Frontend UI Engineering

Frontend UI Engineering

Overview

Production-quality UI engineering: component architecture, accessible markup, responsive design, and state management. The bar is not "does it render?" — the bar is "would a user notice a difference from a product they paid for?"

When to Use

• Building or modifying any user-facing component
• Implementing layouts, navigation, or data displays
• Adding or changing state management
• Any change that affects what a user sees or can interact with

Component Architecture

Hierarchy of Concerns

Page (route-level)
  └── Layout (grid/columns)
        ├── Feature Component (data + logic)
        │     └── UI Component (pure display, no data fetching)
        └── Shared Component (reusable, generic)

• Page components handle routing. No business logic.
• Feature components fetch data and own state. Not reusable.
• UI components receive props, render output. Fully reusable, no side effects.
• Shared components are the design system: Button, Input, Modal, etc.

Component Rules

// GOOD: UI component — pure, testable, reusable
interface SkillCardProps {
  name: string;
  description: string;
  onExport: () => void;
}

export function SkillCard({ name, description, onExport }: SkillCardProps) {
  return (
    <article className="skill-card">
      <h3>{name}</h3>
      <p>{description}</p>
      <button aria-label={`Export ${name} skill`}>
        Export
      </button>
    </article>
  );
}

// BAD: UI component that fetches its own data — not reusable, hard to test
export function SkillCard({ skillId }: { skillId: string }) {
  const { data } = useQuery(['skill', skillId], fetchSkill);
  // ...
}

Accessibility (Non-Negotiable)

Every interactive element must be keyboard-accessible and screen-reader-friendly.

Minimum Requirements (WCAG 2.1 AA)

// Interactive elements use semantic HTML
<button Skill</button>  // ✓
<div Skill</div>        // ✗

// Images have alt text
<img src={logo} alt="GitScape logo" />
<img src={decoration} alt="" />  // decorative: explicit empty string

// Forms have associated labels
<label htmlFor="repo-url">Repository URL</label>
<input id="repo-url" type="url" required />

// Icon-only buttons have aria-label
<button aria-label="Close dialog">✕</button>

// Loading states are announced
<div aria-busy={isLoading} aria-label="Loading skills">
  {isLoading ? <Spinner /> : <SkillList />}
</div>

Focus Management

• Tab order should follow visual order
• Modals trap focus while open; return focus on close
• Never remove focus outlines — style them instead

State Management

Where State Lives

Rules

• Never duplicate server state in local state — React Query is the cache
• Never put derived values in state — compute them during render
• Lift state only as high as necessary (not straight to global)

// BAD: Derived value stored in state
const [filteredSkills, setFilteredSkills] = useState(skills);
useEffect(() => setFilteredSkills(skills.filter(...)), [skills, filter]);

// GOOD: Derived during render
const filteredSkills = skills.filter(...);

Performance in Components

Avoid Common Re-render Traps

// BAD: Creates new object on every render, causing children to re-render
function SkillList() {
  return <SkillFilters options={{ sortBy: 'name', order: 'asc' }} />;
}

// GOOD: Stable reference
const DEFAULT_OPTIONS = { sortBy: 'name', order: 'asc' } as const;
function SkillList() {
  return <SkillFilters options={DEFAULT_OPTIONS} />;
}

// Use React.memo only for expensive components that receive stable props
const SkillCard = React.memo(function SkillCard({ skill }: Props) {
  return <div>{/* expensive render */}</div>;
});

File Preview Truncation (GitScape-Specific)

File previews must be truncated to prevent browser freeze. Large files kill the page:

const MAX_PREVIEW_CHARS = 10_000; // ~200 lines

function FilePreview({ content }: { content: string }) {
  const truncated = content.length > MAX_PREVIEW_CHARS;
  const display = truncated ? content.slice(0, MAX_PREVIEW_CHARS) : content;
  return (
    <div>
      <pre>{display}</pre>
      {truncated && (
        <p className="truncation-notice">
          Preview truncated — full content included in export.
        </p>
      )}
    </div>
  );
}

Error States and Loading States

Every async operation needs all three states:

function SkillExport() {
  const { data, isLoading, error } = useQuery(...);

  if (isLoading) return <LoadingSpinner label="Fetching repository files..." />;
  if (error) return <ErrorMessage message="Could not load repository. Check the URL and try again." />;
  return <SkillPreview data={data} />;
}

Never show a blank screen. Never swallow errors silently.

Common Rationalizations

Red Flags

• <div> anywhere
• Missing alt text on images
• Loading state shows blank content
• Error state shows a technical stack trace to the user
• State that could be derived from props/queries stored in useState
• useEffect used to sync local state with server data