Error Handling ​

onError is called when an error is thrown.

It accepts context similar to a handler but includes an additional:

  • error - a thrown error
  • code - error code
typescript
import { Elysia } from 'elysia'

new Elysia()
	.onError(({ code, status }) => {
		if(code === "NOT_FOUND")
			return 'uhe~ are you lost?'

		return status(418, "My bad! But I'm cute so you'll forgive me, right?")
	})
	.get('/', () => 'ok')
	.listen(3000)

You can return a status to override the default error status.

Custom Error ​

You can provide a custom error with error code as follows:

typescript
import { Elysia } from 'elysia'

class NicheError extends Error {
	constructor(message: string) {
		super(message)
	}
}

new Elysia()
	.error({ 
		'NICHE': NicheError 
	}) 
	.onError(({ error, code, status }) => {
		if(code === 'NICHE') {
			// Typed as NicheError
			console.log(error)

			return status(418, "We have no idea how you got here")
		}
	})
	.get('/', () => {
        throw new NicheError('Custom error message')
	})
	.listen(3000)

Elysia uses error codes to narrow down the type of error.

It's recommended to register custom errors as Elysia can narrow down the type.

Error Status Code ​

You can also provide a custom status code by adding a status property to class:

typescript
import { Elysia } from 'elysia'

class NicheError extends Error {
	status = 418

	constructor(message: string) {
		super(message)
	}
}

Elysia will use this status code if the error is thrown, see Custom Status Code.

Error Response ​

You can also define a custom error response directly into the error by providing a toResponse method:

typescript
import { Elysia } from 'elysia'

class NicheError extends Error {
	status = 418

	constructor(message: string) {
		super(message)
	}

	toResponse() { 
		return { message: this.message } 
	} 
}

Elysia will use this response if the error is thrown, see Custom Error Response.

Assignment ​

Let's try to extend Elysia's context.

  1. Your own 404

    Let's override a default 404 response your own

  2. Your Custom Error

    Let's define your own custom error, throw it in GET "/" endpoint, and return a status 418

Show answer
1. You can narrow down the error by "NOT_FOUND" to override 404 response. 2. Provide your error to `.error()` method with status property of 418.
typescript
import { Elysia } from 'elysia'

class YourError extends Error {
	status = 418

	constructor(message: string) {
		super(message)
	}
}

new Elysia()
	.error({
		"YOUR_ERROR": YourError
	})
	.onError(({ code, status }) => {
		if(code === "NOT_FOUND")
			return "Hi there"
	})
	.get('/', () => {
		throw new YourError("A")
	})
	.listen(3000)

  • index.ts