Express Adapter

toExpressHandler() bridges Futon's Web-standard fetch() pipeline to an Express (req, res, next) handler — useful when you're incrementally adopting Futon inside an existing Express app.

Usage

import express from 'express';
import { Futon, toExpressHandler, json } from '@rasenganjs/futon';
 
const app = new Futon();
app.get('/api/health', async () => json({ status: 'ok' }));
 
const expressApp = express();
expressApp.use(toExpressHandler(app));
expressApp.listen(3000);

What It Does

1. Reads the Express req (method, URL, headers, body) and constructs a Web API Request.
2. Calls app.fetch(request, runtime) — the exact same entry point every other adapter uses.
3. Writes the resulting Response's status, headers, and body back onto the Express res.

If Express's own body-parsing middleware already consumed req.body, the adapter re-serializes it as JSON before handing it to Futon — otherwise it streams the raw Node IncomingMessage through as a Web ReadableStream.

Streaming Responses

For SSR or any streamed response body, the adapter pipes the Web Response body directly to the Express response socket chunk-by-chunk rather than buffering the whole thing first — this is what makes streamResponse() work transparently under Express too.

Error Handling

Any error thrown while building the request or running app.fetch() is forwarded to Express's own next(error) — so your existing Express error-handling middleware still applies as a final safety net, in addition to Futon's own onError().