Router Basics & Configuration

Creating a Router

The create() function is the main entry point to Kaito. It creates a new router instance with your configuration and returns it.


import {create} from '@kaito-http/core';
 
const router = create(); // Arguments to `create()` are optional
 
const handle = router.serve();
 
Bun.serve({
	fetch: handle,
	port: 3000,
});

Error Handling

The onError configuration function is called whenever an error is thrown in the request lifecycle. This function should reply with an object that contains a status and message. These will be used to reply to the client.


import {create} from '@kaito-http/core';
 
export const router = create({
	onError: async (error, req) => {
		return {status: 500, message: 'Internal Server Error'};
	},
});

Request Lifecycle

Kaito has a basic lifecycle for transforming/intercepting requests and responses:

.before() runs before the router is handled. You can return early here to stop the request from being handled.
.transform() runs on EVERY response, including those from .before() and the router itself.

Here’s an example implementing CORS:


const ALLOWED_ORIGINS = ['http://localhost:3000', 'https://app.example.com'];
 
export const router = create({
	getContext,
	before: async req => {
		if (req.method === 'OPTIONS') {
			// Return early to skip the router
			return new Response(null, {status: 204});
		}
	},
 
	transform: async (request, response) => {
		const origin = request.headers.get('origin');
 
		// Include CORS headers if the origin is allowed
		if (origin && ALLOWED_ORIGINS.includes(origin)) {
			response.headers.set('Access-Control-Allow-Origin', origin);
			response.headers.set('Access-Control-Allow-Methods', 'GET, POST, PUT, PATCH, DELETE, OPTIONS');
			response.headers.set('Access-Control-Allow-Headers', 'Content-Type, Authorization');
			response.headers.set('Access-Control-Max-Age', '86400');
			response.headers.set('Access-Control-Allow-Credentials', 'true');
		}
	},
});

You can return a response inside .transform() as well, but this is less common. Most often you’ll only be mutating headers.