EXPRESS req — the features you’ll use 80% of the time

Property	Description
`req.body`	Parsed body (JSON, form-data, etc.)
`req.params`	URL parameters: `/users/:id` → `req.params.id`
`req.query`	Query params: `?page=2&limit=10` → `req.query.page`
`req.headers`	All request headers, lowercase keys: `req.headers['authorization']`
`req.method`	`"GET"`, `"POST"`, `"PUT"`, etc.
`req.originalUrl`	Full URL that hit the server
`req.path`	Route path without query string
`req.ip`	Client IP
`req.cookies`	User cookies (if cookie-parser)

Compulsory Request Headers (client → server)

These are headers almost EVERY request sends.

Header	Description	Example
`Host`	Domain being requested (required by HTTP/1.1)	`Host: api.example.com`
`User-Agent`	Identifies browser/app (Chrome, Safari, curl, etc.)	`User-Agent: Mozilla/5.0 (Windows NT 10.0)`
`Accept`	What response format client accepts	`Accept: application/json`
`Connection`	`keep-alive` or `close`	`Connection: keep-alive`
`Accept-Encoding`	`gzip`, `br`, `deflate` (compression)	`Accept-Encoding: gzip, deflate, br`
`Content-Length`	Size of body (for POST/PUT)	`Content-Length: 256`
`Content-Type`	Indicates JSON, form-data, etc. (for POST/PUT)	`Content-Type: application/json`

Browser-only compulsory headers

Header	Description
`Origin`	Sent automatically by browsers (for cross-origin requests)
`Referer`	Previous page or request source
`Sec-Fetch-*`	Browser security metadata

5. Useful Optional Request Headers (client → backend)

These are the headers you’ll use often in real apps.

Header	Description
`Authorization`	JWTs, API keys, tokens: `Authorization: Bearer <token>`
`Cookie`	Session cookies
`X-Requested-With`	Identifies AJAX requests
`X-Real-IP` / `X-Forwarded-For`	Used by proxies to identify real client IP
`Range`	Used for partial downloads/streaming

🎯 These headers are used in authentication, proxies, logs, file downloads.