Elysia LLM Docs

ElysiaJS Development Skill

Always consult elysiajs.com/llms.txt for code examples and latest API.

Overview

ElysiaJS is a TypeScript framework for building Bun-first (but not limited to Bun) type-safe, high-performance backend servers. This skill provides comprehensive guidance for developing with Elysia, including routing, validation, authentication, plugins, integrations, and deployment.

When to Use This Skill

Trigger this skill when the user asks to:

• Create or modify ElysiaJS routes, handlers, or servers
• Setup validation with TypeBox or other schema libraries (Zod, Valibot)
• Implement authentication (JWT, session-based, macros, guards)
• Add plugins (CORS, OpenAPI, Static files, JWT)
• Integrate with external services (Drizzle ORM, Better Auth, Next.js, Eden Treaty)
• Setup WebSocket endpoints for real-time features
• Create unit tests for Elysia instances
• Deploy Elysia servers to production

Quick Start

Quick scaffold:

bun create elysia app

Basic Server

import { Elysia, t, status } from 'elysia'

const app = new Elysia()
    .get('/', () => 'Hello World')
    .post('/user', ({ body }) => body, {
      body: t.Object({
        name: t.String(),
        age: t.Number()
      })
    })
    .get('/id/:id', ({ params: { id } }) => {
      if(id > 1_000_000) return status(404, 'Not Found')
      return id
    }, {
      params: t.Object({
        id: t.Number({
          minimum: 1
        })
      }),
      response: {
        200: t.Number(),
        404: t.Literal('Not Found')
      }
    })
    .listen(3000)

HTTP Methods

import { Elysia } from 'elysia'

new Elysia()
  .get('/', 'GET')
  .post('/', 'POST')
  .put('/', 'PUT')
  .patch('/', 'PATCH')
  .delete('/', 'DELETE')
  .options('/', 'OPTIONS')
  .head('/', 'HEAD')

TypeBox Validation

Basic Types

import { Elysia, t } from 'elysia'

.post('/user', ({ body }) => body, {
  body: t.Object({
    name: t.String(),
    age: t.Number(),
    email: t.String({ format: 'email' }),
    website: t.Optional(t.String({ format: 'uri' }))
  })
})

File Upload

.post('/upload', ({ body }) => body.file, {
  body: t.Object({
    file: t.File({
      type: 'image',
      maxSize: '5m'
    }),
    files: t.Files({
      type: ['image/png', 'image/jpeg']
    })
  })
})

Response Validation

.get('/user/:id', ({ params: { id } }) => ({
  id,
  name: 'John',
  email: 'john@example.com'
}), {
  params: t.Object({
    id: t.Number()
  }),
  response: {
    200: t.Object({
      id: t.Number(),
      name: t.String(),
      email: t.String()
    }),
    404: t.String()
  }
})

Error Handling

.get('/user/:id', ({ params: { id }, status }) => {
  const user = findUser(id)
  if (!user) {
    return status(404, 'User not found')
  }
  return user
})

Guards (Apply to Multiple Routes)

.guard({
  params: t.Object({
    id: t.Number()
  })
}, app => app
  .get('/user/:id', ({ params: { id } }) => id)
  .delete('/user/:id', ({ params: { id } }) => id)
)

Macro

.macro({
  hi: (word: string) => ({
    beforeHandle() { console.log(word) }
  })
})
.get('/', () => 'hi', { hi: 'Elysia' })

Project Structure (Recommended)

src/
├── index.ts              # Main server entry
├── modules/
│   ├── auth/
│   │   ├── index.ts      # Auth routes (Elysia instance)
│   │   ├── service.ts    # Business logic
│   │   └── model.ts      # TypeBox schemas/DTOs
│   └── user/
│       ├── index.ts
│       ├── service.ts
│       └── model.ts
└── plugins/
    └── custom.ts

public/                   # Static files (if using static plugin)
test/                     # Unit tests

Key Concepts

Encapsulation - Isolates by Default

Lifecycles (hooks, middleware) don't leak between instances unless scoped.

Scope levels:

• local (default) - current instance + descendants
• scoped - parent + current + descendants
• global - all instances

.onBeforeHandle(() => {}) // only local instance
.onBeforeHandle({ as: 'global' }, () => {}) // exports to all

Method Chaining - Required for Types

Must chain. Each method returns new type reference.

❌ Don't:

const app = new Elysia()
app.state('build', 1) // loses type
app.get('/', ({ store }) => store.build) // build doesn't exist

✅ Do:

new Elysia()
  .state('build', 1)
  .get('/', ({ store }) => store.build)

Explicit Dependencies

Each instance independent. Declare what you use.

const auth = new Elysia()
  .decorate('Auth', Auth)
  .model(Auth.models)

new Elysia()
  .use(auth) // must declare
  .get('/', ({ Auth }) => Auth.getProfile())

Reference Model

Model can be referenced by name:

new Elysia()
  .model({
    book: t.Object({
      name: t.String()
    })
  })
  .post('/', ({ body }) => body.name, {
    body: 'book'
  })

Model can be renamed using .prefix / .suffix:

new Elysia()
  .model({
    book: t.Object({
      name: t.String()
    })
  })
  .prefix('model', 'Namespace')
  .post('/', ({ body }) => body.name, {
    body: 'Namespace.Book'
  })

Best Practices

• Controller: Prefer Elysia as a controller for HTTP-dependent logic
• Service: Prefer class (or abstract class if possible), return status for errors
• Models: Always export validation model and type of validation model
• Prefer return Error instead of throw Error
• Use onError to handle local custom errors
• Register Model to Elysia instance via Elysia.models({ ...models })

Integrations

• Better Auth: Authentication framework integration
• Drizzle ORM: Database ORM integration
• Next.js: API route integration
• Expo: Mobile API route integration
• Cloudflare Workers: Edge deployment

Related Skills

• effect-patterns - Effect TypeScript integration
• better-auth - Authentication framework
• bun-llm-docs - Bun runtime documentation