I'm exploring the possibility of supporting Express.js compatible middleware in Hono. This would allow for easier migration from Express to Hono and enable the use of the vast ecosystem of Express middleware within Hono applications.

Proposed Implementation

I'm looking for something like this for backward compatibility:

type ExpressMiddleware = (
  req: IncomingMessage,
  res: ServerResponse,
  next: (err?: any) => void
) => void

export function adaptExpressMiddleware(middleware: ExpressMiddleware): MiddlewareHandler {
  return async (c: Context, next: Next) => {
    const req = c.req.raw as unknown as IncomingMessage
    const res = c.res as unknown as ServerResponse

    await new Promise<void>((resolve, reject) => {
      middleware(req, res, (err) => {
        if (err) reject(err)
        else resolve()
      })
    })

    await next()
  }
}

const app = new Hono()
app.use(adaptExpressMiddleware(existingexpressmiddleware()));

Current Issues

So far, I haven't been able to make this work due to type incompatibility issues. The main challenges are:

1. Hono's Context object doesn't directly map to Express's req and res objects.
2. The IncomingMessage and ServerResponse types from Node.js http module don't align perfectly with Hono's request and response handling.

Questions

1. Is it feasible to implement such an adapter in Hono?
2. What would be the best approach to handle the type mismatches between Express and Hono?
3. Are there any performance implications we should consider when adapting Express middleware?

Goals

• Enable the use of popular Express middleware in Hono applications.
• Provide a smooth migration path for developers moving from Express to Hono.
• Maintain Hono's performance advantages while offering this compatibility.

Any insights, suggestions, or assistance on how to achieve this would be greatly appreciated. Thank you!