1. Component Architecture#

Design your application with a clear component hierarchy. Follow the Single Responsibility Principle (SRP) by breaking down your UI into focused, reusable components.

src/
├── components/
│   ├── common/
│   │   ├── Button.svelte
│   │   ├── Input.svelte
│   │   └── Card.svelte
│   ├── layout/
│   │   ├── Header.svelte
│   │   └── Footer.svelte
│   └── features/
│       ├── auth/
│       └── dashboard/

<!-- Button.svelte -->
<script lang="ts">
  export let label: string;
  export let variant: 'primary' | 'secondary' = 'primary';
  export let disabled: boolean = false;
  export let onClick: () => void;
</script>

<button
  class="btn btn-{variant}"
  {disabled}
  on:click={onClick}
>
  {label}
</button>

<style>
  .btn {
    padding: 0.5rem 1rem;
    border-radius: 4px;
    border: none;
    cursor: pointer;
  }
  .btn-primary {
    background: #4CAF50;
    color: white;
  }
  .btn-secondary {
    background: #9E9E9E;
    color: white;
  }
</style>

2. State Management#

Implement a robust state management strategy using Svelte stores. Create separate stores for different domains of your application.

// stores/auth.ts
import { writable } from 'svelte/store';

interface User {
  id: string;
  name: string;
  email: string;
}

interface AuthStore {
  user: User | null;
  isAuthenticated: boolean;
  loading: boolean;
}

function createAuthStore() {
  const { subscribe, set, update } = writable<AuthStore>({
    user: null,
    isAuthenticated: false,
    loading: false
  });

  return {
    subscribe,
    login: async (credentials: { email: string; password: string }) => {
      update(state => ({ ...state, loading: true }));
      try {
        // API call logic here
        const user = await loginAPI(credentials);
        update(state => ({
          ...state,
          user,
          isAuthenticated: true,
          loading: false
        }));
      } catch (error) {
        update(state => ({ ...state, loading: false }));
        throw error;
      }
    },
    logout: () => {
      set({ user: null, isAuthenticated: false, loading: false });
    }
  };
}

export const auth = createAuthStore();

3. Code Splitting and Lazy Loading#

Implement intelligent code splitting to optimize your application’s performance. Here’s a practical example:

// routes/index.ts
import { wrap } from 'svelte-spa-router/wrap';

const routes = {
  '/': wrap({
    asyncComponent: () => import('./routes/Home.svelte')
  }),
  '/dashboard': wrap({
    asyncComponent: () => import('./routes/Dashboard.svelte'),
    conditions: [() => isAuthenticated()]
  }),
  '/settings': wrap({
    asyncComponent: () => import('./routes/Settings.svelte'),
    conditions: [() => isAuthenticated()]
  })
};

4. Performance Optimization Techniques#

<!-- ImageOptimized.svelte -->
<script lang="ts">
  export let src: string;
  export let alt: string;
  export let width: number;
  export let height: number;

  const generateSrcSet = (src: string) => {
    const sizes = [300, 600, 900, 1200];
    return sizes
      .map(size => `${src}?width=${size} ${size}w`)
      .join(', ');
  };
</script>

<picture>
  <source
    type="image/webp"
    srcset={generateSrcSet(src.replace(/\.[^.]+$/, '.webp'))}
  />
  <img
    {src}
    {alt}
    {width}
    {height}
    loading="lazy"
    decoding="async"
    srcset={generateSrcSet(src)}
    sizes="(max-width: 768px) 100vw, 50vw"
  />
</picture>

5. Testing Strategy#

Implement a comprehensive testing strategy using Jest for unit tests and Cypress for end-to-end testing.

// Button.test.ts
import { render, fireEvent } from '@testing-library/svelte';
import Button from './Button.svelte';

describe('Button Component', () => {
  test('renders with correct label', () => {
    const { getByText } = render(Button, { props: { label: 'Click me' } });
    expect(getByText('Click me')).toBeInTheDocument();
  });

  test('calls onClick handler when clicked', async () => {
    const onClick = jest.fn();
    const { getByText } = render(Button, {
      props: { label: 'Click me', onClick }
    });
    
    await fireEvent.click(getByText('Click me'));
    expect(onClick).toHaveBeenCalled();
  });
});

6. Error Handling and Monitoring#

Implement a global error boundary and monitoring system:

<!-- ErrorBoundary.svelte -->
<script lang="ts">
  import { onError } from 'svelte';
  import * as Sentry from '@sentry/browser';

  let error: Error | null = null;

  onError((error) => {
    Sentry.captureException(error);
  });
</script>

{#if error}
  <div class="error-container">
    <h2>Something went wrong</h2>
    <p>{error.message}</p>
    <button on:click={() => window.location.reload()}>
      Reload Page
    </button>
  </div>
{:else}
  <slot />
{/if}

Conclusion#

Building scalable Svelte applications requires a combination of proper architecture, performance optimization, and maintenance practices. By following these patterns and implementing the provided examples, you’ll be well-equipped to build robust and maintainable applications that can grow with your needs.

Remember to:

• Keep components small and focused
• Implement proper state management
• Use code splitting and lazy loading
• Optimize assets and performance
• Write comprehensive tests
• Monitor and handle errors effectively

These practices will help ensure your Svelte application remains maintainable and performant as it scales.