Express JS: Issue with chain promises halting at return statement in JavaScript

Question

Express JS: Issue with chain promises halting at return statement in JavaScript

In my express app, I have a utility function that controls the promise then chain based on a specific condition. However, when I attempt to end the chain using a return statement, Express throws an error stating that headers cannot be set after they are sent. Here is the code snippet:

function submit_reset_password_request(request, response) {
  if (request.session.id) {
    return response.json({ error: true, message: 'password reset cannot be requested during an active session' });
  }

  let { email } = request.body;
  let user, reset_request;
  if (email) {
    email = email.toLowerCase().trim();
  }
  if (!email) {
    return response.json({ error: true, message: 'input is required' });
  }

  models.Users.findOne({ where: { email } })
  .then(user_result => {
    if (!user_result) {
      return response.json({ error: true, message: 'No account found with that email' });
    }
    user = user_result.dataValues;
    return models.ResetPasswordRequests.findOne({ where: { user_email: user.email } })
  })
  .then(request_result => {
    if (request_result) {
      return response.json({ error: true, message: 'A password reset has already been requested for this email' });
    }
    return models.ResetPasswordRequests.create({ user_email: user.email })
  })
  .then(new_reset_request => {
    reset_request = new_reset_request.dataValues;

    // send reset request email
    let host = request.get('host');
    let link = host.endsWith('/') ? (host + 'search') : (host + '/search');
    let email_subject = 'Epsity - Password reset requested';
    let email_html = templateEngine.PasswordReset_EMAIL({ user, reset_request, link });
    return sendgrid_manager.send_email(null, request.session.you.email, email_subject, email_html);
  })
  .then(email_result => {
    return response.json({ success: true, message: 'A password reset request has been sent to the provided email!' });
  })
  .catch(error => {
    console.log(error);
    return response.json({ error, message: 'Could not submit reset password request...' });
  });
}

Despite the if statements intended to stop the chain and send a response, it continues executing regardless (although the response can still be seen on the client side). The cause of this behavior is unclear.

Below is the error message received:

Unhandled rejection Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:494:11)
    at ServerResponse.setHeader (_http_outgoing.js:501:3)
    at ServerResponse.header (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\express\lib\response.js:767:10)
    at ServerResponse.send (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\express\lib\response.js:170:12)
    at ServerResponse.json (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\express\lib\response.js:267:15)
    at models.Users.findOne.then.then.then.then.catch.error (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\server\routers\main\methods\post.js:221:21)
    at tryCatcher (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\util.js:16:23)
    at Promise._settlePromiseFromHandler (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\promise.js:512:31)
    at Promise._settlePromise (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\promise.js:569:18)
    at Promise._settlePromise0 (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\promise.js:614:10)
    at Promise._settlePromises (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\promise.js:690:18)
    at _drainQueueStep (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\async.js:138:12)
    at _drainQueue (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebluebirdird/js/release/async.js:131:9)
    at Async._drainQueues (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\async.js:147:5)
    at Immediate.Async.drainQueues (C:\Users\Waite-Ryan-M\Desktop\_my-apps\rmw-epsity\node_modules\bluebird\js\release\async.js:17:14)
    at runCallback (timers.js:789:20)
    at tryOnImmediate (timers.js:751:5)
    at processImmediate [as _immediateCallback] (timers.js:722:5)

I might consider implementing the async/await syntax as a potential solution.

javascript express sequelize.js

Answer 1

Answer №1

One issue arises when you find yourself within a .then block nested inside a chain of Promises, as the return value is not always of concern to the Promise itself. Consider this snippet:

.then(user_result => {
  if(!user_result) {
    return response.json({ error: true, message: 'No account found by that email' });
  }
  user = user_result.dataValues;
  return models.ResetPasswordRequests.findOne({ where: { user_email: user.email } })
})

The interpreter treats both the return response.json and return models.Reset... in the same way - passing them along to the succeeding .then without breaking the chain. To address this, you can either extract the functions into named functions for better control flow (albeit more verbose), or utilize async/await, which enables a similar behavior with explicit termination upon return and handling asynchronous calls via await.

