Frontend Component Skill

Purpose: Guidance for creating Next.js components following server/client patterns and existing component structures.

Overview

Next.js 16+ uses App Router with Server Components by default. Client Components are only used when interactivity is needed (hooks, event handlers, browser APIs).

Server vs Client Components

Server Components (Default)

When to Use:

• Pages and layouts
• Static content
• Data fetching from API (when possible)
• SEO-optimized content

Pattern:

// No "use client" directive
import { Metadata } from "next";

export const metadata: Metadata = {
  title: "Page Title",
};

export default function PageComponent() {
  return <div>Static content</div>;
}

Example: frontend/app/layout.tsx, frontend/app/page.tsx

Client Components (When Needed)

When to Use:

• Interactive elements (buttons, forms, inputs)
• Event handlers (onClick, onChange, etc.)
• React hooks (useState, useEffect, useRouter, etc.)
• Browser APIs (localStorage, window, document, etc.)
• Real-time updates
• Drag and drop functionality

Pattern:

"use client"; // MUST be first line

import { useState, useEffect } from "react";
import { useRouter } from "next/navigation";

interface ComponentProps {
  prop1: string;
  prop2?: number;
}

export default function ComponentName({ prop1, prop2 }: ComponentProps) {
  const router = useRouter();
  const [state, setState] = useState("");

  useEffect(() => {
    // Side effects
  }, []);

  return <div>{/* Component JSX */}</div>;
}

Example: frontend/components/ProtectedRoute.tsx, frontend/app/signup/page.tsx

Component Structure Template

"use client"; // Only if client component

/**
 * Component Name
 *
 * Brief description of what this component does
 */

import { useState, useEffect } from "react";
import { ComponentType } from "@/types";
import { cn } from "@/lib/utils";

interface ComponentProps {
  prop1: string;
  prop2?: number;
  className?: string;
}

export default function ComponentName({ prop1, prop2, className }: ComponentProps) {
  // State
  const [state, setState] = useState("");

  // Effects
  useEffect(() => {
    // Side effects
  }, []);

  // Handlers
  const handleClick = () => {
    // Handler logic
  };

  // Render
  return (
    <div className={cn("base-classes", className)}>
      {/* Component content */}
    </div>
  );
}

Specific Component Patterns

1. ProtectedRoute Pattern

From: frontend/components/ProtectedRoute.tsx

"use client";

import { useEffect, useState } from "react";
import { useRouter } from "next/navigation";
import { isAuthenticated } from "@/lib/auth";
import LoadingSpinner from "./LoadingSpinner";

interface ProtectedRouteProps {
  children: React.ReactNode;
}

export default function ProtectedRoute({ children }: ProtectedRouteProps) {
  const router = useRouter();
  const [isAuthorized, setIsAuthorized] = useState(false);
  const [isChecking, setIsChecking] = useState(true);

  useEffect(() => {
    async function checkAuth() {
      try {
        const authenticated = await isAuthenticated();

        if (!authenticated) {
          const currentPath = window.location.pathname;
          if (currentPath !== "/signin") {
            sessionStorage.setItem("redirectAfterLogin", currentPath);
          }
          router.push("/signin");
        } else {
          setIsAuthorized(true);
        }
      } catch (error) {
        console.error("Auth check failed:", error);
        router.push("/signin");
      } finally {
        setIsChecking(false);
      }
    }

    checkAuth();
  }, [router]);

  if (isChecking) {
    return (
      <div className="min-h-screen flex items-center justify-center">
        <LoadingSpinner size="large" />
      </div>
    );
  }

  if (!isAuthorized) {
    return null;
  }

  return <>{children}</>;
}

Pattern:

• Check authentication on mount
• Show loading spinner during check
• Store intended destination in sessionStorage
• Redirect to /signin if not authenticated
• Only render children if authorized

