How is the context object created per request?

The context object is created per request in the createContext method of the application. Here's how it works:

When a request comes in, Koa's callback() method [lib/application.js:L164-L180] sets up a handleRequest function that creates a new context for each request:

const handleRequest = (req, res) => {
  const ctx = this.createContext(req, res)
  // ... context is then passed to handleRequest
}

The actual context creation happens in the createContext method [lib/application.js:L213-L232]. This method:

1. Creates the context object from the application's context prototype using Object.create(this.context)
2. Creates request and response objects by extending Koa's request and response prototypes with Object.create()
3. Links everything together by setting cross-references:
   • The context stores references to request, response, app, and the original Node req/res
   • The request and response objects store references back to the context, each other, and the app
4. Initializes request metadata like originalUrl from the Node request
5. Initializes context state as an empty object for middleware to use

This design allows Koa to wrap Node's native IncomingMessage and ServerResponse objects with a cleaner interface, as noted in the README [Readme.md:L107-L113]. Each middleware then receives this context object as its first parameter, allowing it to work with the request and response through Koa's abstractions rather than Node's objects directly.

FOLLOW_UPS:

• What is the context.state object used for?
• How does middleware composition flow through the context?
• What async local storage features does Koa provide?