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You can add routes to Bun.serve() by using the routes property (for static paths, parameters, and wildcards) or by handling unmatched requests with the fetch method. Bun.serve()’s router builds on top uWebSocket’s tree-based approach to add SIMD-accelerated route parameter decoding and JavaScriptCore structure caching to push the performance limits of what modern hardware allows.

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Basic Setup

/icons/typescript.svgserver.ts
Bun.serve({
	routes: {
		'/': () => new Response('Home'),
		'/api': () => Response.json({success: true}),
		'/users': async () => Response.json({users: []}),
	},
	fetch() {
		return new Response('Unmatched route');
	},
});
Routes in Bun.serve() receive a BunRequest (which extends Request) and return a Response or Promise<Response>. This makes it easier to use the same code for both sending & receiving HTTP requests. 
// Simplified for brevity
interface BunRequest<T extends string> extends Request {
	params: Record<T, string>;
	readonly cookies: CookieMap;
}

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Asynchronous Routes

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Async/await

You can use async/await in route handlers to return a Promise<Response>. 
import {sql, serve} from 'bun';

serve({
	port: 3001,
	routes: {
		'/api/version': async () => {
			const [version] = await sql`SELECT version()`;
			return Response.json(version);
		},
	},
});

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Promise

You can also return a Promise<Response> from a route handler. 
import {sql, serve} from 'bun';

serve({
	routes: {
		'/api/version': () => {
			return new Promise(resolve => {
				setTimeout(async () => {
					const [version] = await sql`SELECT version()`;
					resolve(Response.json(version));
				}, 100);
			});
		},
	},
});

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Route precedence

Routes are matched in order of specificity:
  1. Exact routes (/users/all)
  2. Parameter routes (/users/:id)
  3. Wildcard routes (/users/*)
  4. Global catch-all (/*)
Bun.serve({
	routes: {
		// Most specific first
		'/api/users/me': () => new Response('Current user'),
		'/api/users/:id': req => new Response(`User ${req.params.id}`),
		'/api/*': () => new Response('API catch-all'),
		'/*': () => new Response('Global catch-all'),
	},
});

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Type-safe route parameters

TypeScript parses route parameters when passed as a string literal, so that your editor will show autocomplete when accessing request.params.
index.ts
import type {BunRequest} from 'bun';

Bun.serve({
	routes: {
		// TypeScript knows the shape of params when passed as a string literal
		'/orgs/:orgId/repos/:repoId': req => {
			const {orgId, repoId} = req.params;
			return Response.json({orgId, repoId});
		},

		'/orgs/:orgId/repos/:repoId/settings': (
			// optional: you can explicitly pass a type to BunRequest:
			req: BunRequest<'/orgs/:orgId/repos/:repoId/settings'>,
		) => {
			const {orgId, repoId} = req.params;
			return Response.json({orgId, repoId});
		},
	},
});
Percent-encoded route parameter values are automatically decoded. Unicode characters are supported. Invalid unicode is replaced with the unicode replacement character &0xFFFD;.

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Static responses

Routes can also be Response objects (without the handler function). Bun.serve() optimizes it for zero-allocation dispatch - perfect for health checks, redirects, and fixed content: 
Bun.serve({
	routes: {
		// Health checks
		'/health': new Response('OK'),
		'/ready': new Response('Ready', {
			headers: {
				// Pass custom headers
				'X-Ready': '1',
			},
		}),

		// Redirects
		'/blog': Response.redirect('https://bun.com/blog'),

		// API responses
		'/api/config': Response.json({
			version: '1.0.0',
			env: 'production',
		}),
	},
});
Static responses do not allocate additional memory after initialization. You can generally expect at least a 15% performance improvement over manually returning a Response object. Static route responses are cached for the lifetime of the server object. To reload static routes, call server.reload(options).

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File Responses vs Static Responses

When serving files in routes, there are two distinct behaviors depending on whether you buffer the file content or serve it directly: 
Bun.serve({
	routes: {
		// Static route - content is buffered in memory at startup
		'/logo.png': new Response(await Bun.file('./logo.png').bytes()),

		// File route - content is read from filesystem on each request
		'/download.zip': new Response(Bun.file('./download.zip')),
	},
});
Static routes (new Response(await file.bytes())) buffer content in memory at startup:
  • Zero filesystem I/O during requests - content served entirely from memory
  • ETag support - Automatically generates and validates ETags for caching
  • If-None-Match - Returns 304 Not Modified when client ETag matches
  • No 404 handling - Missing files cause startup errors, not runtime 404s
  • Memory usage - Full file content stored in RAM
  • Best for: Small static assets, API responses, frequently accessed files
File routes (new Response(Bun.file(path))) read from filesystem per request:
  • Filesystem reads on each request - checks file existence and reads content
  • Built-in 404 handling - Returns 404 Not Found if file doesn’t exist or becomes inaccessible
  • Last-Modified support - Uses file modification time for If-Modified-Since headers
  • If-Modified-Since - Returns 304 Not Modified when file hasn’t changed since client’s cached version
  • Range request support - Automatically handles partial content requests with Content-Range headers
  • Streaming transfers - Uses buffered reader with backpressure handling for efficient memory usage
  • Memory efficient - Only buffers small chunks during transfer, not entire file
  • Best for: Large files, dynamic content, user uploads, files that change frequently

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Streaming files

To stream a file, return a Response object with a BunFile object as the body. 
Bun.serve({
	fetch(req) {
		return new Response(Bun.file('./hello.txt'));
	},
});
⚡️ Speed — Bun automatically uses the sendfile(2) system call when possible, enabling zero-copy file transfers in the kernel—the fastest way to send files.
You can send part of a file using the slice(start, end) method on the Bun.file object. This automatically sets the Content-Range and Content-Length headers on the Response object. 
Bun.serve({
	fetch(req) {
		// parse `Range` header
		const [start = 0, end = Infinity] = req.headers
			.get('Range') // Range: bytes=0-100
			.split('=') // ["Range: bytes", "0-100"]
			.at(-1) // "0-100"
			.split('-') // ["0", "100"]
			.map(Number); // [0, 100]

		// return a slice of the file
		const bigFile = Bun.file('./big-video.mp4');
		return new Response(bigFile.slice(start, end));
	},
});

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fetch request handler

The fetch handler handles incoming requests that weren’t matched by any route. It receives a Request object and returns a Response or Promise<Response>. 
Bun.serve({
	fetch(req) {
		const url = new URL(req.url);
		if (url.pathname === '/') return new Response('Home page!');
		if (url.pathname === '/blog') return new Response('Blog!');
		return new Response('404!');
	},
});
The fetch handler supports async/await: 
import {sleep, serve} from 'bun';

serve({
	async fetch(req) {
		const start = performance.now();
		await sleep(10);
		const end = performance.now();
		return new Response(`Slept for ${end - start}ms`);
	},
});
Promise-based responses are also supported: 
Bun.serve({
	fetch(req) {
		// Forward the request to another server.
		return fetch('https://example.com');
	},
});
You can also access the Server object from the fetch handler. It’s the second argument passed to the fetch function. 
// `server` is passed in as the second argument to `fetch`.
const server = Bun.serve({
	fetch(req, server) {
		const ip = server.requestIP(req);
		return new Response(`Your IP is ${ip}`);
	},
});
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