('Not found'); }); ```

Mocking

```javascript // Mock a module jest.mock('./database.js'); import { getUser } from './database.js'; import { processUser } from './service.js';

test('processes user from database', async () => { // Setup mock return value getUser.mockResolvedValue({ id: '1', name: 'Alice', active: true });

const result = await processUser('1'); expect(result.processed).toBe(true); expect(getUser).toHaveBeenCalledWith('1'); expect(getUser).toHaveBeenCalledTimes(1); });

// Mock fetch global.fetch = jest.fn();

test('calls API with correct params', async () => { fetch.mockResolvedValue({ ok: true, json: async () => ({ data: 'test' }), });

const result = await myApiCall('/endpoint'); expect(fetch).toHaveBeenCalledWith('/endpoint', expect.objectContaining({ method: 'GET', })); });

// Spy on existing method (don't replace, just observe) const consoleSpy = jest.spyOn(console, 'log').mockImplementation(); // ... run code ... expect(consoleSpy).toHaveBeenCalledWith('expected message'); consoleSpy.mockRestore(); ```

Coverage

```bash # Jest npx jest --coverage

# Vitest npx vitest --coverage

# Check coverage thresholds (jest.config.js) # coverageThreshold: { global: { branches: 80, functions: 80, lines: 80, statements: 80 } } ```

Python (pytest)

Setup

```bash pip install pytest pytest-cov ```

Unit Tests

```python # calculator.py def add(a, b): return a + b

def divide(a, b): if b == 0: raise ValueError("Division by zero") return a / b

# test_calculator.py import pytest from calculator import add, divide

def test_add(): assert add(2, 3) == 5

def test_add_negative(): assert add(-1, 1) == 0

def test_divide(): assert divide(10, 2) == 5.0

def test_divide_by_zero(): with pytest.raises(ValueError, match="Division by zero"): divide(10, 0)

def test_divide_float(): assert divide(1, 3) == pytest.approx(0.333, abs=0.001) ```

Parametrized Tests

```python @pytest.mark.parametrize("a,b,expected", [ (2, 3, 5), (-1, 1, 0), (0, 0, 0), (100, -50, 50), ]) def test_add_cases(a, b, expected): assert add(a, b) == expected ```

Fixtures

```python import pytest import json import tempfile import os

@pytest.fixture def sample_users(): """Provide test user data.""" return [ {"id": 1, "name": "Alice", "email": "[email protected]"}, {"id": 2, "name": "Bob", "email": "[email protected]"}, ]

@pytest.fixture def temp_db(tmp_path): """Provide a temporary SQLite database.""" import sqlite3 db_path = tmp_path / "test.db" conn = sqlite3.connect(str(db_path)) conn.execute("CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT, email TEXT)") conn.commit() yield conn conn.close()

def test_insert_users(temp_db, sample_users): for user in sample_users: temp_db.execute("INSERT INTO users VALUES (?, ?, ?)", (user["id"], user["name"], user["email"])) temp_db.commit() count = temp_db.execute("SELECT COUNT(*) FROM users").fetchone()[0] assert count == 2

# Fixture with cleanup @pytest.fixture def temp_config_file(): path = tempfile.mktemp(suffix=".json") with open(path, "w") as f: json.dump({"key": "value"}, f) yield path os.unlink(path) ```

Mocking

```python from unittest.mock import patch, MagicMock, AsyncMock

# Mock a function @patch('mymodule.requests.get') def test_fetch_data(mock_get): mock_get.return_value.status_code = 200 mock_get.return_value.json.return_value = {"data": "test"}

result = fetch_data("https://api.example.com") assert result == {"data": "test"} mock_get.assert_called_once_with("https://api.example.com")

# Mock async @patch('mymodule.aiohttp.ClientSession.get', new_callable=AsyncMock) async def test_async_fetch(mock_get): mock_get.return_value.__aenter__.return_value.json = AsyncMock(return_value={"ok": True}) result = await async_fetch("/endpoint") assert result["ok"] is True

# Context manager mock def test_file_reader(): with patch("builtins.open", MagicMock(return_value=MagicMock( read=MagicMock(return_value='{"key": "val"}'), __enter__=MagicMock(return_value=MagicMock(read=MagicMock(return_value='{"key": "val"}'))), __exit__=MagicMock(return_value=False), ))): result = read_config("fake.json") assert result["key"] == "val" ```

Coverage

