Systems

The Express core is six files. Each one carries a tightly-scoped responsibility, and together they make up the entire express runtime. This section walks through each file as its own subsystem.

File	Page	What lives there
lib/express.js	Factory	The createApplication() factory, default exports, re-exports
lib/application.js	Application	The app prototype: settings, mount, handle, use, listen, render
lib/request.js	Request	The req prototype: accepts*, is, ip, host, query, fresh, ...
lib/response.js	Response	The res prototype: send, json, sendFile, redirect, cookie, render
lib/view.js	View	Filesystem view lookup and engine dispatch
lib/utils.js	Utils	ETag, query parser, trust proxy, content-type helpers

How they connect

graph TD
    Factory[lib/express.js] -->|exposes proto| App[lib/application.js]
    Factory -->|exposes proto| Req[lib/request.js]
    Factory -->|exposes proto| Res[lib/response.js]
    App -->|new View(name)| View[lib/view.js]
    App -->|compileETag, compileTrust, compileQueryParser| Utils[lib/utils.js]
    Res -->|setCharset, normalizeType, normalizeTypes| Utils
    App -->|delegates routing| RouterPkg[router npm package]

lib/express.js is the only file that is directly required by user code. Everything else is reached through it.

Reading order for new contributors

1. Factory — 81 lines, gives you the whole shape.
2. Application — the biggest and most important file in core.
3. Response — the largest by LOC and where most user-visible behaviour lives.
4. Request — short, all helpers, no surprises.
5. View — quick read, useful for understanding template engines.
6. Utils — read last; you'll already have seen each helper called from the others.

Related pages

• Architecture — high-level view of how systems and external packages interact.
• Features — capabilities that span multiple systems.