test-levels

Test Levels Guide

Explains test types & guides test selection using car analogy.

When to Use

Invoke when:

• Explaining test concepts to team members
• Deciding which test type to write
• Reviewing test coverage strategy
• Onboarding new developers to testing

The 3 Test Levels

1. Unit Test (Test Case)

The specific instruction.

Single fn/component tested in isolation. No external deps (DB, API, browser).

Aspect	Description
Analogy	"Check if left turn signal blinks when I push lever down"
Scope	Tiny detail - one fn, one input/output
Speed	Fast (ms)
Location	tests/unit/

When to write:

• Pure functions (formatters, validators, utils)
• Data transformations
• Business logic w/o side effects

Project example:

// tests/unit/utils/formatters.spec.ts
import {expect, test} from 'next/experimental/testmode/playwright';
import {leadingZero, formatCurrency} from '@/app/_utils/formatters';

test.describe('formatters', () => {
    test('leadingZero adds zero to single-digit numbers', () => {
        expect(leadingZero(7)).toBe('07');
        expect(leadingZero(10)).toBe('10');
    });

    test('formatCurrency formats correctly', () => {
        const result = formatCurrency({value: 1234.56, locale: 'en', currency: 'USD'});
        expect(result).toBe('1,234.56 $');
    });
});

---

2. Integration Test

The handshake.

Tests if 2+ parts communicate correctly. Focuses on connections, not full system.

Aspect	Description
Analogy	"Does engine make wheels turn?" (Engine → Transmission)
Scope	Connections between components
Speed	Medium (100ms-few seconds)
Location	tests/integration/

When to write:

• Validators w/ schemas (Zod)
• API route handlers
• Service-to-service communication
• DB queries w/ mocked data

Project example: