expressjs/express
Glossary
Express has a small, deliberate vocabulary. Many terms below correspond to specific files or settings rather than abstract concepts.
| Term | Meaning |
|---|---|
| app / application | The callable function returned by express(). It is both a Node HTTP request listener and an EventEmitter. Defined in lib/express.js (factory) and lib/application.js (prototype). |
| router | The middleware/route stack. Express delegates to the external router package; express.Router and express.Route are re-exports. |
| route | A binding of an HTTP method + path pattern to one or more handler functions. Created via app.METHOD(path, ...), app.route(path).METHOD(...), or router.route(path). |
| middleware | A function with the signature (req, res, next) (or (err, req, res, next) for error middleware). Mounted via app.use() or router.use(). |
| mount | Attaching a sub-app or router under a path prefix via app.use('/admin', subApp). Triggers the 'mount' event so the sub-app inherits settings. |
| handle | Internal verb for "dispatch this request through the stack". app.handle(req, res, next) is what app(req, res, next) ultimately calls. |
| setting | A key/value entry in app.settings, manipulated via app.set, app.get, app.enable, app.disable. See Reference: configuration. |
| locals | Two distinct objects: app.locals (per-app) and res.locals (per-request). Both are passed into view rendering as template data. |
| engine | A template engine registered via app.engine(ext, fn). Looked up by file extension when rendering. |
| view | An instance of View (lib/view.js) representing a template file. Created by app.render/res.render and cached when view cache is enabled. |
| trust proxy | A setting controlling whether req.ip, req.protocol, req.host, etc. trust X-Forwarded-* headers. Compiled by compileTrust in lib/utils.js. |
| query parser | The function that parses req.url's query string. Either 'simple' (Node's querystring.parse) or 'extended' (qs.parse). Configured via the 'query parser' setting. |
| etag fn | Function that produces an ETag for a response body. 'weak' (default), 'strong', false, or a custom function. |
| finalhandler | The fallback used by app.handle when no middleware ends the response. Sends 404 or formats errors. From the finalhandler package. |
| body-parser | External package re-exported as express.json, express.urlencoded, express.text, express.raw. See Features / Body parsing. |
| serve-static | External package re-exported as express.static. See Features / Static files. |
| send | External package used by res.sendFile to stream files with caching, range, and ETag support. |
| mountpath | The path under which a mounted sub-app was attached. Available as subApp.mountpath. |
| fresh / stale | req.fresh returns true if the cached response (per If-None-Match/If-Modified-Since) is still valid; req.stale is its inverse. Driven by the fresh package. |
| JSONP | "JSON with padding" — res.jsonp wraps a JSON body in a callback function whose name comes from the jsonp callback name setting. |
| View cache | A setting ('view cache') automatically enabled in production. Caches resolved View instances on app.cache. |
| acceptance test | Higher-level Mocha tests under test/acceptance/ that run the bundled examples to verify they boot. |
| TC | Technical Committee — the project's governance body, listed in Readme.md. |
| Triager | A community member with permissions to triage incoming issues. Listed in Readme.md. |
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