diff --git a/.agent/skills/test-driven-development/SKILL.md b/.agent/skills/test-driven-development/SKILL.md new file mode 100644 index 00000000..33fb8512 --- /dev/null +++ b/.agent/skills/test-driven-development/SKILL.md @@ -0,0 +1,389 @@ +--- +name: test-driven-development +description: Use when implementing any feature or bugfix, before writing implementation code +--- + +# Test-Driven Development (TDD) + +## Overview + +Write the test first. Watch it fail. Write minimal code to pass. + +**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing. + +**Violating the letter of the rules is violating the spirit of the rules.** + +## When to Use + +**Always:** + +- New features +- Bug fixes +- Refactoring +- Behavior changes + +**Exceptions (ask your human partner):** + +- Throwaway prototypes +- Generated code +- Configuration files + +Thinking "skip TDD just this once"? Stop. That's rationalization. + +## The Iron Law + +``` +NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST +``` + +Write code before the test? Delete it. Start over. + +**No exceptions:** + +- Don't keep it as "reference" +- Don't "adapt" it while writing tests +- Don't look at it +- Delete means delete + +Implement fresh from tests. Period. + +## Red-Green-Refactor + +```dot +digraph tdd_cycle { + rankdir=LR; + red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"]; + verify_red [label="Verify fails\ncorrectly", shape=diamond]; + green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"]; + verify_green [label="Verify passes\nAll green", shape=diamond]; + refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"]; + next [label="Next", shape=ellipse]; + + red -> verify_red; + verify_red -> green [label="yes"]; + verify_red -> red [label="wrong\nfailure"]; + green -> verify_green; + verify_green -> refactor [label="yes"]; + verify_green -> green [label="no"]; + refactor -> verify_green [label="stay\ngreen"]; + verify_green -> next; + next -> red; +} +``` + +### RED - Write Failing Test + +Write one minimal test showing what should happen. + + +```typescript +test('retries failed operations 3 times', async () => { + let attempts = 0; + const operation = () => { + attempts++; + if (attempts < 3) throw new Error('fail'); + return 'success'; + }; + +const result = await retryOperation(operation); + +expect(result).toBe('success'); +expect(attempts).toBe(3); +}); + +```` +Clear name, tests real behavior, one thing + + + +```typescript +test('retry works', async () => { + const mock = jest.fn() + .mockRejectedValueOnce(new Error()) + .mockRejectedValueOnce(new Error()) + .mockResolvedValueOnce('success'); + await retryOperation(mock); + expect(mock).toHaveBeenCalledTimes(3); +}); +```` + +Vague name, tests mock not code + + +**Requirements:** + +- One behavior +- Clear name +- Real code (no mocks unless unavoidable) + +### Verify RED - Watch It Fail + +**MANDATORY. Never skip.** + +```bash +npm test path/to/test.test.ts +``` + +Confirm: + +- Test fails (not errors) +- Failure message is expected +- Fails because feature missing (not typos) + +**Test passes?** You're testing existing behavior. Fix test. + +**Test errors?** Fix error, re-run until it fails correctly. + +### GREEN - Minimal Code + +Write simplest code to pass the test. + + +```typescript +async function retryOperation(fn: () => Promise): Promise { + for (let i = 0; i < 3; i++) { + try { + return await fn(); + } catch (e) { + if (i === 2) throw e; + } + } + throw new Error('unreachable'); +} +``` +Just enough to pass + + + +```typescript +async function retryOperation( + fn: () => Promise, + options?: { + maxRetries?: number; + backoff?: 'linear' | 'exponential'; + onRetry?: (attempt: number) => void; + } +): Promise { + // YAGNI +} +``` +Over-engineered + + +Don't add features, refactor other code, or "improve" beyond the test. + +### Verify GREEN - Watch It Pass + +**MANDATORY.** + +```bash +npm test path/to/test.test.ts +``` + +Confirm: + +- Test passes +- Other tests still pass +- Output pristine (no errors, warnings) + +**Test fails?** Fix code, not test. + +**Other tests fail?** Fix now. + +### REFACTOR - Clean Up + +After green only: + +- Remove duplication +- Improve names +- Extract helpers + +Keep tests green. Don't add behavior. + +### Repeat + +Next failing test for next feature. + +## Good Tests + +| Quality | Good | Bad | +| ---------------- | ----------------------------------- | --------------------------------------------------- | +| **Minimal** | One thing. "and" in name? Split it. | `test('validates email and domain and whitespace')` | +| **Clear** | Name describes behavior | `test('test1')` | +| **Shows intent** | Demonstrates desired API | Obscures what code should do | + +## Why Order Matters + +**"I'll write tests after to verify it works"** + +Tests written after code pass immediately. Passing immediately proves nothing: + +- Might test wrong thing +- Might test implementation, not behavior +- Might miss edge cases you forgot +- You never saw it catch the bug + +Test-first forces you to see the test fail, proving it actually tests something. + +**"I already manually tested all the edge cases"** + +Manual testing is ad-hoc. You think you tested everything but: + +- No record of what you tested +- Can't re-run when code changes +- Easy to forget cases under pressure +- "It worked when I tried it" ≠ comprehensive + +Automated tests are systematic. They run the same way every time. + +**"Deleting X hours of work is wasteful"** + +Sunk cost fallacy. The time is already gone. Your choice now: + +- Delete and rewrite with TDD (X more hours, high confidence) +- Keep it and add tests after (30 min, low confidence, likely bugs) + +The "waste" is keeping code you can't trust. Working code without real tests is technical debt. + +**"TDD is dogmatic, being pragmatic means adapting"** + +TDD IS pragmatic: + +- Finds bugs before commit (faster than debugging after) +- Prevents regressions (tests catch breaks immediately) +- Documents behavior (tests show how to use code) +- Enables refactoring (change freely, tests catch breaks) + +"Pragmatic" shortcuts = debugging in production = slower. + +**"Tests after achieve the same goals - it's spirit not ritual"** + +No. Tests-after answer "What does this do?" Tests-first answer "What should this do?" + +Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones. + +Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't). + +30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work. + +## Common Rationalizations + +| Excuse | Reality | +| -------------------------------------- | ----------------------------------------------------------------------- | +| "Too simple to test" | Simple code breaks. Test takes 30 seconds. | +| "I'll test after" | Tests passing immediately prove nothing. | +| "Tests after achieve same goals" | Tests-after = "what does this do?" Tests-first = "what should this do?" | +| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. | +| "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is technical debt. | +| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete. | +| "Need to explore first" | Fine. Throw away exploration, start with TDD. | +| "Test hard = design unclear" | Listen to test. Hard to test = hard to use. | +| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. | +| "Manual test faster" | Manual doesn't prove edge cases. You'll re-test every change. | +| "Existing code has no tests" | You're improving it. Add tests for existing code. | + +## Red Flags - STOP and Start Over + +- Code before test +- Test after implementation +- Test passes immediately +- Can't explain why test failed +- Tests added "later" +- Rationalizing "just this once" +- "I already manually tested it" +- "Tests after achieve the same purpose" +- "It's about spirit not ritual" +- "Keep as reference" or "adapt existing code" +- "Already spent X hours, deleting is wasteful" +- "TDD is dogmatic, I'm being pragmatic" +- "This is different because..." + +**All of these mean: Delete code. Start over with TDD.