# Astro Documentation

Astro is an all-in-one web framework for building fast, content-focused websites. It uses a component-based architecture where `.astro` components render to HTML at build time or on-demand, with zero JavaScript sent to the browser by default. Astro's "Islands Architecture" allows you to add interactive UI components from any framework (React, Vue, Svelte, etc.) as isolated "islands" of interactivity while keeping the rest of the page static.

The framework excels at content-driven sites like blogs, documentation, portfolios, and marketing pages. Key features include Content Collections for type-safe content management, built-in Markdown/MDX support, automatic image optimization, server-side rendering (SSR) with adapters for various hosting platforms, and View Transitions for smooth page navigation. Astro integrates seamlessly with headless CMSs, databases, and authentication providers.

## CLI Commands

The Astro CLI provides commands for development, building, and previewing your site. The dev server uses Hot Module Replacement for instant updates.

```bash
# Create a new Astro project
npm create astro@latest

# Start development server (default port 4321)
npx astro dev

# Build for production
npx astro build

# Preview production build locally
npx astro preview

# Type-check your project
npx astro check

# Generate TypeScript types for Astro modules
npx astro sync

# Add an integration (React, Tailwind, etc.)
npx astro add react

# Common flags
npx astro dev --port 8080 --host
npx astro build --verbose
```

## Astro Components

Astro components are `.astro` files with a frontmatter script (JavaScript/TypeScript) and HTML template. They render on the server with no client-side JavaScript by default.

```astro
---
// src/components/Greeting.astro
// Component Script - runs at build time or on request

// Import other components
import Button from './Button.astro';

// Accept props with TypeScript interface
interface Props {
  name: string;
  greeting?: string;
}

const { name, greeting = "Hello" } = Astro.props;

// Fetch data (runs server-side only)
const response = await fetch('https://api.example.com/user');
const user = await response.json();
---

<!-- Component Template - HTML with JS expressions -->
<div class="greeting-card">
  <h1>{greeting}, {name}!</h1>
  <p>Welcome back, {user.displayName}</p>

  <!-- Use other components -->
  <Button title="Click me" />

  <!-- Conditional rendering -->
  {user.isAdmin && <span class="badge">Admin</span>}

  <!-- Loop through arrays -->
  <ul>
    {user.posts.map((post) => (
      <li><a href={post.url}>{post.title}</a></li>
    ))}
  </ul>
</div>

<style>
  /* Scoped CSS - only applies to this component */
  .greeting-card {
    padding: 1rem;
    border-radius: 8px;
  }
</style>
```

## Slots

Slots allow you to pass child content into components, similar to React's children prop. Named slots enable multiple content areas.

```astro
---
// src/components/Card.astro
interface Props {
  title: string;
}
const { title } = Astro.props;
---

<article class="card">
  <header>
    <slot name="header">
      <!-- Fallback content if no header provided -->
      <h2>{title}</h2>
    </slot>
  </header>

  <main>
    <!-- Default slot for main content -->
    <slot />
  </main>

  <footer>
    <slot name="footer" />
  </footer>
</article>
```

```astro
---
// Usage in another component
import Card from './Card.astro';
---

<Card title="My Post">
  <span slot="header">Custom Header Content</span>

  <p>This goes in the default slot.</p>
  <p>So does this paragraph.</p>

  <div slot="footer">
    <button>Read more</button>
  </div>
</Card>
```

## Astro Render Context (Astro Global)

The `Astro` global object provides runtime information and utilities in `.astro` files. In API endpoints, these are available on the `context` parameter.

```astro
---
// src/pages/dashboard.astro
// Access component props passed from parent
const { title } = Astro.props;

// Get URL information
const currentUrl = Astro.url;           // Full URL object
const pathname = Astro.url.pathname;    // e.g., "/dashboard"
const searchParams = Astro.url.searchParams.get('filter');

// Access route parameters for dynamic routes [slug].astro
const { slug } = Astro.params;

// Get the site URL from config
const siteUrl = Astro.site;             // URL or undefined

// Astro version for meta tags
const generator = Astro.generator;      // "Astro v5.x.x"

// Access request object (headers, method, etc.)
const userAgent = Astro.request.headers.get('user-agent');
const method = Astro.request.method;

// Get client IP (SSR only)
const clientIP = Astro.clientAddress;

// Access data from middleware
const userData = Astro.locals.user;

// Set response headers/status
Astro.response.status = 404;
Astro.response.headers.set('Cache-Control', 'max-age=3600');

// Redirect to another page
if (!userData) {
  return Astro.redirect('/login', 302);
}

// Rewrite to serve different content without redirect
if (pathname === '/old-page') {
  return Astro.rewrite('/new-page');
}

// i18n utilities
const locale = Astro.currentLocale;
const preferred = Astro.preferredLocale;
---

<html>
  <head>
    <meta name="generator" content={generator} />
  </head>
  <body>
    <h1>Dashboard for {userData.name}</h1>
    <p>Current path: {pathname}</p>
  </body>
</html>
```

