Structure

Elysia is a pattern-agnostic framework, leaving the decision of which coding patterns to use up to you and your team.

However, there are several concern from trying to adapt an MVC pattern (Model-View-Controller) with Elysia, and found it's hard to decouple and handle types.

This page is a guide to on how to follows Elysia structure best practice combined with MVC pattern but can be adapted to any coding pattern you like.

Method Chaining

Elysia code should always use method chaining.

As Elysia type system is complex, every methods in Elysia returns a new type reference.

This is important to ensure type integrity and inference.

import { Elysia } from 'elysia'

new Elysia()
    .state('build', 1)
    // Store is strictly typed
    .get('/', ({ store: { build } }) => build)
    .listen(3000)

In the code above state returns a new ElysiaInstance type, adding a build type.

❌ Don't: Use without method chaining

Without using method chaining, Elysia doesn't save these new types, leading to no type inference.

import { Elysia } from 'elysia'

const app = new Elysia()

app.state('build', 1)

app.get('/', ({ store: { build } }) => build)
Property 'build' does not exist on type '{}'.

app.listen(3000)

We recommend to always use method chaining to provide an accurate type inference.

Controller

1 Elysia instance = 1 controller

Elysia does a lot to ensure type integrity, if you pass an entire Context type to a controller, these might be the problems:

1. Elysia type is complex and heavily depends on plugin and multiple level of chaining.
2. Hard to type, Elysia type could change at anytime, especially with decorators, and store
3. Type casting may lead to a loss of type integrity or an inability to ensure consistency between types and runtime code.
4. This makes it more challenging for Sucrose (Elysia's "kind of" compiler) to statically analyze your code

❌ Don't: Create a separate controller

Don't create a separate controller, use Elysia itself as a controller instead:

import { Elysia, t, type Context } from 'elysia'

abstract class Controller {
    static root(context: Context) {
        return Service.doStuff(context.stuff)
    }
}

// ❌ Don't
new Elysia()
    .get('/', Controller.hi)

By passing an entire Controller.method to Elysia is an equivalent of having 2 controllers passing data back and forth. It's against the design of framework and MVC pattern itself.

✅ Do: Use Elysia as a controller

Instead treat an Elysia instance as a controller itself instead.

import { Elysia } from 'elysia'
import { Service } from './service'

new Elysia()
    .get('/', ({ stuff }) => {
        Service.doStuff(stuff)
    })

Service

Service is a set of utility/helper functions decoupled as a business logic to use in a module/controller, in our case, an Elysia instance.

Any technical logic that can be decoupled from controller may live inside a Service.

There're 2 types of service in Elysia:

1. Non-request dependent service
2. Request dependent service

✅ Do: Non-request dependent service

This kind of service doesn't need to access any property from the request or Context, and can be initiated as a static class same as usual MVC service pattern.

import { Elysia, t } from 'elysia'

abstract class Service {
    static fibo(number: number): number {
        if(number < 2)
            return number

        return Service.fibo(number - 1) + Service.fibo(number - 2)
    }
}

new Elysia()
    .get('/fibo', ({ body }) => {
        return Service.fibo(body)
    }, {
        body: t.Numeric()
    })

If your service doesn't need to store a property, you may use abstract class and static instead to avoid allocating class instance.

Request Dependent Service

This kind of service may require some property from the request, and should be initiated as an Elysia instance.

❌ Don't: Pass entire Context to a service

Context is a highly dynamic type that can be inferred from Elysia instance.

Do not pass an entire Context to a service, instead use object destructuring to extract what you need and pass it to the service.

import type { Context } from 'elysia'

class AuthService {
	constructor() {}

	// ❌ Don't do this
	isSignIn({ cookie: { session } }: Context) {
		if (session.value)
			return error(401)
	}
}

As Elysia type is complex, and heavily depends on plugin and multiple level of chaining, it can be challenging to manually type as it's highly dynamic.

