How to Build High-Performance APIs with Elysia and Bun

Elysia is a fast, ergonomic web framework built specifically for the Bun runtime. It combines Bun’s native speed with an elegant API inspired by Express and Fastify. This tutorial walks you through creating a simple REST API using Elysia, including routes, validation, and middleware.

Before you start, ensure you have Bun installed (version 1.0.0 or later). You can verify with bun --version. Create a new directory and run bun init to scaffold a project.

Setting Up Your Elysia Project

Install Elysia using Bun’s package manager:

bun add elysia

Create an index.ts file and import Elysia:

import { Elysia } from 'elysia';

const app = new Elysia();

app.listen(3000, () => {
  console.log('🔥 Elysia running on http://localhost:3000');
});

Run your server with bun run index.ts. You now have a working Elysia server.

Defining Routes and Handling Requests

Add routes using familiar HTTP methods. Elysia uses a chainable API:

app.get('/', () => 'Hello Bun!');
app.post('/data', ({ body }) => {
  return { received: body };
});

Access route parameters with square brackets:

app.get('/user/:id', ({ params: { id } }) => `User ${id}`);

Validation and Middleware

Elysia integrates with @elysiajs/validator for schema validation. Install it:

bun add @elysiajs/validator

Use it to validate request bodies:

import { validator } from '@elysiajs/validator';

app.post('/signup', 
  validator({
    body: t.Object({
      email: t.String({ format: 'email' }),
      password: t.String({ minLength: 8 })
    })
  }),
  ({ body }) => `Welcome ${body.email}!`
);

For middleware, simply call .use() with a function that modifies request or set:

app.use((app) => app.onBeforeHandle(({ request }) => {
  console.log(`Request: ${request.method}`);
}));

Running in Production

Elysia supports hot reloading with bun --watch. For production, use bun build --target=bun index.ts to compile a single binary. Then run the output:

bun run dist/index.js

Deploy to any Bun‑compatible host (e.g., Fly.io, Railway).

Elysia combines Bun’s blistering speed with a developer‑friendly API. Start small, add validation and middleware as needed, and you’ll have a production‑ready API in minutes.