Design-Led Development

Build systems that feel inevitable, trustworthy, and delightful. Every line of code serves a human outcome.

Core Decision Framework

Before writing any feature, answer these questions in order:

1. User outcome: "This helps [user] achieve [outcome] by [mechanism]"
2. Anxiety/control: Does this reduce user anxiety or increase user control?
3. Simplicity: Is this the simplest solution?
4. Measurability: Can we measure success?
5. Failure mode: What's the failure mode? If catastrophic, add safeguards
6. Recovery: Can users recover from errors?

If you cannot articulate the user outcome in one sentence, do not code it yet.

Code Principles

Clarity Over Cleverness

// ✅ DO: Name for humans reading at 2am
const userAuthenticationStatus = checkAuth(userId);
const formattedOrderDate = formatDate(order.createdAt);

// ❌ DON'T: Clever but obscure
const x = chk(u);
const d = fmt(o.c);

Comments explain why, not what. Red flag phrases: "just", "simply", "obviously".

Explicit Error Handling

// ✅ DO: Error states as return types
type Result<T> = 
  | { success: true; data: T } 
  | { success: false; error: UserFacingError };

async function fetchUser(id: string): Promise<Result<User>> {
  try {
    const user = await api.getUser(id);
    return { success: true, data: user };
  } catch (error) {
    return { 
      success: false, 
      error: { 
        message: "Unable to load user profile",
        action: "Please try again or contact support"
      }
    };
  }
}

// ❌ NEVER: Generic errors or silent failures
throw new Error("Something went wrong");

Network Resilience

// ✅ DO: Exponential backoff with jitter
const retryWithBackoff = async <T>(
  fn: () => Promise<T>, 
  maxRetries = 3
): Promise<T> => {
  for (let i = 0; i < maxRetries; i++) {
    try {
      return await fn();
    } catch (error) {
      if (i === maxRetries - 1) throw error;
      const delay = Math.min(1000 * 2 ** i + Math.random() * 1000, 10000);
      await new Promise(resolve => setTimeout(resolve, delay));
    }
  }
  throw new Error('Max retries exceeded');
};

// ✅ DO: Timeout promises
const withTimeout = <T>(promise: Promise<T>, ms: number): Promise<T> =>
  Promise.race([
    promise,
    new Promise<never>((_, reject) =>
      setTimeout(() => reject(new Error('Request timeout')), ms)
    )
  ]);

Performance Budgets

Set these before coding:

Metric	Budget
Cold start	< 2s on median device
Interaction response	< 100ms perceived
Animations	< 16ms per frame (60fps)
API calls (p95)	< 500ms

Treat performance regressions as P0 bugs. Profile on low-end devices.

UI Component States

Every interactive component MUST handle all states:

✅ Default (idle)
✅ Hover (pointer devices)
✅ Active/pressed
✅ Focus (keyboard navigation)
✅ Disabled (with explanation why)
✅ Loading (with progress indication)
✅ Error (with recovery action)
✅ Success (with next step)
✅ Empty (with helpful onboarding)

Visual System

Spacing (8pt Grid)

4px:  Tight grouping (icon + label)
8px:  Related items (form fields in group)
16px: Section separation
24px: Component boundaries
32px: Major sections
48px: Screen-level padding

Typography Scale

H1: 32-40px, bold, line-height 1.2
H2: 24-28px, semibold, line-height 1.3
H3: 20-24px, semibold, line-height 1.4
Body: 16-18px, regular, line-height 1.5
Caption: 14px, regular, line-height 1.4

RULE: Never below 14px for body text (accessibility)

Motion

Purpose	Duration	Easing
Micro-interactions	100-200ms	ease-out
Screen transitions	300-400ms	ease-in-out
Loading states	600ms+	linear

Rules:

• Animation explains, never decorates
• Respect prefers-reduced-motion
• Never delay user actions

Accessibility Requirements

Non-negotiable checklist:

• Semantic HTML with ARIA labels
• Keyboard navigation for all interactions
• Color contrast: 4.5:1 minimum (7:1 for body text)
• Touch targets: 44x44pt minimum
• Screen reader tested
• Respects prefers-reduced-motion
• Respects prefers-color-scheme

Trust Architecture

Every feature must answer:

1. Data collected: What data? (Collect minimum)
2. Failure modes: What could go wrong? (Design failures first)
3. Trust signals: How do I prove safety? (Make visible)
4. Reversibility: Can users undo? (Preview before commit)
5. Data fate: What happens to their data? (Explicit, not ToS)

Privacy Defaults

// ✅ DO: Default private, opt-in sharing
const defaultSettings = {
  shareAnalytics: false,
  publicProfile: false,
  dataRetention: 'minimum'
};

// ✅ DO: Redact PII in logs
logger.info('user_action', {
  action: 'profile_update',
  userId: hashUserId(user.id), // Never raw PII
  duration_ms: 234,
  success: true
});

Form Validation

• Validate on blur, not on every keystroke
• Show errors inline, near the field
• Preserve user input on errors (never clear)
• Auto-save drafts for long forms
• Disable submit only if invalid, explain why

Feedback Loops

Every user action needs acknowledgment:

Type	Timing	Example
Immediate	< 100ms	Button press visual
Progress	> 1s operations	Loading indicator
Completion	After success	"Saved" with next step
Failure	On error	What happened + how to fix

Never blame the user in error messages.

Anti-Patterns

Code Anti-Patterns (Never Do)

❌ Magic numbers without constants
❌ Functions over 50 lines
❌ God objects over 300 lines
❌ Mutable global state
❌ Side effects not in function name
❌ Catching errors without handling
❌ Copy-pasted code

UX Anti-Patterns (Never Do)

❌ Forced account creation before value
❌ Dark patterns (hidden costs, trick questions)
❌ Generic error messages ("Error 500")
❌ Modal dialogs for everything
❌ Destroying data without confirmation
❌ Disabling paste in password fields
❌ Auto-playing video/audio
❌ Infinite scroll without pagination option

Security Checklist

• Sanitize all user input (XSS prevention)
• Parameterized queries (SQL injection prevention)
• Rate limit all endpoints
• CSRF tokens for state-changing operations
• Encrypt PII at rest (AES-256)
• TLS 1.3 for all network traffic
• Hash passwords with bcrypt/Argon2
• HttpOnly, Secure, SameSite cookies

Quality Gates (Before Ship)

• Lighthouse score > 90
• Zero critical/high security vulnerabilities
• Core flows work offline or degrade gracefully
• Keyboard navigation works
• Screen reader tested (VoiceOver + NVDA)
• Error states tested
• Load tested at 2x expected peak
• Mobile tested on real devices
• Privacy review completed
• Rollback procedure documented

Final Mandate

Every piece of code should make someone's life measurably better.

• If you cannot explain the user benefit, do not ship it
• If you cannot measure the outcome, instrument it
• If you cannot maintain it, simplify it
• If it does not feel inevitable, redesign it

Quality is not negotiable. Speed is achieved through clarity, not shortcuts.