```bash # Run with coverage pytest --cov=mypackage --cov-report=term-missing

# HTML report pytest --cov=mypackage --cov-report=html # Open htmlcov/index.html

# Fail if coverage below threshold pytest --cov=mypackage --cov-fail-under=80 ```

Go

Unit Tests

```go // math.go package math

import "errors"

func Add(a, b int) int { return a + b }

func Divide(a, b float64) (float64, error) { if b == 0 { return 0, errors.New("division by zero") } return a / b, nil }

// math_test.go package math

import ( "testing" "math" )

func TestAdd(t *testing.T) { tests := []struct { name string a, b int expected int }{ {"positive", 2, 3, 5}, {"negative", -1, 1, 0}, {"zeros", 0, 0, 0}, } for _, tt := range tests { t.Run(tt.name, func(t *testing.T) { got := Add(tt.a, tt.b) if got != tt.expected { t.Errorf("Add(%d, %d) = %d, want %d", tt.a, tt.b, got, tt.expected) } }) } }

func TestDivide(t *testing.T) { result, err := Divide(10, 2) if err != nil { t.Fatalf("unexpected error: %v", err) } if math.Abs(result-5.0) > 0.001 { t.Errorf("Divide(10, 2) = %f, want 5.0", result) } }

func TestDivideByZero(t *testing.T) { _, err := Divide(10, 0) if err == nil { t.Error("expected error for division by zero") } } ```

Run Tests

```bash # All tests go test ./...

# Verbose go test -v ./...

# Specific package go test ./pkg/math/

# With coverage go test -cover ./... go test -coverprofile=coverage.out ./... go tool cover -html=coverage.out

# Run specific test go test -run TestAdd ./...

# Race condition detection go test -race ./...

# Benchmark go test -bench=. ./... ```

Rust

Unit Tests

```rust // src/math.rs pub fn add(a: i64, b: i64) -> i64 { a + b }

pub fn divide(a: f64, b: f64) -> Result<f64, String> { if b == 0.0 { return Err("division by zero".into()); } Ok(a / b) }

#[cfg(test)] mod tests { use super::*;

#[test] fn test_add() { assert_eq!(add(2, 3), 5); assert_eq!(add(-1, 1), 0); }

#[test] fn test_divide() { let result = divide(10.0, 2.0).unwrap(); assert!((result - 5.0).abs() < f64::EPSILON); }

#[test] fn test_divide_by_zero() { assert!(divide(10.0, 0.0).is_err()); }

#[test] #[should_panic(expected = "overflow")] fn test_overflow_panics() { let _ = add(i64::MAX, 1); // Will panic on overflow in debug } } ```

```bash cargo test cargo test -- --nocapture # Show println output cargo test test_add # Run specific test cargo tarpaulin # Coverage (install: cargo install cargo-tarpaulin) ```