## API Endpoints

Server endpoints handle API requests with full access to the request/response cycle. Export functions named after HTTP methods.

```typescript
// src/pages/api/users/[id].ts
import type { APIContext } from 'astro';

// Handle GET requests
export async function GET({ params, request }: APIContext) {
  const { id } = params;

  try {
    const user = await db.users.findById(id);

    if (!user) {
      return new Response(JSON.stringify({ error: 'User not found' }), {
        status: 404,
        headers: { 'Content-Type': 'application/json' }
      });
    }

    return new Response(JSON.stringify(user), {
      status: 200,
      headers: { 'Content-Type': 'application/json' }
    });
  } catch (error) {
    return new Response(JSON.stringify({ error: 'Server error' }), {
      status: 500,
      headers: { 'Content-Type': 'application/json' }
    });
  }
}

// Handle POST requests
export async function POST({ request, cookies, redirect }: APIContext) {
  const data = await request.json();

  // Validate input
  if (!data.email || !data.name) {
    return new Response(JSON.stringify({ error: 'Missing fields' }), {
      status: 400,
      headers: { 'Content-Type': 'application/json' }
    });
  }

  const newUser = await db.users.create(data);

  // Set a cookie
  cookies.set('user-id', newUser.id, {
    path: '/',
    httpOnly: true,
    secure: true,
    maxAge: 60 * 60 * 24 * 7  // 1 week
  });

  return new Response(JSON.stringify(newUser), {
    status: 201,
    headers: { 'Content-Type': 'application/json' }
  });
}

// Handle DELETE requests
export async function DELETE({ params }: APIContext) {
  await db.users.delete(params.id);
  return new Response(null, { status: 204 });
}
```

## Content Collections

Content Collections provide type-safe content management with Zod schema validation. Define collections in `src/content.config.ts`.

```typescript
// src/content.config.ts
import { defineCollection, reference } from 'astro:content';
import { glob, file } from 'astro/loaders';
import { z } from 'astro/zod';

// Blog collection from markdown files
const blog = defineCollection({
  loader: glob({ pattern: '**/*.md', base: './src/content/blog' }),
  schema: z.object({
    title: z.string(),
    description: z.string(),
    pubDate: z.coerce.date(),
    updatedDate: z.coerce.date().optional(),
    author: reference('authors'),  // Reference another collection
    tags: z.array(z.string()),
    draft: z.boolean().default(false),
    image: z.object({
      src: z.string(),
      alt: z.string(),
    }).optional(),
  })
});

// Authors collection from JSON
const authors = defineCollection({
  loader: file('src/data/authors.json'),
  schema: z.object({
    id: z.string(),
    name: z.string(),
    email: z.string().email(),
    avatar: z.string().url(),
    bio: z.string().optional(),
  })
});

// Remote data with custom loader
const products = defineCollection({
  loader: async () => {
    const response = await fetch('https://api.example.com/products');
    const data = await response.json();
    return data.map((product: any) => ({
      id: product.sku,
      ...product,
    }));
  },
  schema: z.object({
    name: z.string(),
    price: z.number(),
    category: z.enum(['electronics', 'clothing', 'books']),
  })
});

export const collections = { blog, authors, products };
```