** + +## Example: Bug Fix + +**Bug:** Empty email accepted + +**RED** + +```typescript +test('rejects empty email', async () => { + const result = await submitForm({ email: '' }); + expect(result.error).toBe('Email required'); +}); +``` + +**Verify RED** + +```bash +$ npm test +FAIL: expected 'Email required', got undefined +``` + +**GREEN** + +```typescript +function submitForm(data: FormData) { + if (!data.email?.trim()) { + return { error: 'Email required' }; + } + // ... +} +``` + +**Verify GREEN** + +```bash +$ npm test +PASS +``` + +**REFACTOR** +Extract validation for multiple fields if needed. + +## Verification Checklist + +Before marking work complete: + +- [ ] Every new function/method has a test +- [ ] Watched each test fail before implementing +- [ ] Each test failed for expected reason (feature missing, not typo) +- [ ] Wrote minimal code to pass each test +- [ ] All tests pass +- [ ] Output pristine (no errors, warnings) +- [ ] Tests use real code (mocks only if unavoidable) +- [ ] Edge cases and errors covered + +Can't check all boxes? You skipped TDD. Start over. + +## When Stuck + +| Problem | Solution | +| ---------------------- | -------------------------------------------------------------------- | +| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. | +| Test too complicated | Design too complicated. Simplify interface. | +| Must mock everything | Code too coupled. Use dependency injection. | +| Test setup huge | Extract helpers. Still complex? Simplify design. | + +## Debugging Integration + +Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression. + +Never fix bugs without a test. + +## Testing Anti-Patterns + +When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls: + +- Testing mock behavior instead of real behavior +- Adding test-only methods to production classes +- Mocking without understanding dependencies + +## Final Rule + +``` +Production code → test exists and failed first +Otherwise → not TDD +``` + +No exceptions without your human partner's permission. diff --git a/.agent/skills/test-driven-development/testing-anti-patterns.md b/.agent/skills/test-driven-development/testing-anti-patterns.md new file mode 100644 index 00000000..d0a654fa --- /dev/null +++ b/.agent/skills/test-driven-development/testing-anti-patterns.md @@ -0,0 +1,317 @@ +# Testing Anti-Patterns + +**Load this reference when:** writing or changing tests, adding mocks, or tempted to add test-only methods to production code. + +## Overview + +Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested. + +**Core principle:** Test what the code does, not what the mocks do. + +**Following strict TDD prevents these anti-patterns.** + +## The Iron Laws + +``` +1. NEVER test mock behavior +2. NEVER add test-only methods to production classes +3. NEVER mock without understanding dependencies +``` + +## Anti-Pattern 1: Testing Mock Behavior + +**The violation:** + +```typescript +// ❌ BAD: Testing that the mock exists +test('renders sidebar', () => { + render(); + expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument(); +}); +``` + +**Why this is wrong:** + +- You're verifying the mock works, not that the component works +- Test passes when mock is present, fails when it's not +- Tells you nothing about real behavior + +**your human partner's correction:** "Are we testing the behavior of a mock?" + +**The fix:** + +```typescript +// ✅ GOOD: Test real component or don't mock it +test('renders sidebar', () => { + render(); // Don't mock sidebar + expect(screen.getByRole('navigation')).toBeInTheDocument(); +}); + +// OR if sidebar must be mocked for isolation: +// Don't assert on the mock - test Page's behavior with sidebar present +``` + +### Gate Function + +``` +BEFORE asserting on any mock element: + Ask: "Am I testing real component behavior or just mock existence?" + + IF testing mock existence: + STOP - Delete the assertion or unmock the component + + Test real behavior instead +``` + +## Anti-Pattern 2: Test-Only Methods in Production + +**The violation:** + +```typescript +// ❌ BAD: destroy() only used in tests +class Session { + async destroy() { + // Looks like production API! + await this._workspaceManager?.destroyWorkspace(this.id); + // ... cleanup + } +} + +// In tests +afterEach(() => session.destroy()); +``` + +**Why this is wrong:** + +- Production class polluted with test-only code +- Dangerous if accidentally called in production +- Violates YAGNI and separation of concerns +- Confuses object lifecycle with entity lifecycle + +**The fix:** + +```typescript +// ✅ GOOD: Test utilities handle test cleanup +// Session has no destroy() - it's stateless in production + +// In test-utils/ +export async function cleanupSession(session: Session) { + const workspace = session.getWorkspaceInfo(); + if (workspace) { + await workspaceManager.destroyWorkspace(workspace.id); + } +} + +// In tests +afterEach(() => cleanupSession(session)); +``` + +### Gate Function + +``` +BEFORE adding any method to production class: + Ask: "Is this only used by tests?" + + IF yes: + STOP - Don't add it + Put it in test utilities instead + + Ask: "Does this class own this resource's lifecycle?" + + IF no: + STOP - Wrong class for this method +``` + +## Anti-Pattern 3: Mocking Without Understanding + +**The violation:** + +```typescript +// ❌ BAD: Mock breaks test logic +test('detects duplicate server', () => { + // Mock prevents config write that test depends on! + vi.mock('ToolCatalog', () => ({ + discoverAndCacheTools: vi.fn().mockResolvedValue(undefined), + })); + + await addServer(config); + await addServer(config); // Should throw - but won't! +}); +``` + +**Why this is wrong:** + +- Mocked method had side effect test depended on (writing config) +- Over-mocking to "be safe" breaks actual behavior +- Test passes for wrong reason or fails mysteriously + +**The fix:** + +```typescript +// ✅ GOOD: Mock at correct level +test('detects duplicate server', () => { + // Mock the slow part, preserve behavior test needs + vi.mock('MCPServerManager'); // Just mock slow server startup + + await addServer(config); // Config written + await addServer(config); // Duplicate detected ✓ +}); +``` + +### Gate Function + +``` +BEFORE