2. LoadingSpinner Pattern

From: frontend/components/LoadingSpinner.tsx

interface LoadingSpinnerProps {
  size?: "small" | "medium" | "large";
  color?: string;
  label?: string;
}

export default function LoadingSpinner({
  size = "medium",
  color = "blue",
  label = "Loading...",
}: LoadingSpinnerProps) {
  const sizeClasses = {
    small: "w-4 h-4 border-2",
    medium: "w-8 h-8 border-3",
    large: "w-12 h-12 border-4",
  };

  const colorClasses = {
    blue: "border-blue-600 border-t-transparent",
    gray: "border-gray-600 border-t-transparent",
    white: "border-white border-t-transparent",
  };

  const spinnerClass = `${sizeClasses[size]} ${
    colorClasses[color as keyof typeof colorClasses] || colorClasses.blue
  } rounded-full animate-spin`;

  return (
    <div
      className="flex items-center justify-center"
      role="status"
      aria-label={label}
      aria-live="polite"
    >
      <div className={spinnerClass}></div>
      <span className="sr-only">{label}</span>
    </div>
  );
}

Pattern:

• Multiple sizes (small, medium, large)
• Multiple colors (blue, gray, white)
• Accessibility labels (role="status", aria-label, aria-live)
• Screen reader text with sr-only class

3. Form Handling Pattern

From: frontend/app/signup/page.tsx

"use client";

import { useState } from "react";
import { useRouter } from "next/navigation";
import { api } from "@/lib/api";
import { isValidEmail, getPasswordStrength } from "@/lib/utils";

export default function SignupPage() {
  const router = useRouter();
  const [formData, setFormData] = useState({
    name: "",
    email: "",
    password: "",
  });
  const [errors, setErrors] = useState<Record<string, string>>({});
  const [isLoading, setIsLoading] = useState(false);
  const [apiError, setApiError] = useState("");

  const validateForm = (): boolean => {
    const newErrors: Record<string, string> = {};

    if (!formData.name.trim()) {
      newErrors.name = "Name is required";
    }

    if (!formData.email.trim()) {
      newErrors.email = "Email is required";
    } else if (!isValidEmail(formData.email)) {
      newErrors.email = "Please enter a valid email address";
    }

    // More validation...

    setErrors(newErrors);
    return Object.keys(newErrors).length === 0;
  };

  const handleSubmit = async (e: React.FormEvent) => {
    e.preventDefault();
    setApiError("");

    if (!validateForm()) {
      return;
    }

    setIsLoading(true);

    try {
      const response = await api.signup(formData);
      if (response.success) {
        router.push("/dashboard");
      } else {
        setApiError(response.message || "Signup failed");
      }
    } catch (error: any) {
      setApiError(error.message || "An error occurred");
    } finally {
      setIsLoading(false);
    }
  };

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    const { name, value } = e.target;
    setFormData((prev) => ({ ...prev, [name]: value }));
    // Clear error for this field when user starts typing
    if (errors[name]) {
      setErrors((prev) => ({ ...prev, [name]: "" }));
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      {/* Form fields */}
    </form>
  );
}

Pattern:

• Separate state for form data, errors, loading, API errors
• Validation function that returns boolean
• Clear errors on input change
• Loading state during submission
• Try-catch for error handling
• Redirect on success

4. ToastNotification Pattern

From: frontend/components/ToastNotification.tsx

"use client";

import { useEffect, useState } from "react";
import { ToastMessage, ToastType } from "@/types";

export function useToast() {
  const [toasts, setToasts] = useState<ToastMessage[]>([]);

  const showToast = (type: ToastType, message: string, duration?: number) => {
    const id = `toast-${Date.now()}-${Math.random()}`;
    const newToast: ToastMessage = {
      id,
      type,
      message,
      duration,
    };
    setToasts((prev) => [...prev, newToast]);
  };

  const dismissToast = (id: string) => {
    setToasts((prev) => prev.filter((toast) => toast.id !== id));
  };

  return {
    toasts,
    showToast,
    dismissToast,
    success: (message: string, duration?: number) => showToast("success", message, duration),
    error: (message: string, duration?: number) => showToast("error", message, duration),
    // ...
  };
}

Pattern:

• Custom hook for toast management
• Auto-dismiss with duration
• Stack multiple toasts
• Helper methods (success, error, warning, info)

Tailwind CSS Patterns

1. Utility Classes Only

<div className="flex items-center justify-center gap-4 p-6 bg-white rounded-lg shadow-md">

Pattern: Use Tailwind utility classes, no inline styles

2. Conditional Classes with cn() Utility

import { cn } from "@/lib/utils";

<div className={cn(
  "base-classes",
  condition && "conditional-classes",
  className // Allow prop override
)}>