## Querying Collections

Use `getCollection()` and `getEntry()` to query content collections with full type safety.

```astro
---
// src/pages/blog/index.astro
import { getCollection, getEntry, render } from 'astro:content';

// Get all non-draft blog posts, sorted by date
const posts = (await getCollection('blog', ({ data }) => {
  return data.draft !== true;
})).sort((a, b) => b.data.pubDate.valueOf() - a.data.pubDate.valueOf());

// Get a single entry by ID
const featuredPost = await getEntry('blog', 'welcome-post');
---

<h1>Blog</h1>
<ul>
  {posts.map((post) => (
    <li>
      <a href={`/blog/${post.id}`}>
        <h2>{post.data.title}</h2>
        <time datetime={post.data.pubDate.toISOString()}>
          {post.data.pubDate.toLocaleDateString()}
        </time>
        <p>{post.data.description}</p>
      </a>
    </li>
  ))}
</ul>
```

```astro
---
// src/pages/blog/[...slug].astro
import { getCollection, getEntry, render } from 'astro:content';

// Generate static paths for all posts
export async function getStaticPaths() {
  const posts = await getCollection('blog');
  return posts.map((post) => ({
    params: { slug: post.id },
    props: { post },
  }));
}

const { post } = Astro.props;

// Render markdown content to HTML
const { Content, headings } = await render(post);

// Get referenced author data
const author = await getEntry(post.data.author);
---

<article>
  <h1>{post.data.title}</h1>
  <p>By {author.data.name}</p>
  <time datetime={post.data.pubDate.toISOString()}>
    {post.data.pubDate.toLocaleDateString()}
  </time>

  <!-- Table of contents from headings -->
  <nav>
    <h2>Contents</h2>
    <ul>
      {headings.map((h) => (
        <li style={`margin-left: ${(h.depth - 1) * 1}rem`}>
          <a href={`#${h.slug}`}>{h.text}</a>
        </li>
      ))}
    </ul>
  </nav>

  <!-- Rendered markdown content -->
  <Content />
</article>
```

## Actions

Actions are type-safe server functions callable from the client with automatic validation. Define actions in `src/actions/index.ts`.

```typescript
// src/actions/index.ts
import { defineAction, ActionError } from 'astro:actions';
import { z } from 'astro/zod';

export const server = {
  // Simple action with JSON input
  createComment: defineAction({
    input: z.object({
      postId: z.string(),
      content: z.string().min(1).max(500),
      authorEmail: z.string().email(),
    }),
    handler: async (input, context) => {
      // Check authentication from middleware locals
      if (!context.locals.user) {
        throw new ActionError({
          code: 'UNAUTHORIZED',
          message: 'You must be logged in to comment',
        });
      }

      const comment = await db.comments.create({
        ...input,
        authorId: context.locals.user.id,
        createdAt: new Date(),
      });

      return { id: comment.id, success: true };
    },
  }),

  // Form action with file upload
  uploadAvatar: defineAction({
    accept: 'form',
    input: z.object({
      file: z.instanceof(File),
      userId: z.string(),
    }),
    handler: async ({ file, userId }) => {
      const buffer = await file.arrayBuffer();
      const url = await uploadToStorage(buffer, file.name);
      await db.users.update(userId, { avatarUrl: url });
      return { url };
    },
  }),

  // Grouped actions
  newsletter: {
    subscribe: defineAction({
      accept: 'form',
      input: z.object({
        email: z.string().email(),
        topics: z.array(z.string()).optional(),
      }),
      handler: async (input) => {
        await mailchimp.subscribe(input.email, input.topics);
        return { subscribed: true };
      },
    }),
    unsubscribe: defineAction({
      input: z.object({ email: z.string().email() }),
      handler: async ({ email }) => {
        await mailchimp.unsubscribe(email);
        return { unsubscribed: true };
      },
    }),
  },
};
```

```astro
---
// src/pages/post/[id].astro - Using actions in Astro components
import { actions } from 'astro:actions';

// Call action from server-side
const result = await Astro.callAction(actions.newsletter.subscribe, {
  email: 'user@example.com'
});
---

<script>
  // Call action from client-side
  import { actions } from 'astro:actions';

  const form = document.querySelector('form');
  form?.addEventListener('submit', async (e) => {
    e.preventDefault();
    const formData = new FormData(form);

    const { data, error } = await actions.createComment({
      postId: '123',
      content: formData.get('content') as string,
      authorEmail: formData.get('email') as string,
    });

    if (error) {
      if (error.code === 'UNAUTHORIZED') {
        window.location.href = '/login';
      } else {
        console.error(error.message);
      }
      return;
    }

    console.log('Comment created:', data.id);
  });
</script>

<!-- HTML form action (zero JS) -->
<form method="POST" action={actions.newsletter.subscribe}>
  <input type="email" name="email" required />
  <button type="submit">Subscribe</button>
</form>
```