mocking any method: + STOP - Don't mock yet + + 1. Ask: "What side effects does the real method have?" + 2. Ask: "Does this test depend on any of those side effects?" + 3. Ask: "Do I fully understand what this test needs?" + + IF depends on side effects: + Mock at lower level (the actual slow/external operation) + OR use test doubles that preserve necessary behavior + NOT the high-level method the test depends on + + IF unsure what test depends on: + Run test with real implementation FIRST + Observe what actually needs to happen + THEN add minimal mocking at the right level + + Red flags: + - "I'll mock this to be safe" + - "This might be slow, better mock it" + - Mocking without understanding the dependency chain +``` + +## Anti-Pattern 4: Incomplete Mocks + +**The violation:** + +```typescript +// ❌ BAD: Partial mock - only fields you think you need +const mockResponse = { + status: 'success', + data: { userId: '123', name: 'Alice' }, + // Missing: metadata that downstream code uses +}; + +// Later: breaks when code accesses response.metadata.requestId +``` + +**Why this is wrong:** + +- **Partial mocks hide structural assumptions** - You only mocked fields you know about +- **Downstream code may depend on fields you didn't include** - Silent failures +- **Tests pass but integration fails** - Mock incomplete, real API complete +- **False confidence** - Test proves nothing about real behavior + +**The Iron Rule:** Mock the COMPLETE data structure as it exists in reality, not just fields your immediate test uses. + +**The fix:** + +```typescript +// ✅ GOOD: Mirror real API completeness +const mockResponse = { + status: 'success', + data: { userId: '123', name: 'Alice' }, + metadata: { requestId: 'req-789', timestamp: 1234567890 }, + // All fields real API returns +}; +``` + +### Gate Function + +``` +BEFORE creating mock responses: + Check: "What fields does the real API response contain?" + + Actions: + 1. Examine actual API response from docs/examples + 2. Include ALL fields system might consume downstream + 3. Verify mock matches real response schema completely + + Critical: + If you're creating a mock, you must understand the ENTIRE structure + Partial mocks fail silently when code depends on omitted fields + + If uncertain: Include all documented fields +``` + +## Anti-Pattern 5: Integration Tests as Afterthought + +**The violation:** + +``` +✅ Implementation complete +❌ No tests written +"Ready for testing" +``` + +**Why this is wrong:** + +- Testing is part of implementation, not optional follow-up +- TDD would have caught this +- Can't claim complete without tests + +**The fix:** + +``` +TDD cycle: +1. Write failing test +2. Implement to pass +3. Refactor +4. THEN claim complete +``` + +## When Mocks Become Too Complex + +**Warning signs:** + +- Mock setup longer than test logic +- Mocking everything to make test pass +- Mocks missing methods real components have +- Test breaks when mock changes + +**your human partner's question:** "Do we need to be using a mock here?" + +**Consider:** Integration tests with real components often simpler than complex mocks + +## TDD Prevents These Anti-Patterns + +**Why TDD helps:** + +1. **Write test first** → Forces you to think about what you're actually testing +2. **Watch it fail** → Confirms test tests real behavior, not mocks +3. **Minimal implementation** → No test-only methods creep in +4. **Real dependencies** → You see what the test actually needs before mocking + +**If you're testing mock behavior, you violated TDD** - you added mocks without watching test fail against real code first. + +## Quick Reference + +| Anti-Pattern | Fix | +| ------------------------------- | --------------------------------------------- | +| Assert on mock elements | Test real component or unmock it | +| Test-only methods in production | Move to test utilities | +| Mock without understanding | Understand dependencies first, mock minimally | +| Incomplete mocks | Mirror real API completely | +| Tests as afterthought | TDD - tests first | +| Over-complex mocks | Consider integration tests | + +## Red Flags + +- Assertion checks for `*-mock` test IDs +- Methods only called in test files +- Mock setup is >50% of test +- Test fails when you remove mock +- Can't explain why mock is needed +- Mocking "just to be safe" + +## The Bottom Line + +**Mocks are tools to isolate, not things to test.** + +If TDD reveals you're testing mock behavior, you've gone wrong. + +Fix: Test real behavior or question why you're mocking at all. diff --git a/.agent/skills/vercel-react-best-practices/AGENTS.md b/.agent/skills/vercel-react-best-practices/AGENTS.md new file mode 100644 index 00000000..ef35f360 --- /dev/null +++ b/.agent/skills/vercel-react-best-practices/AGENTS.md @@ -0,0 +1,2666 @@ +# React Best Practices + +**Version 1.0.0** +Vercel Engineering +January 2026 + +> **Note:** +> This document is mainly for agents and LLMs to follow when maintaining, +> generating, or refactoring React and Next.js codebases at Vercel. Humans +> may also find it useful, but guidance here is optimized for automation +> and consistency by AI-assisted workflows. + +--- + +## Abstract + +Comprehensive performance optimization guide for React and Next.js applications, designed for AI agents and LLMs. Contains 40+ rules across 8 categories, prioritized by impact from critical (eliminating waterfalls, reducing bundle size) to incremental (advanced patterns). Each rule includes detailed explanations, real-world examples comparing incorrect vs. correct implementations, and specific impact metrics to guide automated refactoring and code generation. + +--- + +## Table of Contents + +1. [Eliminating Waterfalls](#1-eliminating-waterfalls) — **CRITICAL** + - 1.1 [Defer Await Until Needed](#11-defer-await-until-needed) + - 1.2 [Dependency-Based Parallelization](#12-dependency-based-parallelization) + - 1.3 [Prevent Waterfall Chains in API Routes](#13-prevent-waterfall-chains-in-api-routes) + - 1.4 [Promise.all() for Independent Operations](#14-promiseall-for-independent-operations) + - 1.5 [Strategic Suspense Boundaries](#15-strategic-suspense-boundaries) +2. [Bundle Size Optimization](#2-bundle-size-optimization) — **CRITICAL** + - 2.1 [Avoid Barrel File Imports](#21-avoid-barrel-file-imports) + - 2.2 [Conditional Module Loading](#22-conditional-module-loading) + - 2.3 [Defer Non-Critical Third-Party Libraries](#23-defer-non-critical-third-party-libraries) + - 2.4 [Dynamic Imports for Heavy Components](#24-dynamic-imports-for-heavy-components) + - 2.5 [Preload Based on User Intent](#25-preload-based-on-user-intent) +3. [Server-Side Performance](#3-server-side-performance) — **HIGH** + - 3.1 [Authenticate Server Actions Like API Routes](#31-authenticate-server-actions-like-api-routes) + - 3.2 [Avoid Duplicate Serialization in RSC Props](#32-avoid-duplicate-serialization-in-rsc-props) + - 3.3 [Cross-Request LRU Caching](#33-cross-request-lru-caching) + - 3.4 [Minimize Serialization at RSC Boundaries](#34-minimize-serialization-at-rsc-boundaries) + - 3.5 [Parallel Data Fetching with Component Composition](#35-parallel-data-fetching-with-component-composition) + - 