请求 - 响应周期是怎样的 · 八股题库 / Drills

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一句话：客户端发请求 → 经过一串中间件 → 匹配到路由 → 处理逻辑（查库等）→发出一个响应→ 结束。

Express 里具体经过什么：

解析——express.json()把请求体解析成 req.body（不加这个中间件req.body 就是undefined，这是最常见的新手问题）
通用中间件—— CORS、日志、关联 id、限流
认证 / 鉴权—— 验 token，把用户挂到req.user
路由匹配—— 按注册顺序找第一个匹配的 method + path
校验 → 业务逻辑 → 查库
响应——res.status(200).json(...)
兜底—— 404 处理 + 错误处理中间件

三个必须知道的规则：

中间件按注册顺序执行，必须调 next()才会往下走 —— 忘了调请求就永远挂着，客户端一直转圈。
一个请求只能响应一次。重复 res.send会报Cannot set headers after they are sent。所以调用响应之后一定要return。
错误处理中间件有四个参数(err, req, res, next)——少一个参数 Express 就认不出它是错误处理器，这个坑很经典。

会追问：「异步函数里抛的错误 Express 会自动捕获吗？」——Express 4 里不会，会变成未处理的 rejection，请求挂死。要么自己 try/catch后 next(err)，要么包一层 asyncHandler。Express 5 才支持自动转发。

In one line: the client sends a request, it walks through a chain of middleware, a route matches, the handler does its work (hits the database and so on), one response goes out, done.

What it actually passes through in Express:

Parsing — express.json() turns the body into req.body (without that middleware req.body is undefined, which is the single most common beginner bug)
Generic middleware — CORS, logging, a correlation id, rate limiting
Authentication and authorisation — verify the token, hang the user off req.user
Route matching — the first registered method + path that matches wins
Validation, then business logic, then the database
Response — res.status(200).json(...)
Catch-alls — the 404 handler and the error-handling middleware

Three rules you have to know:

Middleware runs in registration order and you must call next() for the chain to continue. Forget it and the request hangs forever while the client spins.
One request gets exactly one response. A second res.send throws Cannot set headers after they are sent. So always return after you respond.
Error middleware takes four arguments, (err, req, res, next) — drop one and Express no longer recognises it as an error handler. A classic trap.

Follow-up: “Does Express catch errors thrown inside an async function?” — not in Express 4. It becomes an unhandled rejection and the request hangs. Either try/catch yourself and call next(err), or wrap the handler in an asyncHandler. Express 5 forwards them for you.

JavaScript一个请求经过的全部环节示意Illustrative

1app.use(express.json()); // 1 解析 body

2app.use(cors()); // 2 通用

3app.use(correlationId); // 关联 id，方便串联日志

4app.use(auth); // 3 认证

6app.get("/orders/:id", async (req, res, next) => {

7 try {

8 const order = await db.find(req.params.id);

9 if (!order) return res.status(404).json({ error: "not found" }); // 记得 return

10 res.json(order);

11 } catch (e) {

12 next(e); // Express 4 不会自动接异步错误

13 }

14});

16app.use((req, res) => res.status(404).json({ error: "no route" }));

18// 错误处理中间件：必须是四个参数，少一个就不生效

19app.use((err, req, res, next) => {

20 console.error(err);

21 res.status(500).json({ error: "internal" });

22});

1app.use(express.json()); // 1 parse the body

2app.use(cors()); // 2 general

3app.use(correlationId); // a correlation id, so logs can be tied together

4app.use(auth); // 3 authentication