## Middleware

Middleware intercepts requests and responses to inject behavior, modify data, or perform authentication before pages render.

```typescript
// src/middleware.ts
import { defineMiddleware, sequence } from 'astro:middleware';

// Authentication middleware
const auth = defineMiddleware(async (context, next) => {
  const sessionToken = context.cookies.get('session')?.value;

  if (sessionToken) {
    try {
      const user = await verifySession(sessionToken);
      context.locals.user = user;
    } catch {
      context.cookies.delete('session');
    }
  }

  return next();
});

// Logging middleware
const logging = defineMiddleware(async (context, next) => {
  const start = Date.now();
  const response = await next();
  const duration = Date.now() - start;

  console.log(`${context.request.method} ${context.url.pathname} - ${duration}ms`);

  return response;
});

// Protected routes middleware
const protectRoutes = defineMiddleware(async (context, next) => {
  const protectedPaths = ['/dashboard', '/settings', '/admin'];
  const isProtected = protectedPaths.some(p => context.url.pathname.startsWith(p));

  if (isProtected && !context.locals.user) {
    // Rewrite to login page without changing URL
    return context.rewrite('/login');
  }

  return next();
});

// Response modification middleware
const addHeaders = defineMiddleware(async (context, next) => {
  const response = await next();

  // Add security headers
  response.headers.set('X-Frame-Options', 'DENY');
  response.headers.set('X-Content-Type-Options', 'nosniff');

  return response;
});

// Chain multiple middlewares
export const onRequest = sequence(logging, auth, protectRoutes, addHeaders);
```

```typescript
// src/env.d.ts - Type the locals object
declare namespace App {
  interface Locals {
    user?: {
      id: string;
      email: string;
      role: 'admin' | 'user';
    };
  }
}
```

## Configuration Reference

Configure Astro in `astro.config.mjs` with options for build, server, integrations, and more.

```javascript
// astro.config.mjs
import { defineConfig } from 'astro/config';
import react from '@astrojs/react';
import tailwind from '@astrojs/tailwind';
import mdx from '@astrojs/mdx';
import node from '@astrojs/node';

export default defineConfig({
  // Site URL for sitemap and canonical URLs
  site: 'https://example.com',

  // Base path for deployment to subdirectory
  base: '/docs',

  // Output mode: 'static' (default) or 'server' for SSR
  output: 'server',

  // Server adapter for SSR deployment
  adapter: node({ mode: 'standalone' }),

  // Integrations for frameworks, tools, and features
  integrations: [
    react(),
    tailwind(),
    mdx(),
  ],

  // Trailing slash behavior: 'always', 'never', or 'ignore'
  trailingSlash: 'always',

  // Redirects configuration
  redirects: {
    '/old-page': '/new-page',
    '/blog/[...slug]': '/articles/[...slug]',
  },

  // Development server options
  server: {
    port: 4321,
    host: true,  // Expose to network
  },

  // Build options
  build: {
    format: 'directory',  // or 'file'
    assets: '_assets',
    inlineStylesheets: 'auto',
  },

  // Image optimization
  image: {
    service: { entrypoint: 'astro/assets/services/sharp' },
    domains: ['images.unsplash.com'],
    remotePatterns: [{
      protocol: 'https',
      hostname: '**.cloudinary.com',
    }],
  },

  // Markdown processing
  markdown: {
    syntaxHighlight: 'shiki',
    shikiConfig: {
      theme: 'github-dark',
      wrap: true,
    },
    remarkPlugins: [],
    rehypePlugins: [],
  },

  // i18n configuration
  i18n: {
    defaultLocale: 'en',
    locales: ['en', 'es', 'fr'],
    routing: {
      prefixDefaultLocale: false,
    },
    fallback: {
      fr: 'en',
    },
  },

  // Environment variables schema
  env: {
    schema: {
      API_URL: envField.string({ context: 'server', access: 'secret' }),
      PUBLIC_SITE_NAME: envField.string({ context: 'client', access: 'public' }),
    },
  },

  // Prefetch configuration
  prefetch: {
    prefetchAll: true,
    defaultStrategy: 'hover',
  },

  // Vite configuration passthrough
  vite: {
    plugins: [],
    ssr: {
      external: ['some-package'],
    },
  },
});
```

## Cookie Management

Cookies are available in SSR contexts for reading and writing session data, authentication tokens, and user preferences.

```astro
---
// src/pages/api/auth.ts or .astro pages
export const prerender = false; // Required for cookie access

// Read cookies
const sessionId = Astro.cookies.get('session')?.value;
const preferences = Astro.cookies.get('prefs')?.json();
const visitCount = Astro.cookies.get('visits')?.number() ?? 0;

// Set cookies with options
Astro.cookies.set('session', 'abc123', {
  path: '/',
  httpOnly: true,
  secure: true,
  sameSite: 'lax',
  maxAge: 60 * 60 * 24 * 7,  // 1 week in seconds
});

// Set JSON cookie
Astro.cookies.set('preferences', { theme: 'dark', language: 'en' }, {
  path: '/',
});

// Delete cookie
Astro.cookies.delete('session');

// Check if cookie exists
const hasSession = Astro.cookies.has('session');
---
```

