Extends context

Elysia provides a context with small utilities to help you get started.

You can extend Elysia’s context with:

Decorate

Singleton, and immutable properties that are shared across all requests.

import { Elysia } from 'elysia'

class Logger {
    log(value: string) {
        console.log(value)
    }
}

new Elysia()
    .decorate('logger', new Logger())
    .get('/', ({ logger }) => {
        logger.log('hi')

        return 'hi'
    })

Decorated value it will be available in the context as a read-only property, see Decorate.

State

A mutable reference that is shared across all requests.

import { Elysia } from 'elysia'

new Elysia()
  .state('count', 0)
  .get('/', ({ store }) => {
    store.count++

    return store.count
  })

State will be available in context.store that is shared across every request, see State.

The first two requests return 1, then 2. Increment store.count to update the shared state. Destructuring a number with ({ store: { count } }) copies its current value into a local variable. Incrementing that variable does not change store.count.

Derive

Decorate value is registered as a singleton.

Derive adds context values per request, after validation.

import { Elysia } from 'elysia'

new Elysia()
  .derive(({ headers: { authorization } }) => ({
    authorization
  }))
  .get('/', ({ authorization }) => authorization)

Any returned value will be available in context except status, which will be send to client directly, and abort the subsequent handlers.

derive runs before the handler, after input has been validated and converted. Return an object to add values to context, or return status() to end the request early.

Scope

State, and Decorate are shared across all requests, and instances.

Derive values are per request, and have an encapsulation scope (as they’re based on life-cycle event).

If you want to use a derived value from a plugin, you would have to declare a Scope.

import { Elysia } from 'elysia'

const plugin = new Elysia()
  .derive(
    'plugin',
    ({ headers: { authorization } }) => ({
      authorization
    })
  )

new Elysia()
  .use(plugin)
  .get('/', ({ authorization }) => authorization)
  .listen(3000)

Assignment

Let’s try to extend Elysia’s context.

Assignments

0 of 2 assignments passed

Extract "query.age"
Not passed yet
Let's extract a "query.age" as "age". If it doesn't existed, return 401.
Use "age"
Not passed yet
In a main handler of GET "/profile", return a value of "age" from the context

We can use derive to extract age from query.

import { Elysia, t } from 'elysia'

class Logger {
  log(info: string) {
    console.log(info)
  }
}

new Elysia()
  .decorate('logger', new Logger())
  .onRequest(({ request, logger }) => {
    logger.log(`Request to ${request.url}`)
  })
  .guard({
    query: t.Optional(
      t.Object({
        age: t.Number({ minimum: 15 })
      })
    )
  })
  .derive(({ query: { age }, status }) => {
    if(!age) return status(401)

    return { age }
  })
  .get('/profile', ({ age }) => age)
  .listen(3000)

One sec, warming things up…

Extract "query.age"

Use "age"