3.6 [Per-Request Deduplication with React.cache()](#36-per-request-deduplication-with-reactcache) + - 3.7 [Use after() for Non-Blocking Operations](#37-use-after-for-non-blocking-operations) +4. [Client-Side Data Fetching](#4-client-side-data-fetching) — **MEDIUM-HIGH** + - 4.1 [Deduplicate Global Event Listeners](#41-deduplicate-global-event-listeners) + - 4.2 [Use Passive Event Listeners for Scrolling Performance](#42-use-passive-event-listeners-for-scrolling-performance) + - 4.3 [Use SWR for Automatic Deduplication](#43-use-swr-for-automatic-deduplication) + - 4.4 [Version and Minimize localStorage Data](#44-version-and-minimize-localstorage-data) +5. [Re-render Optimization](#5-re-render-optimization) — **MEDIUM** + - 5.1 [Defer State Reads to Usage Point](#51-defer-state-reads-to-usage-point) + - 5.2 [Do not wrap a simple expression with a primitive result type in useMemo](#52-do-not-wrap-a-simple-expression-with-a-primitive-result-type-in-usememo) + - 5.3 [Extract Default Non-primitive Parameter Value from Memoized Component to Constant](#53-extract-default-non-primitive-parameter-value-from-memoized-component-to-constant) + - 5.4 [Extract to Memoized Components](#54-extract-to-memoized-components) + - 5.5 [Narrow Effect Dependencies](#55-narrow-effect-dependencies) + - 5.6 [Subscribe to Derived State](#56-subscribe-to-derived-state) + - 5.7 [Use Functional setState Updates](#57-use-functional-setstate-updates) + - 5.8 [Use Lazy State Initialization](#58-use-lazy-state-initialization) + - 5.9 [Use Transitions for Non-Urgent Updates](#59-use-transitions-for-non-urgent-updates) +6. [Rendering Performance](#6-rendering-performance) — **MEDIUM** + - 6.1 [Animate SVG Wrapper Instead of SVG Element](#61-animate-svg-wrapper-instead-of-svg-element) + - 6.2 [CSS content-visibility for Long Lists](#62-css-content-visibility-for-long-lists) + - 6.3 [Hoist Static JSX Elements](#63-hoist-static-jsx-elements) + - 6.4 [Optimize SVG Precision](#64-optimize-svg-precision) + - 6.5 [Prevent Hydration Mismatch Without Flickering](#65-prevent-hydration-mismatch-without-flickering) + - 6.6 [Use Activity Component for Show/Hide](#66-use-activity-component-for-showhide) + - 6.7 [Use Explicit Conditional Rendering](#67-use-explicit-conditional-rendering) + - 6.8 [Use useTransition Over Manual Loading States](#68-use-usetransition-over-manual-loading-states) +7. [JavaScript Performance](#7-javascript-performance) — **LOW-MEDIUM** + - 7.1 [Avoid Layout Thrashing](#71-avoid-layout-thrashing) + - 7.2 [Build Index Maps for Repeated Lookups](#72-build-index-maps-for-repeated-lookups) + - 7.3 [Cache Property Access in Loops](#73-cache-property-access-in-loops) + - 7.4 [Cache Repeated Function Calls](#74-cache-repeated-function-calls) + - 7.5 [Cache Storage API Calls](#75-cache-storage-api-calls) + - 7.6 [Combine Multiple Array Iterations](#76-combine-multiple-array-iterations) + - 7.7 [Early Length Check for Array Comparisons](#77-early-length-check-for-array-comparisons) + - 7.8 [Early Return from Functions](#78-early-return-from-functions) + - 7.9 [Hoist RegExp Creation](#79-hoist-regexp-creation) + - 7.10 [Use Loop for Min/Max Instead of Sort](#710-use-loop-for-minmax-instead-of-sort) + - 7.11 [Use Set/Map for O(1) Lookups](#711-use-setmap-for-o1-lookups) + - 7.12 [Use toSorted() Instead of sort() for Immutability](#712-use-tosorted-instead-of-sort-for-immutability) +8. [Advanced Patterns](#8-advanced-patterns) — **LOW** + - 8.1 [Store Event Handlers in Refs](#81-store-event-handlers-in-refs) + - 8.2 [useEffectEvent for Stable Callback Refs](#82-useeffectevent-for-stable-callback-refs) + +--- + +## 1. Eliminating Waterfalls + +**Impact: CRITICAL** + +Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains. + +### 1.1 Defer Await Until Needed + +**Impact: HIGH (avoids blocking unused code paths)** + +Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them. + +**Incorrect: blocks both branches** + +```typescript +async function handleRequest(userId: string, skipProcessing: boolean) { + const userData = await fetchUserData(userId); + + if (skipProcessing) { + // Returns immediately but still waited for userData + return { skipped: true }; + } + + // Only this branch uses userData + return processUserData(userData); +} +``` + +**Correct: only blocks when needed** + +```typescript +async function handleRequest(userId: string, skipProcessing: boolean) { + if (skipProcessing) { + // Returns immediately without waiting + return { skipped: true }; + } + + // Fetch only when needed + const userData = await fetchUserData(userId); + return processUserData(userData); +} +``` + +**Another example: early return optimization** + +```typescript +// Incorrect: always fetches permissions +async function updateResource(resourceId: string, userId: string) { + const permissions = await fetchPermissions(userId); + const resource = await getResource(resourceId); + + if (!resource) { + return { error: 'Not found' }; + } + + if (!permissions.canEdit) { + return { error: 'Forbidden' }; + } + + return await updateResourceData(resource, permissions); +} + +// Correct: fetches only when needed +async function updateResource(resourceId: string, userId: string) { + const resource = await getResource(resourceId); + + if (!resource) { + return { error: 'Not found' }; + } + + const permissions = await fetchPermissions(userId); + + if (!permissions.canEdit) { + return { error: 'Forbidden' }; + } + + return await updateResourceData(resource, permissions); +} +``` + +This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive. + +### 1.2 Dependency-Based Parallelization + +**Impact: CRITICAL (2-10× improvement)** + +For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment. + +**Incorrect: profile waits for config unnecessarily** + +```typescript +const [user, config] = await Promise.all([fetchUser(), fetchConfig()]); +const profile = await fetchProfile(user.id); +``` + +**Correct: config and profile run in parallel** + +```typescript +import { all } from 'better-all'; + +const { user, config, profile } = await all({ + async user() { + return fetchUser(); + }, + async config() { + return fetchConfig(); + }, + async profile() { + return fetchProfile((await this.