✅ Do: Use Elysia instance as a service

We recommended to use Elysia instance as a service to ensure type integrity and inference:

import { Elysia } from 'elysia'

// ✅ Do
const AuthService = new Elysia({ name: 'Service.Auth' })
    .derive({ as: 'scoped' }, ({ cookie: { session } }) => ({
    	// This is equivalent to dependency injection
        Auth: {
            user: session.value
        }
    }))
    .macro(({ onBeforeHandle }) => ({
     	// This is declaring a service method
        isSignIn(value: boolean) {
            onBeforeHandle(({ Auth, error }) => {
                if (!Auth?.user || !Auth.user) return error(401)
            })
        }
    }))

const UserController = new Elysia()
    .use(AuthService)
    .get('/profile', ({ Auth: { user } }) => user, {
    	isSignIn: true
    })

TIP

Elysia handle plugin deduplication by default so you don't have to worry about performance, as it's going to be Singleton if you specified a "name" property.

⚠️ Infers Context from Elysia instance

In case of absolute necessity, you may infer the Context type from the Elysia instance itself:

import { Elysia, type InferContext } from 'elysia'

const setup = new Elysia()
	.state('a', 'a')
	.decorate('b', 'b')

class AuthService {
	constructor() {}

	// ✅ Do
	isSignIn({ cookie: { session } }: InferContext<typeof setup>) {
		if (session.value)
			return error(401)
	}
}

However we recommend to avoid this if possible, and use Elysia as a service instead.

You may find more about InferContext in Essential: Handler.

Model

Model or DTO (Data Transfer Object) is handle by Elysia.t (Validation).

Elysia has a validation system built-in which can infers type from your code and validate it at runtime.

❌ Don't: Declare a class instance as a model

Do not declare a class instance as a model:

// ❌ Don't
class CustomBody {
	username: string
	password: string

	constructor(username: string, password: string) {
		this.username = username
		this.password = password
	}
}

// ❌ Don't
interface ICustomBody {
	username: string
	password: string
}

✅ Do: Use Elysia's validation system

Instead of declaring a class or interface, use Elysia's validation system to define a model:

// ✅ Do
import { Elysia, t } from 'elysia'

const customBody = t.Object({
	username: t.String(),
	password: t.String()
})

// Optional if you want to get the type of the model
// Usually if we didn't use the type, as it's already inferred by Elysia
type CustomBody = typeof customBody.static



export { customBody }

We can get type of model by using typeof with .static property from the model.

Then you can use the CustomBody type to infer the type of the request body.

// ✅ Do
new Elysia()
	.post('/login', ({ body }) => {
		return body
	}, {
		body: customBody
	})

❌ Don't: Declare type separate from the model

Do not declare a type separate from the model, instead use typeof with .static property to get the type of the model.

// ❌ Don't
import { Elysia, t } from 'elysia'

const customBody = t.Object({
	username: t.String(),
	password: t.String()
})

type CustomBody = {
	username: string
	password: string
}

// ✅ Do
const customBody = t.Object({
	username: t.String(),
	password: t.String()
})

type customBody = typeof customBody.static

Group

You can group multiple models into a single object to make it more organized.

import { Elysia, t } from 'elysia'

export const AuthModel = {
	sign: t.Object({
		username: t.String(),
		password: t.String()
	})
}

Model Injection

Though this is optional, if you are strictly following MVC pattern, you may want to inject like a service into a controller. We recommended using Elysia reference model

Using Elysia's model reference

import { Elysia, t } from 'elysia'

const customBody = t.Object({
	username: t.String(),
	password: t.String()
})

const AuthModel = new Elysia()
    .model({
        'auth.sign': customBody
    })

const UserController = new Elysia({ prefix: '/auth' })
    .use(AuthModel)
    .post('/sign-in', async ({ body, cookie: { session } }) => {

        return true
    }, {
        body: 'auth.sign'
    })

This approach provide several benefits:

1. Allow us to name a model and provide auto-completion.
2. Modify schema for later usage, or perform remapping.
3. Show up as "models" in OpenAPI compliance client, eg. Swagger.
4. Improve TypeScript inference speed as model type will be cached during registration.

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As mentioned, Elysia is a pattern-agnostic framework, and we only provide a recommendation guide for handling Elysia with the MVC pattern.

It’s entirely up to you and your team whether to follow this recommendation based on your preferences and agreement.