koa-as-middleware.js

class ConnectKoa extends Koa {
  callback() {
    // compose middleware stack
    const fn = compose(this.middleware);
    
    // expose a connect-conforming function
    return (req, res, next) => {
      // create context
      const ctx = this.createContext(req, res);
      
      // dispatch context through middleware stack
      return fn(ctx).then(() => {
        
        // if part of the middleware stack is going to handle the response,
        // it should announce it has done so by setting ctx.respond = false
        if (ctx.respond === false) return ctx;
        
        // otherwise, pass control back to the enlcosing app
        if (next && res.statusCode == null) return next();
        
        // as a sanity check, ensure none of the middleware set a status code
        // but DIDN'T set ctx.respond = false -- they must do both or none!
        throw new Error("ctx.respond not set to `false`, but no status code set");
        
      }).catch(err => {
        // if next was provided, pass the error back to the enclosing application
        if (next) return next(err);
        
        // otherwise, try to surface the error -- if nobody is waiting on this promise
        // it the error will be printed by default (process.onUnhandledRejection behavior)
        throw err
      });
    };
  }
}