$.user).id); + }, +}); +``` + +**Alternative without extra dependencies:** + +```typescript +const userPromise = fetchUser(); +const profilePromise = userPromise.then((user) => fetchProfile(user.id)); + +const [user, config, profile] = await Promise.all([userPromise, fetchConfig(), profilePromise]); +``` + +We can also create all the promises first, and do `Promise.all()` at the end. + +Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all) + +### 1.3 Prevent Waterfall Chains in API Routes + +**Impact: CRITICAL (2-10× improvement)** + +In API routes and Server Actions, start independent operations immediately, even if you don't await them yet. + +**Incorrect: config waits for auth, data waits for both** + +```typescript +export async function GET(request: Request) { + const session = await auth(); + const config = await fetchConfig(); + const data = await fetchData(session.user.id); + return Response.json({ data, config }); +} +``` + +**Correct: auth and config start immediately** + +```typescript +export async function GET(request: Request) { + const sessionPromise = auth(); + const configPromise = fetchConfig(); + const session = await sessionPromise; + const [config, data] = await Promise.all([configPromise, fetchData(session.user.id)]); + return Response.json({ data, config }); +} +``` + +For operations with more complex dependency chains, use `better-all` to automatically maximize parallelism (see Dependency-Based Parallelization). + +### 1.4 Promise.all() for Independent Operations + +**Impact: CRITICAL (2-10× improvement)** + +When async operations have no interdependencies, execute them concurrently using `Promise.all()`. + +**Incorrect: sequential execution, 3 round trips** + +```typescript +const user = await fetchUser(); +const posts = await fetchPosts(); +const comments = await fetchComments(); +``` + +**Correct: parallel execution, 1 round trip** + +```typescript +const [user, posts, comments] = await Promise.all([fetchUser(), fetchPosts(), fetchComments()]); +``` + +### 1.5 Strategic Suspense Boundaries + +**Impact: HIGH (faster initial paint)** + +Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads. + +**Incorrect: wrapper blocked by data fetching** + +```tsx +async function Page() { + const data = await fetchData(); // Blocks entire page + + return ( +
+
Sidebar
+
Header
+
+ +
+
Footer
+
+ ); +} +``` + +The entire layout waits for data even though only the middle section needs it. + +**Correct: wrapper shows immediately, data streams in** + +```tsx +function Page() { + return ( +
+
Sidebar
+
Header
+
+ }> + + +
+
Footer
+
+ ); +} + +async function DataDisplay() { + const data = await fetchData(); // Only blocks this component + return
{data.content}
; +} +``` + +Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data. + +**Alternative: share promise across components** + +```tsx +function Page() { + // Start fetch immediately, but don't await + const dataPromise = fetchData(); + + return ( +
+
Sidebar
+
Header
+ }> + + + +
Footer
+
+ ); +} + +function DataDisplay({ dataPromise }: { dataPromise: Promise }) { + const data = use(dataPromise); // Unwraps the promise + return
{data.content}
; +} + +function DataSummary({ dataPromise }: { dataPromise: Promise }) { + const data = use(dataPromise); // Reuses the same promise + return
{data.summary}
; +} +``` + +Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together. + +**When NOT to use this pattern:** + +- Critical data needed for layout decisions (affects positioning) + +- SEO-critical content above the fold + +- Small, fast queries where suspense overhead isn't worth it + +- When you want to avoid layout shift (loading → content jump) + +**Trade-off:** Faster initial paint vs potential layout shift. Choose based on your UX priorities. + +--- + +## 2. Bundle Size Optimization + +**Impact: CRITICAL** + +Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint. + +### 2.1 Avoid Barrel File Imports + +**Impact: CRITICAL (200-800ms import cost, slow builds)** + +Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`). + +Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts. + +**Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph. + +**Incorrect: imports entire library** + +```tsx +import { Check, X, Menu } from 'lucide-react'; +// Loads 1,583 modules, takes ~2.8s extra in dev +// Runtime cost: 200-800ms on every cold start + +import { Button, TextField } from '@mui/material'; +// Loads 2,225 modules, takes ~4.2s extra in dev +``` + +**Correct: imports only what you need** + +```tsx +import Check from 'lucide-react/dist/esm/icons/check'; +import X from 'lucide-react/dist/esm/icons/x'; +import Menu from 'lucide-react/dist/esm/icons/menu'; +// Loads only 3 modules (~2KB vs ~1MB) + +import Button from '@mui/material/Button'; +import TextField from '@mui/material/TextField'; +// Loads only what you use +``` + +**Alternative: Next.js 13.5+** + +```js +// next.config.js - use optimizePackageImports +module.exports = { + experimental: { + optimizePackageImports: ['lucide-react', '@mui/material'], + }, +}; + +// Then you can keep the ergonomic barrel imports: +import { Check, X, Menu } from 'lucide-react'; +// Automatically transformed to direct imports at build time +``` + +Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR. + +Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`. + +Reference: [https://vercel.com/blog/how-we-optimized-package-imports-in-next-js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js) + +### 2.2 Conditional Module Loading + +**Impact: HIGH (loads large data only when needed)** + +Load large data or modules only when a feature is activated. + +**Example: lazy-load animation frames** + +```tsx +function AnimationPlayer({ + enabled, + setEnabled, +}: { + enabled: boolean; + setEnabled: React.Dispatch>; +}) { + const [frames, setFrames] = useState(null); + + useEffect(() => { + if (enabled && !frames && typeof window !== 'undefined') { + import('./animation-frames.js').then((mod) => setFrames(mod.frames)).catch(() => setEnabled(false)); + } + }, [enabled, frames, setEnabled]); + + if (!frames) return ; + return ; +} +``` + +The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed. + +### 2.3 Defer Non-Critical Third-Party Libraries + +**Impact: MEDIUM (loads after hydration)** + +Analytics, logging, and error tracking don't block user interaction. Load them after hydration. + +**Incorrect: blocks initial bundle** + +```tsx +import { Analytics } from '@vercel/analytics/react'; + +export default function RootLayout({ children }) { + return ( + + + {children} + + + + ); +} +``` + +**Correct: loads after hydration** + +```tsx +import dynamic from 'next/dynamic'; + +const Analytics = dynamic(() => import('@vercel/analytics/react').then((m) => m.Analytics), { ssr: false }); + +export default function RootLayout({ children }) { + return ( + + + {children} + + + + ); +} +``` + +### 2.4 Dynamic Imports for Heavy Components + +**Impact: CRITICAL (directly affects TTI and LCP)** + +Use `next/dynamic` to lazy-load large components not needed on initial render. + +**Incorrect: Monaco bundles with main chunk ~300KB** + +```tsx +import { MonacoEditor } from './monaco-editor'; + +function CodePanel({ code }: { code: string }) { + return ; +} +``` + +**Correct: Monaco loads on demand** + +```tsx +import dynamic from 'next/dynamic'; + +const MonacoEditor = dynamic(() => import('./monaco-editor').then((m) => m.MonacoEditor), { ssr: false }); + +function CodePanel({ code }: { code: string }) { + return ; +} +``` + +### 2.5 Preload Based on User Intent + +**Impact: MEDIUM (reduces perceived latency)** + +Preload heavy bundles before they're needed to reduce perceived latency. + +**Example: preload on hover/focus** + +```tsx +function EditorButton({ onClick }: { onClick: () => void }) { + const preload = () => { + if (typeof window !== 'undefined') { + void import('./monaco-editor'); + } + }; + + return ( + + ); +} +``` + +**Example: preload when feature flag is enabled** + +```tsx +function FlagsProvider({ children, flags }: Props) { + useEffect(() => { + if (flags.editorEnabled && typeof window !