## Sessions

Sessions provide persistent server-side storage for user data across requests, configured with various storage drivers.

```javascript
// astro.config.mjs
export default defineConfig({
  output: 'server',
  session: {
    driver: 'redis',
    options: {
      url: process.env.REDIS_URL,
    },
    ttl: 3600,  // Session expires after 1 hour
    cookie: {
      name: 'app-session',
      sameSite: 'lax',
      httpOnly: true,
      secure: true,
    },
  },
});
```

```astro
---
// src/pages/cart.astro
export const prerender = false;

// Get session data
const cart = await Astro.session?.get('cart') ?? [];
const lastViewed = await Astro.session?.get('lastViewedProduct');

// Set session data (with optional per-key TTL)
Astro.session?.set('cart', [...cart, newItem]);
Astro.session?.set('flash', 'Item added!', { ttl: 60 }); // 60 seconds

// On login - regenerate session ID to prevent fixation
Astro.session?.regenerate();

// On logout - destroy session entirely
Astro.session?.destroy();
---

<h1>Your Cart ({cart.length} items)</h1>
```

## Dynamic Routes

Dynamic routes use bracket syntax in filenames to match variable URL segments and generate multiple pages from data.

```astro
---
// src/pages/blog/[slug].astro - Single dynamic segment
import { getCollection } from 'astro:content';

export async function getStaticPaths() {
  const posts = await getCollection('blog');
  return posts.map((post) => ({
    params: { slug: post.id },
    props: { post },
  }));
}

const { post } = Astro.props;
---

<h1>{post.data.title}</h1>
```

```astro
---
// src/pages/[...path].astro - Catch-all route (rest parameter)
export async function getStaticPaths() {
  const pages = await fetchPagesFromCMS();
  return pages.map((page) => ({
    params: { path: page.slug || undefined }, // undefined matches root
    props: { page },
  }));
}

const { page } = Astro.props;
---
```

```astro
---
// src/pages/products/[category]/[id].astro - Multiple segments
export async function getStaticPaths() {
  const products = await getProducts();
  return products.map((p) => ({
    params: { category: p.category, id: p.id },
    props: { product: p },
  }));
}
---
```

## UI Framework Integration

Astro supports multiple UI frameworks (React, Vue, Svelte, etc.) as interactive "islands" with client directives controlling when components hydrate.

```astro
---
// src/pages/index.astro
import ReactCounter from '../components/ReactCounter.jsx';
import VueCard from '../components/VueCard.vue';
import SvelteForm from '../components/SvelteForm.svelte';
---

<!-- Static by default - no JS sent to browser -->
<ReactCounter />

<!-- Hydrate when page loads -->
<ReactCounter client:load />

<!-- Hydrate when component enters viewport -->
<VueCard client:visible />

<!-- Hydrate when browser is idle -->
<SvelteForm client:idle />

<!-- Hydrate on specific media query -->
<ReactCounter client:media="(max-width: 768px)" />

<!-- Only render on server, never hydrate -->
<ReactCounter client:only="react" />
```

```jsx
// src/components/ReactCounter.jsx
import { useState } from 'react';

export default function Counter({ initialCount = 0 }) {
  const [count, setCount] = useState(initialCount);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}
```

## Summary

Astro is ideal for building content-rich websites where performance matters. Its zero-JS-by-default approach delivers fast page loads while still supporting interactive components through the Islands Architecture. Content Collections provide a robust, type-safe way to manage Markdown, MDX, JSON, or remote content with automatic validation and TypeScript integration. Actions simplify server-client communication with type-safe RPC calls.

Common integration patterns include using Astro for documentation sites with MDX content, blogs with markdown and frontmatter, e-commerce storefronts with SSR for dynamic pricing, and marketing sites with forms handled by Actions. The framework works seamlessly with headless CMSs (Contentful, Sanity, Strapi), databases (Prisma, Drizzle), authentication providers (Auth.js, Lucia), and deployment platforms (Vercel, Netlify, Cloudflare, Node.js servers) through its adapter system.