A refactor example using async/await:

async function submit_reset_password_request (request, response) {
  if(request.session.id) {
    return response.json({ error: true, message: 'password reset cannot be requested during an active session' });
  }
  let { email } = request.body;
  if(email) {
    email = email.toLowerCase().trim();
  }
  if(!email) {
    return response.json({ error: true, message: 'input is required' });
  }

  try {
    const user_result = await models.Users.findOne({ where: { email } });
    if(!user_result) {
      return response.json({ error: true, message: 'No account found by that email' });
    }
    const user = user_result.dataValues;
    const request_result = await models.ResetPasswordRequests.findOne({ where: { user_email: user.email } });
    if(request_result) {
      return response.json({ error: true, message: 'A password reset has already been requested for this email' });
    }
    const new_reset_request = await models.ResetPasswordRequests.create({ user_email: user.email });
    const reset_request = new_reset_request.dataValues;

    // send reset request email
    const host = request.get('host');
    const link = host.endsWith('/') ? (host + 'search') : (host + '/search');
    const email_subject = 'Epsity - Password reset requested';
    const email_html = templateEngine.PasswordReset_EMAIL({ user, reset_request, link });
    const email_result = await sendgrid_manager.send_email(null, request.session.you.email, email_subject, email_html);
    return response.json({ success: true, message: 'A password reset request has been sent to the provided email!' });
  } catch(error) {
    console.log(error);
    return response.json({ error, message: 'Could not submit reset password request...' });
  }
}

Answer 2

One issue arises when you find yourself within a .then block nested inside a chain of Promises, as the return value is not always of concern to the Promise itself. Consider this snippet:

.then(user_result => {
  if(!user_result) {
    return response.json({ error: true, message: 'No account found by that email' });
  }
  user = user_result.dataValues;
  return models.ResetPasswordRequests.findOne({ where: { user_email: user.email } })
})

The interpreter treats both the return response.json and return models.Reset... in the same way - passing them along to the succeeding .then without breaking the chain. To address this, you can either extract the functions into named functions for better control flow (albeit more verbose), or utilize async/await, which enables a similar behavior with explicit termination upon return and handling asynchronous calls via await.

A refactor example using async/await:

async function submit_reset_password_request (request, response) {
  if(request.session.id) {
    return response.json({ error: true, message: 'password reset cannot be requested during an active session' });
  }
  let { email } = request.body;
  if(email) {
    email = email.toLowerCase().trim();
  }
  if(!email) {
    return response.json({ error: true, message: 'input is required' });
  }

  try {
    const user_result = await models.Users.findOne({ where: { email } });
    if(!user_result) {
      return response.json({ error: true, message: 'No account found by that email' });
    }
    const user = user_result.dataValues;
    const request_result = await models.ResetPasswordRequests.findOne({ where: { user_email: user.email } });
    if(request_result) {
      return response.json({ error: true, message: 'A password reset has already been requested for this email' });
    }
    const new_reset_request = await models.ResetPasswordRequests.create({ user_email: user.email });
    const reset_request = new_reset_request.dataValues;

    // send reset request email
    const host = request.get('host');
    const link = host.endsWith('/') ? (host + 'search') : (host + '/search');
    const email_subject = 'Epsity - Password reset requested';
    const email_html = templateEngine.PasswordReset_EMAIL({ user, reset_request, link });
    const email_result = await sendgrid_manager.send_email(null, request.session.you.email, email_subject, email_html);
    return response.json({ success: true, message: 'A password reset request has been sent to the provided email!' });
  } catch(error) {
    console.log(error);
    return response.json({ error, message: 'Could not submit reset password request...' });
  }
}

Express JS: Issue with chain promises halting at return statement in JavaScript

Answer №1

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