Transform

Executed just before Validation process, designed to mutate context to conform with the validation or appending new value.

It's recommended to use transform for the following:

• Mutate existing context to conform with validation.
• derive is based on onTransform with support for providing type.

Example

Below is an example of using transform to mutate params to be numeric values.

import { Elysia, t } from 'elysia'

new Elysia()
    .get('/id/:id', ({ params: { id } }) => id, {
        params: t.Object({
            id: t.Number()
        }),
        transform({ params }) {
            const id = +params.id

            if(!Number.isNaN(id))
                params.id = id
        }
    })
    .listen(3000)

Derive

Designed to append new value to context directly before validation process storing in the same stack as transform.

Unlike state and decorate that assigned value before the server started. derive assigns a property when each request happens. Allowing us to extract a piece of information into a property instead.

import { Elysia } from 'elysia'

new Elysia()
    .derive(({ headers }) => {
        const auth = headers['Authorization']

        return {
            bearer: auth?.startsWith('Bearer ') ? auth.slice(7) : null
        }
    })
    .get('/', ({ bearer }) => bearer)

Because derive is assigned once a new request starts, derive can access Request properties like headers, query, body where store, and decorate can't.

Unlike state, and decorate. Properties which assigned by derive is unique and not shared with another request.

Queue twoslash

Using derived and transform is stored in the same queue.

import { Elysia } from 'elysia'

new Elysia()
    .onTransform(() => {
        console.log(1)
    })
    .derive(() => {
        console.log(2)

        return {}
    })

The console should log as the following:

1
2