frontend-ui-engineering

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---
name: frontend-ui-engineering
description: Builds production-quality, accessible, responsive user-facing UIs. Use when building or modifying interfaces and pages, creating components, implementing layouts, meeting WCAG accessibility requirements, managing state, or when the output needs to look and feel production-quality rather than AI-generated.
---

# Frontend UI Engineering

## Overview

Build production-quality user interfaces that are accessible, performant, and visually polished. The goal is UI that looks like it was built by a design-aware engineer at a top company — not like it was generated by an AI. This means real design system adherence, proper accessibility, thoughtful interaction patterns, and no generic "AI aesthetic."

## When to Use

- Building new UI components or pages
- Modifying existing user-facing interfaces
- Implementing responsive layouts
- Adding interactivity or state management
- Fixing visual or UX issues

## Component Architecture

### File Structure

Colocate everything related to a component:

```
src/components/
  TaskList/
    TaskList.tsx          # Component implementation
    TaskList.test.tsx     # Tests
    TaskList.stories.tsx  # Storybook stories (if using)
    use-task-list.ts      # Custom hook (if complex state)
    types.ts              # Component-specific types (if needed)
```

### Component Patterns

**Prefer composition over configuration:**

```tsx
// Good: Composable
<Card>
  <CardHeader>
    <CardTitle>Tasks</CardTitle>
  </CardHeader>
  <CardBody>
    <TaskList tasks={tasks} />
  </CardBody>
</Card>

// Avoid: Over-configured
<Card
  title="Tasks"
  headerVariant="large"
  bodyPadding="md"
  content={<TaskList tasks={tasks} />}
/>
```

**Keep components focused:**

```tsx
// Good: Does one thing
export function TaskItem({ task, onToggle, onDelete }: TaskItemProps) {
  return (
    <li className="flex items-center gap-3 p-3">
      <Checkbox checked={task.done} onChange={() => onToggle(task.id)} />
      <span className={task.done ? 'line-through text-muted' : ''}>{task.title}</span>
      <Button variant="ghost" size="sm" onClick={() => onDelete(task.id)}>
        <TrashIcon />
      </Button>
    </li>
  );
}
```

**Separate data fetching from presentation:**

```tsx
// Container: handles data
export function TaskListContainer() {
  const { tasks, isLoading, error } = useTasks();

  if (isLoading) return <TaskListSkeleton />;
  if (error) return <ErrorState message="Failed to load tasks" retry={refetch} />;
  if (tasks.length === 0) return <EmptyState message="No tasks yet" />;

  return <TaskList tasks={tasks} />;
}

// Presentation: handles rendering
export function TaskList({ tasks }: { tasks: Task[] }) {
  return (
    <ul role="list" className="divide-y">
      {tasks.map(task => <TaskItem key={task.id} task={task} />)}
    </ul>
  );
}
```

## State Management

**Choose the simplest approach that works:**

```
Local state (useState)           → Component-specific UI state
Lifted state                     → Shared between 2-3 sibling components
Context                          → Theme, auth, locale (read-heavy, write-rare)
URL state (searchParams)         → Filters, pagination, shareable UI state
Server state (React Query, SWR)  → Remote data with caching
Global store (Zustand, Redux)    → Complex client state shared app-wide
```

**Avoid prop drilling deeper than 3 levels.** If you're passing props through components that don't use them, introduce context or restructure the component tree.

## Design System Adherence

### Avoid the AI Aesthetic

AI-generated UI has recognizable patterns. Avoid all of them:

| AI Default | Why It Is a Problem | Production Quality |
|---|---|---|
| Purple/indigo everything | Models default to visually "safe" palettes, making every app look identical | Use the project's actual color palette |
| Excessive gradients | Gradients add visual noise and clash with most design systems | Flat or subtle gradients matching the design system |