What steps should I take to address the ES6 promise chain issue within Express?

Question

What steps should I take to address the ES6 promise chain issue within Express?

Currently, I am delving into Promise chaining and experimenting with some code involving express and mongoose in a node environment. Below is the snippet of code that I am working on:

const register = (req, res, next) => {
  User.findOne(req.params.email).exec().then(user => {
    if (user) {
      return res.status(409).json({ error: 409 });
    }
    return User.create(req.body);
  }).then(user => {
    const token = jwt.sign({ sub: user._id }, 'test-secret');
    return res.status(200).json({ token });
  }).catch(err => {
    return next(err);
  });
};

This piece of code simplifies the process of registering a user and providing them with a token. My objective is to first check if the user already exists, and if not, proceed with the registration.

Line 6 seems incorrect as it does not appear to be returning any Promise, causing the code execution to continue after line 4. I aim to steer clear of callback hell, so I am seeking guidance on how to achieve this. Thank you!

javascript express mongoose ecmascript-6 promise

Answer 1

Answer №1

It's not crucial that you're not returning a Promise. Both the return values from then() and catch() are automatically enclosed in Promise objects.

The issue lies in the linear flow of your code, where you attempt to create branching paths for specific scenarios. You actually require two distinct execution paths:

const register = (req, res, next) => {
  User.findOne(req.params.email).exec().then(user => {
    if (user) {
      // First path leads here:
      return res.status(409).json({ error: 409 });

    } else {
      // Second path continues this Promise chain:
      return User.create(req.body).then(user => {
        const token = jwt.sign({ sub: user._id }, 'test-secret');
        return res.status(200).json({ token });
      })
    }

  }).catch(err => {
    // Both paths converge on this error handler
    return next(err);
  });
};

To avoid excessive nesting, you have a couple of alternatives:

Encapsulate the different paths into functions, enabling you to simplify your main logic like so:
```
if (user) {
  return sendHttpError(res, 409)
} else {
  return sendNewUser(res, req.body)
}
```

You might find that only utilizing the sendHttpError function suffices, while the other branch can be incorporated directly within your route. This approach is quite standard.

Given that one branch handles error scenarios exclusively, consider throwing an Error and catching it below or through an error-handling middleware. This method could look like the following:

if (user) {
  // This throw will halt all subsequent executions. With a custom Error class, you can appropriately manage it via a catch() handler or Express middleware:
  throw new APIError(409)
}

// ... proceed with creating the user as usual

This practice is commonly employed.

Answer 2

It's not crucial that you're not returning a Promise. Both the return values from then() and catch() are automatically enclosed in Promise objects.

The issue lies in the linear flow of your code, where you attempt to create branching paths for specific scenarios. You actually require two distinct execution paths:

const register = (req, res, next) => {
  User.findOne(req.params.email).exec().then(user => {
    if (user) {
      // First path leads here:
      return res.status(409).json({ error: 409 });

    } else {
      // Second path continues this Promise chain:
      return User.create(req.body).then(user => {
        const token = jwt.sign({ sub: user._id }, 'test-secret');
        return res.status(200).json({ token });
      })
    }

  }).catch(err => {
    // Both paths converge on this error handler
    return next(err);
  });
};

To avoid excessive nesting, you have a couple of alternatives:

Encapsulate the different paths into functions, enabling you to simplify your main logic like so:
```
if (user) {
  return sendHttpError(res, 409)
} else {
  return sendNewUser(res, req.body)
}
```

You might find that only utilizing the sendHttpError function suffices, while the other branch can be incorporated directly within your route. This approach is quite standard.

Given that one branch handles error scenarios exclusively, consider throwing an Error and catching it below or through an error-handling middleware. This method could look like the following:

if (user) {
  // This throw will halt all subsequent executions. With a custom Error class, you can appropriately manage it via a catch() handler or Express middleware:
  throw new APIError(409)
}

// ... proceed with creating the user as usual

This practice is commonly employed.

Answer 3

Answer №2

To streamline the process, simply trigger an error if the user is already in existence and proceed directly to the catch function.

const register = (req, res, next) => {
  User.findOne(req.params.email).exec().then(user => {
    if (user) {
      throw new Error({ error: 409, msg: 'User already exists' });
    }
    return User.create(req.body);
  }).then(user => {
    const token = jwt.sign({ sub: user._id }, 'test-secret');
    return res.status(200).json({ token });
  }).catch(err => {
    return next(err);
  });
};

Answer 4

To streamline the process, simply trigger an error if the user is already in existence and proceed directly to the catch function.

const register = (req, res, next) => {
  User.findOne(req.params.email).exec().then(user => {
    if (user) {
      throw new Error({ error: 409, msg: 'User already exists' });
    }
    return User.create(req.body);
  }).then(user => {
    const token = jwt.sign({ sub: user._id }, 'test-secret');
    return res.status(200).json({ token });
  }).catch(err => {
    return next(err);
  });
};

What steps should I take to address the ES6 promise chain issue within Express?

Answer №1

Answer №2

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