Pattern: Use cn() from @/lib/utils for conditional classes

3. Dark Mode Support

<div className="bg-white dark:bg-gray-800 text-gray-900 dark:text-white">

Pattern: Use dark: prefix for dark mode styles

4. Responsive Design

<div className="w-full sm:w-1/2 md:w-1/3 lg:w-1/4">

Pattern: Use breakpoint prefixes (sm:, md:, lg:, xl:)

Accessibility Patterns (WCAG 2.1 AA)

1. ARIA Labels

<button aria-label="Close dialog">×</button>
<div role="status" aria-live="polite" aria-label="Loading...">

Pattern: Always provide aria-label for icon-only buttons

2. Semantic HTML

<nav>
  <ul>
    <li><a href="/">Home</a></li>
  </ul>
</nav>

Pattern: Use semantic HTML elements (nav, main, section, article, etc.)

3. Keyboard Navigation

<button
  onClick={handleClick}
  onKeyDown={(e) => {
    if (e.key === "Enter" || e.key === " ") {
      handleClick();
    }
  }}
>

Pattern: Ensure keyboard accessibility for all interactive elements

4. Screen Reader Text

<span className="sr-only">Loading content</span>

Pattern: Use sr-only class for screen reader-only text

5. Focus Management

<input
  autoFocus
  className="focus:outline-none focus:ring-2 focus:ring-blue-500"
/>

Pattern: Always provide visible focus indicators

File Naming Conventions

• Pages: kebab-case (e.g., signup.tsx, signin.tsx)
• Components: PascalCase (e.g., TaskList.tsx, TaskItem.tsx)
• Layouts: layout.tsx
• Error Pages: error.tsx, not-found.tsx

Constitution Requirements

• FR-033: Next.js 16+ App Router structure ✅
• FR-034: Server components default, client when needed ✅
• FR-035: Error boundaries ✅
• FR-036: WCAG 2.1 AA compliance ✅
• FR-037: TypeScript strict mode ✅
• FR-038: Prettier formatting ✅
• FR-039: ESLint rules ✅

References

• Specification: specs/002-frontend-todo-app/spec.md - Component specifications
• Existing Components: frontend/components/*.tsx - Component examples
• Existing Pages: frontend/app/*.tsx - Page examples

Advanced Component Patterns

1. Drag and Drop Pattern (Phase 7 - T065)

Library: @dnd-kit/core

"use client";

import { DndContext, closestCenter, KeyboardSensor, PointerSensor, useSensor, useSensors } from "@dnd-kit/core";
import { SortableContext, sortableKeyboardCoordinates, useSortable, verticalListSortingStrategy } from "@dnd-kit/sortable";

export default function SortableTaskList({ tasks, onReorder }: Props) {
  const sensors = useSensors(
    useSensor(PointerSensor),
    useSensor(KeyboardSensor, {
      coordinateGetter: sortableKeyboardCoordinates,
    })
  );

  const handleDragEnd = (event: DragEndEvent) => {
    const { active, over } = event;
    if (over && active.id !== over.id) {
      onReorder(active.id, over.id);
    }
  };

  return (
    <DndContext sensors={sensors} collisionDetection={closestCenter} onDragEnd={handleDragEnd}>
      <SortableContext items={tasks.map(t => t.id)} strategy={verticalListSortingStrategy}>
        {tasks.map(task => (
          <SortableTaskItem key={task.id} task={task} />
        ))}
      </SortableContext>
    </DndContext>
  );
}

Pattern: Use @dnd-kit/core for drag and drop, handle reorder on drag end

2. Undo/Redo Pattern (Phase 7 - T066)

"use client";

import { useReducer } from "react";

interface HistoryState<T> {
  past: T[];
  present: T;
  future: T[];
}

function historyReducer<T>(state: HistoryState<T>, action: { type: string; newPresent?: T }): HistoryState<T> {
  const { past, present, future } = state;

  switch (action.type) {
    case "UNDO":
      if (past.length === 0) return state;
      return {
        past: past.slice(0, past.length - 1),
        present: past[past.length - 1],
        future: [present, ...future],
      };
    case "REDO":
      if (future.length === 0) return state;
      return {
        past: [...past, present],
        present: future[0],
        future: future.slice(1),
      };
    case "SET":
      if (action.newPresent === present) return state;
      return {
        past: [...past, present],
        present: action.newPresent!,
        future: [],
      };
    default:
      return state;
  }
}

export function useHistory<T>(initialPresent: T) {
  const [state, dispatch] = useReducer(historyReducer, {
    past: [],
    present: initialPresent,
    future: [],
  });

  const undo = () => dispatch({ type: "UNDO" });
  const redo = () => dispatch({ type: "REDO" });
  const set = (newPresent: T) => dispatch({ type: "SET", newPresent });

  return { state: state.present, set, undo, redo, canUndo: state.past.length > 0, canRedo: state.future.length > 0 };
}

