> For the complete documentation index, see [llms.txt](https://adam-37.gitbook.io/joomadeung/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://adam-37.gitbook.io/joomadeung/node.js/node.js-express-mysql/next.md).

# next 활용법

* undefined인 asdasdad에서 error를 catch해서 그 다음 미들웨어인 `res.send('에러났습니다!!!!!!!!!!!!!!!!!!!!!!!')` 가 보여진다.

```jsx
app.use(
  (req, res, next) => {
    console.log('1 모든 요청에 실행하고 싶어요');
    next();
  },
  (req, res, next) => {
    try {
      console.log('에러 try catch');
      console.log(asdasdad);
    } catch (err) {
      console.log(err);
      next(err); // next에 인수가 들어가있으면 error처리 미들웨어로 넘어감.
    }
  }
);

// 에러처리 미들웨어
app.use((err, req, res, next) => {
  console.error(err);
  res.send('에러났습니다!!!!!!!!!!!!!!!!!!!!!!!');
});
```

* 중복을 제거할 수 있다.
  * if문 안에 true이면 다음 미들웨어인 `console.log('예, 물론 실행됩니다.')`가 실행되고
  * false라면 바로 아래 미들웨어인 `console.log('실행되나요?')` 가 실행된다. 중복을 줄이기에 용이하다.

```jsx
app.get(
  '/',
  (req, res, next) => {
    res.sendFile(path.join(__dirname, 'index.html'));
    if (true) {
      next('route'); // 예, 물론 실행됩니다.
    } else {
      next(); // 실행되나요?
    }
  },
  (req, res) => {
    console.log('실행되나요?');
  }
);

app.get('/', (req, res) => {
  console.log('예, 물론 실행됩니다.');
});
```

```jsx
app.get(
  '/',
  (req, res, next) => {
    res.sendFile(path.join(__dirname, 'index.html'));
    if (false) {
      next('route'); // 예, 물론 실행됩니다.
    } else {
      next(); // 실행되나요?
    }
  },
  (req, res) => {
    console.log('실행되나요?'); // false일 경우 이 부분이 실행됨
  }
);

app.get('/', (req, res) => {
  console.log('예, 물론 실행됩니다.');
});
```


---

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