Bash Tests

Simple Test Runner

```bash #!/bin/bash # test.sh - Minimal bash test framework PASS=0 FAIL=0

assert_eq() { local actual="$1" expected="$2" label="$3" if [ "$actual" = "$expected" ]; then echo " PASS: $label" ((PASS++)) else echo " FAIL: $label (got '$actual', expected '$expected')" ((FAIL++)) fi }

assert_exit_code() { local cmd="$1" expected="$2" label="$3" eval "$cmd" >/dev/null 2>&1 assert_eq "$?" "$expected" "$label" }

assert_contains() { local actual="$1" substring="$2" label="$3" if echo "$actual" | grep -q "$substring"; then echo " PASS: $label" ((PASS++)) else echo " FAIL: $label ('$actual' does not contain '$substring')" ((FAIL++)) fi }

# --- Tests --- echo "Running tests..."

# Test your scripts output=$(./my-script.sh --help 2>&1) assert_exit_code "./my-script.sh --help" "0" "help flag exits 0" assert_contains "$output" "Usage" "help shows usage"

output=$(./my-script.sh --invalid 2>&1) assert_exit_code "./my-script.sh --invalid" "1" "invalid flag exits 1"

# Test command outputs assert_eq "$(echo 'hello' | wc -c | tr -d ' ')" "6" "echo hello is 6 bytes"

echo "" echo "Results: $PASS passed, $FAIL failed" [ "$FAIL" -eq 0 ] && exit 0 || exit 1 ```

Integration Testing Patterns

API Integration Test (any language)

```bash #!/bin/bash # test-api.sh - Start server, run tests, tear down SERVER_PID="" cleanup() { [ -n "$SERVER_PID" ] && kill "$SERVER_PID" 2>/dev/null; } trap cleanup EXIT

# Start server in background npm start & SERVER_PID=$! sleep 2 # Wait for server

# Run tests against live server BASE_URL=http://localhost:3000 npx jest --testPathPattern=integration EXIT_CODE=$?

exit $EXIT_CODE ```

Database Integration Test (Python)

```python import pytest import sqlite3

@pytest.fixture def db(): """Fresh database for each test.""" conn = sqlite3.connect(":memory:") conn.execute("CREATE TABLE items (id INTEGER PRIMARY KEY, name TEXT, price REAL)") yield conn conn.close()

def test_insert_and_query(db): db.execute("INSERT INTO items (name, price) VALUES (?, ?)", ("Widget", 9.99)) db.commit() row = db.execute("SELECT name, price FROM items WHERE name = ?", ("Widget",)).fetchone() assert row == ("Widget", 9.99)

def test_empty_table(db): count = db.execute("SELECT COUNT(*) FROM items").fetchone()[0] assert count == 0 ```

TDD Workflow

The red-green-refactor cycle:

1. Red: Write a failing test for the next piece of behavior
2. Green: Write the minimum code to make it pass
3. Refactor: Clean up without changing behavior (tests stay green)

```bash # Tight feedback loop # Jest watch mode npx jest --watch

# Vitest watch (default) npx vitest

# pytest watch (with pytest-watch) pip install pytest-watch ptw

# Go (with air or entr) ls *.go | entr -c go test ./... ```

Debugging Failed Tests

Common Issues

• Test passes alone, fails in suite → shared state. Check for:
• Global variables modified between tests
• Database not cleaned up
• Mocks not restored (`afterEach` / `teardown`)

• Test fails intermittently (flaky) → timing or ordering issue:
• Async operations without proper `await`
• Tests depending on execution order
• Time-dependent logic (use clock mocking)
• Network calls in unit tests (should be mocked)

• Coverage shows uncovered branches → missing edge cases:
• Error paths (what if the API returns 500?)
• Empty inputs (empty string, null, empty array)
• Boundary values (0, -1, MAX_INT)

Run Single Test

```bash # Jest npx jest -t "test name substring"

# pytest pytest -k "test_divide_by_zero"

# Go go test -run TestDivideByZero ./...

# Rust cargo test test_divide ```

Tips

• Test behavior, not implementation. Tests should survive refactors.
• One assertion per concept (not necessarily one `assert` per test, but one logical check).
• Name tests descriptively: `test_returns_empty_list_when_no_users_exist` beats `test_get_users_2`.
• Don't mock what you don't own — write thin wrappers around external libraries, mock the wrapper.
• Integration tests catch bugs unit tests miss. Don't skip them.
• Use `tmp_path` (pytest), `t.TempDir()` (Go), or `tempfile` (Node) for file-based tests.
• Snapshot tests are great for detecting unintended changes, bad for evolving formats.