Pattern: Use useReducer with history pattern for undo/redo functionality

3. Real-time Updates with Polling (Phase 7 - T067)

"use client";

import { useEffect, useRef } from "react";

export function usePolling(callback: () => Promise<void>, interval: number = 5000) {
  const intervalRef = useRef<NodeJS.Timeout | null>(null);

  useEffect(() => {
    const poll = async () => {
      try {
        await callback();
      } catch (error) {
        console.error("Polling error:", error);
      }
    };

    // Initial call
    poll();

    // Set up polling interval
    intervalRef.current = setInterval(poll, interval);

    // Cleanup
    return () => {
      if (intervalRef.current) {
        clearInterval(intervalRef.current);
      }
    };
  }, [callback, interval]);
}

Pattern: Use setInterval for polling, cleanup on unmount, handle errors gracefully

4. Inline Editing Pattern (Phase 7 - T068)

"use client";

import { useState } from "react";

export default function InlineEditable({ value, onSave }: Props) {
  const [isEditing, setIsEditing] = useState(false);
  const [editValue, setEditValue] = useState(value);

  const handleSave = () => {
    onSave(editValue);
    setIsEditing(false);
  };

  const handleCancel = () => {
    setEditValue(value);
    setIsEditing(false);
  };

  if (isEditing) {
    return (
      <input
        value={editValue}
        onChange={(e) => setEditValue(e.target.value)}
        onBlur={handleSave}
        onKeyDown={(e) => {
          if (e.key === "Enter") handleSave();
          if (e.key === "Escape") handleCancel();
        }}
        autoFocus
      />
    );
  }

  return (
    <span onClick={() => setIsEditing(true)} className="cursor-pointer">
      {value}
    </span>
  );
}

Pattern: Toggle edit mode, save on blur/Enter, cancel on Escape

5. Performance Optimization Patterns (Phase 8 - T072)

Code Splitting with next/dynamic

import dynamic from "next/dynamic";

// Lazy load heavy components
const TaskStatistics = dynamic(() => import("@/components/TaskStatistics"), {
  loading: () => <LoadingSpinner />,
  ssr: false, // Disable SSR if not needed
});

const TaskDetailModal = dynamic(() => import("@/components/TaskDetailModal"), {
  loading: () => <LoadingSpinner />,
});

Pattern: Use next/dynamic for code splitting, provide loading fallback

Image Optimization

import Image from "next/image";

<Image
  src="/image.jpg"
  alt="Description"
  width={500}
  height={300}
  loading="lazy"
  placeholder="blur"
/>

Pattern: Use Next.js Image component for automatic optimization

6. Error Boundary Pattern (Phase 8 - T074)

"use client";

import React, { Component, ErrorInfo, ReactNode } from "react";

interface Props {
  children: ReactNode;
  fallback?: ReactNode;
}

interface State {
  hasError: boolean;
  error?: Error;
}

export default class ErrorBoundary extends Component<Props, State> {
  constructor(props: Props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error: Error): Partial<State> {
    return { hasError: true, error };
  }

  componentDidCatch(error: Error, errorInfo: ErrorInfo): void {
    console.error("ErrorBoundary caught an error:", error, errorInfo);
    // Log to error tracking service
  }

  render() {
    if (this.state.hasError) {
      return (
        this.props.fallback || (
          <div className="p-4 bg-red-50 border border-red-200 rounded">
            <h2 className="text-red-800 font-bold">Something went wrong</h2>
            <p className="text-red-600">{this.state.error?.message}</p>
            <button onClick={() => this.setState({ hasError: false })}>Try again</button>
          </div>
        )
      );
    }

    return this.props.children;
  }
}

Pattern: Class component, catch errors, provide fallback UI, log errors

7. PWA Patterns (Phase 8 - T069, T070, T071)

Service Worker Setup

// public/sw.js or use next-pwa
if ("serviceWorker" in navigator) {
  window.addEventListener("load", () => {
    navigator.serviceWorker
      .register("/sw.js")
      .then((registration) => {
        console.log("SW registered:", registration);
      })
      .catch((error) => {
        console.error("SW registration failed:", error);
      });
  });
}