== 'undefined') { + void import('./monaco-editor').then((mod) => mod.init()); + } + }, [flags.editorEnabled]); + + return {children}; +} +``` + +The `typeof window !== 'undefined'` check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed. + +--- + +## 3. Server-Side Performance + +**Impact: HIGH** + +Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times. + +### 3.1 Authenticate Server Actions Like API Routes + +**Impact: CRITICAL (prevents unauthorized access to server mutations)** + +Server Actions (functions with `"use server"`) are exposed as public endpoints, just like API routes. Always verify authentication and authorization **inside** each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly. + +Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation." + +**Incorrect: no authentication check** + +```typescript +'use server'; + +export async function deleteUser(userId: string) { + // Anyone can call this! No auth check + await db.user.delete({ where: { id: userId } }); + return { success: true }; +} +``` + +**Correct: authentication inside the action** + +```typescript +'use server'; + +import { verifySession } from '@/lib/auth'; +import { unauthorized } from '@/lib/errors'; + +export async function deleteUser(userId: string) { + // Always check auth inside the action + const session = await verifySession(); + + if (!session) { + throw unauthorized('Must be logged in'); + } + + // Check authorization too + if (session.user.role !== 'admin' && session.user.id !== userId) { + throw unauthorized('Cannot delete other users'); + } + + await db.user.delete({ where: { id: userId } }); + return { success: true }; +} +``` + +**With input validation:** + +```typescript +'use server'; + +import { verifySession } from '@/lib/auth'; +import { z } from 'zod'; + +const updateProfileSchema = z.object({ + userId: z.string().uuid(), + name: z.string().min(1).max(100), + email: z.string().email(), +}); + +export async function updateProfile(data: unknown) { + // Validate input first + const validated = updateProfileSchema.parse(data); + + // Then authenticate + const session = await verifySession(); + if (!session) { + throw new Error('Unauthorized'); + } + + // Then authorize + if (session.user.id !== validated.userId) { + throw new Error('Can only update own profile'); + } + + // Finally perform the mutation + await db.user.update({ + where: { id: validated.userId }, + data: { + name: validated.name, + email: validated.email, + }, + }); + + return { success: true }; +} +``` + +Reference: [https://nextjs.org/docs/app/guides/authentication](https://nextjs.org/docs/app/guides/authentication) + +### 3.2 Avoid Duplicate Serialization in RSC Props + +**Impact: LOW (reduces network payload by avoiding duplicate serialization)** + +RSC→client serialization deduplicates by object reference, not value. Same reference = serialized once; new reference = serialized again. Do transformations (`.toSorted()`, `.filter()`, `.map()`) in client, not server. + +**Incorrect: duplicates array** + +```tsx +// RSC: sends 6 strings (2 arrays × 3 items) + +``` + +**Correct: sends 3 strings** + +```tsx +// RSC: send once +; + +// Client: transform there +('use client'); +const sorted = useMemo(() => [...usernames].sort(), [usernames]); +``` + +**Nested deduplication behavior:** + +```tsx +// string[] - duplicates everything +usernames={['a','b']} sorted={usernames.toSorted()} // sends 4 strings + +// object[] - duplicates array structure only +users={[{id:1},{id:2}]} sorted={users.toSorted()} // sends 2 arrays + 2 unique objects (not 4) +``` + +Deduplication works recursively. Impact varies by data type: + +- `string[]`, `number[]`, `boolean[]`: **HIGH impact** - array + all primitives fully duplicated + +- `object[]`: **LOW impact** - array duplicated, but nested objects deduplicated by reference + +**Operations breaking deduplication: create new references** + +- Arrays: `.toSorted()`, `.filter()`, `.map()`, `.slice()`, `[...arr]` + +- Objects: `{...obj}`, `Object.assign()`, `structuredClone()`, `JSON.parse(JSON.stringify())` + +**More examples:** + +```tsx +// ❌ Bad + u.active)} /> + + +// ✅ Good + + +// Do filtering/destructuring in client +``` + +**Exception:** Pass derived data when transformation is expensive or client doesn't need original. + +### 3.3 Cross-Request LRU Caching + +**Impact: HIGH (caches across requests)** + +`React.cache()` only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache. + +**Implementation:** + +```typescript +import { LRUCache } from 'lru-cache'; + +const cache = new LRUCache({ + max: 1000, + ttl: 5 * 60 * 1000, // 5 minutes +}); + +export async function getUser(id: string) { + const cached = cache.get(id); + if (cached) return cached; + + const user = await db.user.findUnique({ where: { id } }); + cache.set(id, user); + return user; +} + +// Request 1: DB query, result cached +// Request 2: cache hit, no DB query +``` + +Use when sequential user actions hit multiple endpoints needing the same data within seconds. + +**With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis. + +**In traditional serverless:** Each invocation runs in isolation, so consider Redis for cross-process caching. + +Reference: [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache) + +### 3.4 Minimize Serialization at RSC Boundaries + +**Impact: HIGH (reduces data transfer size)** + +The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses. + +**Incorrect: serializes all 50 fields** + +```tsx +async function Page() { + const user = await fetchUser(); // 50 fields + return ; +} + +('use client'); +function Profile({ user }: { user: User }) { + return
{user.name}
; // uses 1 field +} +``` + +**Correct: serializes only 1 field** + +```tsx +async function Page() { + const user = await fetchUser(); + return ; +} + +('use client'); +function Profile({ name }: { name: string }) { + return
{name}
; +} +``` + +### 3.5 Parallel Data Fetching with Component Composition + +**Impact: CRITICAL (eliminates server-side waterfalls)** + +React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching. + +**Incorrect: Sidebar waits for Page's fetch to complete** + +```tsx +export default async function Page() { + const header = await fetchHeader(); + return ( +
+
{header}
+ +
+ ); +} + +async function Sidebar() { + const items = await fetchSidebarItems(); + return ; +} +``` + +**Correct: both fetch simultaneously** + +```tsx +async function Header() { + const data = await fetchHeader(); + return
{data}
; +} + +async function Sidebar() { + const items = await fetchSidebarItems(); + return ; +} + +export default function Page() { + return ( +
+
+ +
+ ); +} +``` + +**Alternative with children prop:** + +```tsx +async function Header() { + const data = await fetchHeader(); + return
{data}
; +} + +async function Sidebar() { + const items = await fetchSidebarItems(); + return ; +} + +function Layout({ children }: { children: ReactNode }) { + return ( +
+
+ {children} +
