Test File Conventions

When to Apply This Skill

Use this pattern when you need to:

• Write or refactor *.test.ts files in this codebase.
• Structure tests with setup() functions instead of mutable beforeEach setup.
• Split large test files into focused behavior/type/scenario files.
• Enforce behavior-based test naming and clear failure intent.
• Add or review negative type tests using @ts-expect-error.

References

Load these on demand based on what you're working on:

• If working with negative type tests (@ts-expect-error, bun:test type strategy, no as any), read references/type-testing.md
• If working with test setup architecture (setup() patterns, composable setup, beforeEach avoidance, shared schemas), read references/setup-pattern.md
• If working with test organization structure (flat tests, describe() boundaries, helper-over-nesting), read references/test-structure.md

External reading:

• Kent C. Dodds, "Avoid Nesting When You're Testing" — setup functions over beforeEach, flat tests
• Kent C. Dodds, "AHA Testing" — avoid hasty abstractions in tests
• Kent C. Dodds, Testing JavaScript — Test Object Factory Pattern
• Matt Pocock, "How to test your types" — vitest expectTypeOf for type testing
• Matt Pocock, shoehorn — partial mocks for test ergonomics

Related Skills: See services-layer for the service patterns being tested. See typescript for type testing conventions.

File-Level Doc Comments

Every .test.ts file MUST start with a JSDoc block explaining what is being tested and the key behaviors verified. This serves as documentation for the module's contract.

Structure

/**
 * [Module Name] Tests
 *
 * [1-3 sentences explaining what this file tests and why these tests matter.]
 *
 * Key behaviors:
 * - [Behavior 1]
 * - [Behavior 2]
 * - [Behavior 3]
 *
 * See also:
 * - `related-file.test.ts` for [related aspect]
 */

Good Example

/**
 * Cell-Level LWW CRDT Sync Tests
 *
 * Verifies cell-level LWW conflict resolution where each field
 * has its own timestamp. Unlike row-level LWW, concurrent edits to
 * DIFFERENT fields merge independently.
 *
 * Key behaviors:
 * - Concurrent edits to SAME field: latest timestamp wins
 * - Concurrent edits to DIFFERENT fields: BOTH preserved (merge)
 * - Delete removes all cells for a row
 */

Bad Example (Too Minimal)

// Tests for create-tables

Section Headers

For long test files (100+ lines), use comment headers to separate logical sections:

// ============================================================================
// MESSAGE_SYNC Tests
// ============================================================================

Multi-Aspect Test File Splitting

When a module has distinct behavioral aspects, split into focused test files rather than one monolithic file:

When to Split

• File exceeds ~500 lines
• Tests cover genuinely distinct concerns (CRUD vs sync vs types)
• Different setup requirements per concern

When NOT to Split

• Splitting would create files with fewer than 3 tests
• All tests share the same setup and concern

Test Naming

Test descriptions MUST be behavior assertions, not vague descriptions. The name should tell you what broke when the test fails.

Rules

1. State what happens, not "should work" or "handles correctly"
2. Include the condition when testing edge cases
3. No filler words: "should", "correctly", "properly" add nothing

Good Names

test('upsert stores row and get retrieves it', () => { ... });
test('filter returns only published posts', () => { ... });
test('concurrent edits to different fields: both preserved', () => { ... });
test('delete vs update race: update wins (rightmost entry)', () => { ... });
test('observer fires once per transaction, not per operation', () => { ... });
test('get() throws for undefined tables with helpful message', () => { ... });

Bad Names

test('should work correctly', () => { ... });         // What works? What's correct?
test('should handle batch operations', () => { ... }); // Handle how?
test('basic test', () => { ... });                     // Says nothing
test('should create and retrieve rows correctly', () => { ... }); // Vague "correctly"

Pattern: {action} {outcome} [condition]

"upsert stores row and get retrieves it"
 ^^^^^^ ^^^^^^^^^^   ^^^ ^^^^^^^^^^^^^
 action  outcome    action  outcome

"observer fires once per transaction, not per operation"
 ^^^^^^^^ ^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 subject   outcome              condition

"get() returns not_found for non-existent rows"
 ^^^^^ ^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^
 action     outcome            condition