# Behavior traces — koajs/koa

> Canonical page: https://codevetter.com/unpack/koa/behavior-traces

# Behavior traces — koajs/koa

Scanned commit 824c1cf8de9a91a2941973b25dc8a3d3029b9e4f (2026-09-18). Generated by codevetter codevetter 1.15.0 on 2026-10-02.

Source inspection; upstream code and tests were not executed.

The inspected flows cover startup, a JSON response, signed-cookie output, error handling, and streamed responses. Each follows the implementation from entry boundary through context delegation to the transport. These are source walkthroughs, not executed traces.
- Startup: new Application initializes prototypes/settings → use(fn) appends handlers → listen() calls callback() → callback() composes middleware and installs a default error listener if needed → http.createServer(...).listen(...). — [lib/application.js#L87-115](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L87-L115) [lib/application.js#L128-132](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L128-L132) [lib/application.js#L167-185](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L167-L185)
- Request entry: callback's listener → createContext(req,res) → optional ctxStorage.run(ctx) → handleRequest(), which sets 404 and registers onFinished before invoking middleware. — [lib/application.js#L187-195](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L187-L195) [lib/application.js#L213-243](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L213-L243)
- Middleware cascade: the first handler calls next() → the second runs → the second's continuation runs → the first's continuation runs; the compose test asserts [1,2,3,4] and a final 404 when neither sets a body. — [__tests__/application/compose.test.js#L9-31](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/__tests__/application/compose.test.js#L9-L31) [lib/application.js#L213-219](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L213-L219)
- JSON response: middleware assigns ctx.body → response.body stores the object, defaults status to 200 unless explicit, removes stale length, and chooses JSON type → respond() stringifies and res.end() sends bytes. — [lib/context.js#L205-209](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/context.js#L205-L209) [lib/response.js#L135-137](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L135-L137) [lib/response.js#L167-171](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L167-L171) [lib/response.js#L228-231](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L228-L231) [lib/application.js#L346-351](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L346-L351)
- Signed-cookie output: app.keys is configured → middleware accesses ctx.cookies → the getter constructs Cookies(req,res,{keys,secure}) → cookies.set(...,{signed:true}) emits value and .sig cookies; setting 204 ends without a body. — [lib/context.js#L164-171](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/context.js#L164-L171) [__tests__/context/cookies.test.js#L45-62](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/__tests__/context/cookies.test.js#L45-L62) [lib/application.js#L295-299](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L295-L299)
- Error response: ctx.throw() creates and throws an HTTP error → handleRequest's catch invokes ctx.onerror → the application error event fires → unsent headers are cleared/replaced → validated status and exposed-or-generic text are sent. — [lib/context.js#L95-96](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/context.js#L95-L96) [lib/application.js#L213-219](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L213-L219) [lib/context.js#L120-161](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/context.js#L120-L161) [__tests__/context/onerror.test.js#L10-23](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/__tests__/context/onerror.test.js#L10-L23)
- Stream response: body assignment recognizes a Node stream and registers destruction on response completion → middleware resolves → respond() selects the stream → Stream.pipeline writes to res and forwards errors to ctx.onerror. — [lib/is-stream.js#L7-19](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/is-stream.js#L7-L19) [lib/response.js#L189-198](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L189-L198) [lib/application.js#L334-343](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/application.js#L334-L343)
- Return navigation: ctx.back() delegates to response.back() → Referrer/Referer is read → a URL is resolved against ctx.href → matching hosts redirect to the referrer, otherwise alt or '/' → redirect() sets encoded Location and negotiated text/HTML. — [lib/context.js#L195-204](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/context.js#L195-L204) [lib/request.js#L689-697](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/request.js#L689-L697) [lib/response.js#L338-350](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L338-L350) [lib/response.js#L302-322](https://github.com/koajs/koa/blob/824c1cf8de9a91a2941973b25dc8a3d3029b9e4f/lib/response.js#L302-L322)

Every claim links to its cited files at the pinned commit. Citations identify source evidence; they do not establish runtime correctness. Independent CodeVetter scan — not reviewed by upstream maintainers.

## Public product links

- [CodeVetter](https://codevetter.com/)
- [Download](https://codevetter.com/download)
- [Documentation](https://codevetter.com/docs/)
- [Source](https://github.com/Codevetter/codevetter)