+ ); +} + +export default function Page() { + return ( + + + + ); +} +``` + +### 3.6 Per-Request Deduplication with React.cache() + +**Impact: MEDIUM (deduplicates within request)** + +Use `React.cache()` for server-side request deduplication. Authentication and database queries benefit most. + +**Usage:** + +```typescript +import { cache } from 'react'; + +export const getCurrentUser = cache(async () => { + const session = await auth(); + if (!session?.user?.id) return null; + return await db.user.findUnique({ + where: { id: session.user.id }, + }); +}); +``` + +Within a single request, multiple calls to `getCurrentUser()` execute the query only once. + +**Avoid inline objects as arguments:** + +`React.cache()` uses shallow equality (`Object.is`) to determine cache hits. Inline objects create new references each call, preventing cache hits. + +**Incorrect: always cache miss** + +```typescript +const getUser = cache(async (params: { uid: number }) => { + return await db.user.findUnique({ where: { id: params.uid } }); +}); + +// Each call creates new object, never hits cache +getUser({ uid: 1 }); +getUser({ uid: 1 }); // Cache miss, runs query again +``` + +**Correct: cache hit** + +```typescript +const params = { uid: 1 }; +getUser(params); // Query runs +getUser(params); // Cache hit (same reference) +``` + +If you must pass objects, pass the same reference: + +**Next.js-Specific Note:** + +In Next.js, the `fetch` API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need `React.cache()` for `fetch` calls. However, `React.cache()` is still essential for other async tasks: + +- Database queries (Prisma, Drizzle, etc.) + +- Heavy computations + +- Authentication checks + +- File system operations + +- Any non-fetch async work + +Use `React.cache()` to deduplicate these operations across your component tree. + +Reference: [https://react.dev/reference/react/cache](https://react.dev/reference/react/cache) + +### 3.7 Use after() for Non-Blocking Operations + +**Impact: MEDIUM (faster response times)** + +Use Next.js's `after()` to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response. + +**Incorrect: blocks response** + +```tsx +import { logUserAction } from '@/app/utils'; + +export async function POST(request: Request) { + // Perform mutation + await updateDatabase(request); + + // Logging blocks the response + const userAgent = request.headers.get('user-agent') || 'unknown'; + await logUserAction({ userAgent }); + + return new Response(JSON.stringify({ status: 'success' }), { + status: 200, + headers: { 'Content-Type': 'application/json' }, + }); +} +``` + +**Correct: non-blocking** + +```tsx +import { after } from 'next/server'; +import { headers, cookies } from 'next/headers'; +import { logUserAction } from '@/app/utils'; + +export async function POST(request: Request) { + // Perform mutation + await updateDatabase(request); + + // Log after response is sent + after(async () => { + const userAgent = (await headers()).get('user-agent') || 'unknown'; + const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous'; + + logUserAction({ sessionCookie, userAgent }); + }); + + return new Response(JSON.stringify({ status: 'success' }), { + status: 200, + headers: { 'Content-Type': 'application/json' }, + }); +} +``` + +The response is sent immediately while logging happens in the background. + +**Common use cases:** + +- Analytics tracking + +- Audit logging + +- Sending notifications + +- Cache invalidation + +- Cleanup tasks + +**Important notes:** + +- `after()` runs even if the response fails or redirects + +- Works in Server Actions, Route Handlers, and Server Components + +Reference: [https://nextjs.org/docs/app/api-reference/functions/after](https://nextjs.org/docs/app/api-reference/functions/after) + +--- + +## 4. Client-Side Data Fetching + +**Impact: MEDIUM-HIGH** + +Automatic deduplication and efficient data fetching patterns reduce redundant network requests. + +### 4.1 Deduplicate Global Event Listeners + +**Impact: LOW (single listener for N components)** + +Use `useSWRSubscription()` to share global event listeners across component instances. + +**Incorrect: N instances = N listeners** + +```tsx +function useKeyboardShortcut(key: string, callback: () => void) { + useEffect(() => { + const handler = (e: KeyboardEvent) => { + if (e.metaKey && e.key === key) { + callback(); + } + }; + window.addEventListener('keydown', handler); + return () => window.removeEventListener('keydown', handler); + }, [key, callback]); +} +``` + +When using the `useKeyboardShortcut` hook multiple times, each instance will register a new listener. + +**Correct: N instances = 1 listener** + +```tsx +import useSWRSubscription from 'swr/subscription'; + +// Module-level Map to track callbacks per key +const keyCallbacks = new Map void>>(); + +function useKeyboardShortcut(key: string, callback: () => void) { + // Register this callback in the Map + useEffect(() => { + if (!keyCallbacks.has(key)) { + keyCallbacks.set(key, new Set()); + } + keyCallbacks.get(key)!.add(callback); + + return () => { + const set = keyCallbacks.get(key); + if (set) { + set.delete(callback); + if (set.size === 0) { + keyCallbacks.delete(key); + } + } + }; + }, [key, callback]); + + useSWRSubscription('global-keydown', () => { + const handler = (e: KeyboardEvent) => { + if (e.metaKey && keyCallbacks.has(e.key)) { + keyCallbacks.get(e.key)!.forEach((cb) => cb()); + } + }; + window.addEventListener('keydown', handler); + return () => window.removeEventListener('keydown', handler); + }); +} + +function Profile() { + // Multiple shortcuts will share the same listener + useKeyboardShortcut('p', () => { + /* ... */ + }); + useKeyboardShortcut('k', () => { + /* ... */ + }); + // ... +} +``` + +### 4.2 Use Passive Event Listeners for Scrolling Performance + +**Impact: MEDIUM (eliminates scroll delay caused by event listeners)** + +Add `{ passive: true }` to touch and wheel event listeners to enable immediate scrolling. Browsers normally wait for listeners to finish to check if `preventDefault()` is called, causing scroll delay. + +**Incorrect:** + +```typescript +useEffect(() => { + const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX); + const handleWheel = (e: WheelEvent) => console.log(e.deltaY); + + document.addEventListener('touchstart', handleTouch); + document.addEventListener('wheel', handleWheel); + + return () => { + document.removeEventListener('touchstart', handleTouch); + document.removeEventListener('wheel', handleWheel); + }; +}, []); +``` + +**Correct:** + +```typescript +useEffect(() => { + const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX); + const handleWheel = (e: WheelEvent) => console.log(e.deltaY); + + document.addEventListener('touchstart', handleTouch, { passive: true }); + document.addEventListener('wheel', handleWheel, { passive: true }); + + return () => { + document.removeEventListener('touchstart', handleTouch); + document.removeEventListener('wheel', handleWheel); + }; +}, []); +``` + +**Use passive when:** tracking/analytics, logging, any listener that doesn't call `preventDefault()`. + +**Don't use passive when:** implementing custom swipe gestures, custom zoom