Pattern: Register service worker on page load, handle registration errors

IndexedDB for Offline Storage

import { openDB, DBSchema, IDBPDatabase } from "idb";

interface TaskDB extends DBSchema {
  tasks: {
    key: number;
    value: Task;
    indexes: { "by-user-id": string };
  };
}

export async function getDB(): Promise<IDBPDatabase<TaskDB>> {
  return openDB<TaskDB>("todo-db", 1, {
    upgrade(db) {
      const taskStore = db.createObjectStore("tasks", { keyPath: "id" });
      taskStore.createIndex("by-user-id", "user_id");
    },
  });
}

export async function saveTaskOffline(task: Task) {
  const db = await getDB();
  await db.put("tasks", task);
}

export async function getTasksOffline(userId: string): Promise<Task[]> {
  const db = await getDB();
  return db.getAllFromIndex("tasks", "by-user-id", userId);
}

Pattern: Use idb library for IndexedDB, create stores and indexes, handle offline data

Offline Sync Mechanism

export async function syncOfflineChanges(userId: string) {
  const db = await getDB();
  const offlineTasks = await db.getAllFromIndex("tasks", "by-user-id", userId);

  for (const task of offlineTasks) {
    if (task.syncStatus === "pending") {
      try {
        await api.createTask(userId, task);
        await db.put("tasks", { ...task, syncStatus: "synced" });
      } catch (error) {
        console.error("Sync failed for task:", task.id, error);
      }
    }
  }
}

// Call on connection restore
window.addEventListener("online", () => {
  syncOfflineChanges(currentUserId);
});

Pattern: Track sync status, sync on connection restore, handle sync errors

8. Caching Strategies (Phase 8 - T073)

// Cache API responses
const cache = new Map<string, { data: any; timestamp: number }>();
const CACHE_DURATION = 5 * 60 * 1000; // 5 minutes

export async function getCachedData<T>(key: string, fetcher: () => Promise<T>): Promise<T> {
  const cached = cache.get(key);
  if (cached && Date.now() - cached.timestamp < CACHE_DURATION) {
    return cached.data;
  }

  const data = await fetcher();
  cache.set(key, { data, timestamp: Date.now() });
  return data;
}

Pattern: Use Map for in-memory cache, check expiration, update cache on fetch

9. Error Logging and Tracking (Phase 8 - T075)

export function logError(error: Error, context?: Record<string, any>) {
  console.error("Error:", error, context);

  // Send to error tracking service (e.g., Sentry)
  if (typeof window !== "undefined" && (window as any).Sentry) {
    (window as any).Sentry.captureException(error, {
      extra: context,
    });
  }

  // Or send to custom endpoint
  fetch("/api/log-error", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({
      message: error.message,
      stack: error.stack,
      context,
      timestamp: new Date().toISOString(),
    }),
  }).catch((err) => console.error("Failed to log error:", err));
}

Pattern: Log to console, send to error tracking service, include context

Common Patterns Summary

1. ✅ Use Server Components by default
2. ✅ Add "use client" only when needed
3. ✅ Use TypeScript interfaces for props
4. ✅ Use cn() utility for conditional classes
5. ✅ Always include accessibility attributes
6. ✅ Use semantic HTML elements
7. ✅ Provide loading states
8. ✅ Handle errors gracefully
9. ✅ Use Tailwind utility classes
10. ✅ Support dark mode with dark: prefix
11. ✅ Use @dnd-kit/core for drag and drop
12. ✅ Use useReducer for undo/redo
13. ✅ Use setInterval for polling with cleanup
14. ✅ Use next/dynamic for code splitting
15. ✅ Use ErrorBoundary for error handling
16. ✅ Use IndexedDB for offline storage
17. ✅ Sync offline changes on connection restore
18. ✅ Cache API responses with expiration
19. ✅ Log errors to tracking service