Calling an async middleware inside of another async middleware in Express.js?

Question

Is it possible to call an async middleware inside of another async middleware in Express.js?

Whenever I try to do so, it doesn't execute in the order I would like it to.

I want them to execute in order and have an output of First, Second, Third.

Could anyone explain why or how I can achieve what I'm trying to?

const first = async (req, res, next) => {  
  console.log('First');
  next()
}

const second = async (req, res, next) => {
  await first(req, res, next);
  console.log('Second');
  next();
}

router.get('/logs', second, async (req, res) => {
  console.log('Third');
  res.send('Done');
});

// Console Output:
// First
// Third
// Second

If I don't execute first() inside of second() it works just fine, but for something I'm doing I want to be able to execute the middleware inside of the other.

const first = async (req, res, next) => {  
  console.log('First');
  next()
}

const second = async (req, res, next) => {
  console.log('Second');
  next();
}

router.get('/logs', first, second, async (req, res) => {
  console.log('Third');
  res.send('Done');
});

// Console Output:
// First
// Second
// Third

Konrad · Accepted Answer · 2022-12-07 22:21:56Z

2

You passed next to the first function, which will call the route handler, instead of second code

You have to pass a new callback

const first = async (req, res, next) => {  
  console.log('First');
  next()
}

const second = async (req, res, next) => {
  await first(req, res, () => {
    console.log('Second');
    next();
  });
}

router.get('/logs', second, async (req, res) => {
  console.log('Third');
  res.send('Done');
});

or

const first = async (req, res, next) => {  
  console.log('First');
  next()
}

const second = async (req, res, next) => {
  await new Promise(r => first(req, res, r))
  console.log('Second');
  next();
}

router.get('/logs', second, async (req, res) => {
  console.log('Third');
  res.send('Done');
});

answered Dec 7, 2022 at 22:21

Konrad

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1 Comment

Adrian D. Over a year ago

Oh wow, it worked! Thank you, you are incredible. Are you able to go into a bit of a more in-depth explanation as to why you wouldn't pass in next to the middleware and add it to your answer? Also, am I able to use async for that callback function replacing the next?

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