controls, or any listener that needs `preventDefault()`. + +### 4.3 Use SWR for Automatic Deduplication + +**Impact: MEDIUM-HIGH (automatic deduplication)** + +SWR enables request deduplication, caching, and revalidation across component instances. + +**Incorrect: no deduplication, each instance fetches** + +```tsx +function UserList() { + const [users, setUsers] = useState([]); + useEffect(() => { + fetch('/api/users') + .then((r) => r.json()) + .then(setUsers); + }, []); +} +``` + +**Correct: multiple instances share one request** + +```tsx +import useSWR from 'swr'; + +function UserList() { + const { data: users } = useSWR('/api/users', fetcher); +} +``` + +**For immutable data:** + +```tsx +import { useImmutableSWR } from '@/lib/swr'; + +function StaticContent() { + const { data } = useImmutableSWR('/api/config', fetcher); +} +``` + +**For mutations:** + +```tsx +import { useSWRMutation } from 'swr/mutation'; + +function UpdateButton() { + const { trigger } = useSWRMutation('/api/user', updateUser); + return ; +} +``` + +Reference: [https://swr.vercel.app](https://swr.vercel.app) + +### 4.4 Version and Minimize localStorage Data + +**Impact: MEDIUM (prevents schema conflicts, reduces storage size)** + +Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data. + +**Incorrect:** + +```typescript +// No version, stores everything, no error handling +localStorage.setItem('userConfig', JSON.stringify(fullUserObject)); +const data = localStorage.getItem('userConfig'); +``` + +**Correct:** + +```typescript +const VERSION = 'v2'; + +function saveConfig(config: { theme: string; language: string }) { + try { + localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config)); + } catch { + // Throws in incognito/private browsing, quota exceeded, or disabled + } +} + +function loadConfig() { + try { + const data = localStorage.getItem(`userConfig:${VERSION}`); + return data ? JSON.parse(data) : null; + } catch { + return null; + } +} + +// Migration from v1 to v2 +function migrate() { + try { + const v1 = localStorage.getItem('userConfig:v1'); + if (v1) { + const old = JSON.parse(v1); + saveConfig({ theme: old.darkMode ? 'dark' : 'light', language: old.lang }); + localStorage.removeItem('userConfig:v1'); + } + } catch {} +} +``` + +**Store minimal fields from server responses:** + +```typescript +// User object has 20+ fields, only store what UI needs +function cachePrefs(user: FullUser) { + try { + localStorage.setItem( + 'prefs:v1', + JSON.stringify({ + theme: user.preferences.theme, + notifications: user.preferences.notifications, + }) + ); + } catch {} +} +``` + +**Always wrap in try-catch:** `getItem()` and `setItem()` throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled. + +**Benefits:** Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags. + +--- + +## 5. Re-render Optimization + +**Impact: MEDIUM** + +Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness. + +### 5.1 Defer State Reads to Usage Point + +**Impact: MEDIUM (avoids unnecessary subscriptions)** + +Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks. + +**Incorrect: subscribes to all searchParams changes** + +```tsx +function ShareButton({ chatId }: { chatId: string }) { + const searchParams = useSearchParams(); + + const handleShare = () => { + const ref = searchParams.get('ref'); + shareChat(chatId, { ref }); + }; + + return ; +} +``` + +**Correct: reads on demand, no subscription** + +```tsx +function ShareButton({ chatId }: { chatId: string }) { + const handleShare = () => { + const params = new URLSearchParams(window.location.search); + const ref = params.get('ref'); + shareChat(chatId, { ref }); + }; + + return ; +} +``` + +### 5.2 Do not wrap a simple expression with a primitive result type in useMemo + +**Impact: LOW-MEDIUM (wasted computation on every render)** + +When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in `useMemo`. + +Calling `useMemo` and comparing hook dependencies may consume more resources than the expression itself. + +**Incorrect:** + +```tsx +function Header({ user, notifications }: Props) { + const isLoading = useMemo(() => { + return user.isLoading || notifications.isLoading; + }, [user.isLoading, notifications.isLoading]); + + if (isLoading) return ; + // return some markup +} +``` + +**Correct:** + +```tsx +function Header({ user, notifications }: Props) { + const isLoading = user.isLoading || notifications.isLoading; + + if (isLoading) return ; + // return some markup +} +``` + +### 5.3 Extract Default Non-primitive Parameter Value from Memoized Component to Constant + +**Impact: MEDIUM (restores memoization by using a constant for default value)** + +When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in `memo()`. + +To address this issue, extract the default value into a constant. + +**Incorrect: `onClick` has different values on every rerender** + +```tsx +const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) { + // ... +}) + +// Used without optional onClick + +``` + +**Correct: stable default value** + +```tsx +const NOOP = () => {}; + +const UserAvatar = memo(function UserAvatar({ onClick = NOOP }: { onClick?: () => void }) { + // ... +}) + +// Used without optional onClick + +``` + +### 5.4 Extract to Memoized Components + +**Impact: MEDIUM (enables early returns)** + +Extract expensive work into memoized components to enable early returns before computation. + +**Incorrect: computes avatar even when loading** + +```tsx +function Profile({ user, loading }: Props) { + const avatar = useMemo(() => { + const id = computeAvatarId(user); + return ; + }, [user]); + + if (loading) return ; + return
{avatar}
; +} +``` + +**Correct: skips computation when loading** + +```tsx +const UserAvatar = memo(function UserAvatar({ user }: { user: User }) { + const id = useMemo(() => computeAvatarId(user), [user]); + return ; +}); + +function Profile({ user, loading }: Props) { + if (loading) return ; + return ( +
+ +
+ ); +} +``` + +**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders. + +### 5.5 Narrow Effect Dependencies + +**Impact: LOW (minimizes effect re-runs)** + +Specify primitive dependencies instead of objects to minimize effect re-runs. + +**Incorrect: re-runs on any user field change** + +```tsx +useEffect(() => { + console.log(user.id); +}, [user]); +``` + +**Correct: re-runs only when id changes** + +```tsx +useEffect(() => { + console.log(user.id); +}, [user.id]); +``` + +**For derived state, compute outside effect:** + +```tsx +// Incorrect: runs on width=767, 766, 765... +useEffect(() => { + if (width < 768) { + enableMobileMode(); + } +}, [width]); + +// Correct: runs only on boolean transition +const isMobile = width < 768; +useEffect(() => { + if (isMobile) { + enableMobileMode(); + } +}, [isMobile]); +``` + +### 5.6 Subscribe to Derived State + +**Impact: MEDIUM (reduces re-render frequency)** + +Subscribe to derived boolean state instead of continuous values to reduce re-render frequency. + +**Incorrect: re-renders on every pixel change** + +```tsx +function Sidebar() { + const width = useWindowWidth(); // updates continuously + const isMobile = width < 768; + return