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

# 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

// 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

// 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

// 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

# 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

pip install pytest pytest-cov

Unit Tests

# 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

@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

import pytest
import json
import tempfile
import os
@pytest.fixture
def sample_users():
    """Provide test user data."""
    return [
        {"id": 1, "name": "Alice", "email": "alice@test.com"},
        {"id": 2, "name": "Bob", "email": "bob@test.com"},
    ]
@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

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

# 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

// 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

# 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

// src/math.rs
pub fn add(a: i64, b: i64) -> i64 { a + b }
pub fn divide(a: f64, b: f64) -> Result {
    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
    }
}

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

#!/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)

#!/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)

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:

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

# 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

# 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.