UnhandledPromiseRejection undefined problem

Question

I hope you can help me. I am developing a functionality that reads a series of data (data is taked from csv file) and checks if it has to put it in a list or not. The problem comes when I start to check the data (through promises) since it gives me an error telling me that the rejected promise has not been captured. You will need to use the following:

//  -npm install email-existence
const emailExistence = require("email-existence");

The code:

function checkEmailExistencePromise(element) {
  return new Promise((resolve, reject) => {
    emailExistence.check(element.email, (error, response) => {
      if (error) {
        reject(error);
        return;
      }
      // If the person has false email the promise will be save (as object) with "Exists" attribute in false.
      if (!response) {
        resolve({
          name: element.name,
          phone: element.phone,
          email: element.email,
          document: element.document,
          weight: element.weight,
          tags: element.tags,
          exists: false,
        });
        return;
      }
      // If the person has valid email the promise will be save (as object) with "Exists" attribute in true.
      resolve({
        name: element.name,
        phone: element.phone,
        email: element.email,
        document: element.document,
        weight: element.weight,
        tags: element.tags,
        exists: true,
      });
    });
  }).catch(() => {
    throw console.error();
  });
}

// I call the function that will write the CSV file with valid email records.
checkEmails();

// This function collects the promises of the "checkEmailExistencePromise" function and dumps them into an array.
async function checkEmails() {
  const promises = sinRepetidos.map((element) =>
    checkEmailExistencePromise(element)
  );

  const values = await Promise.all(promises);

  // Here we go through the promises (which are also objects) and those with the true attribute I put them in a new array that will be printed later.
  values.forEach((element) => {
    if (element.exists === true) {
      checked.push(element);
    }
  });

`throw console.error();` is very likely problematic... you handle the promise, but rethrow `undefined` (which makes no sense), so you'll anyway end up with a rejected promise (`console.error`-ing also won't do anything unless you pass the error to it). However, it might be better to let errors propagate to the parent by not handling them, and let the caller (i.e. `checkEmails`) handle them. — FZs, Apr 29 '21 at 07:28

divillysausages · Accepted Answer · 2021-04-29T16:08:19.767

Because checkEmailExistencePromise() can throw an error (both through the reject() and the throw call), you need to wrap your

const values = await Promise.all(promises);

call in checkEmails() in a try..catch as well, like so

let values = null;
try {
    values = await Promise.all(promises)
} catch (e) {
    console.error(e) 
}
// do something with values, if it's not null

Edit

As you most likely don't want checkEmailExistencePromise to throw an error, you can replace it with this:

function checkEmailExistencePromise(element) {
  // NOTE: we're making is so that this promise never rejects - if there's
  // an error in there, we'll assume that the email isn't valid
  return new Promise(resolve => {
    emailExistence.check(element.email, (error, response) => {
      let exists = false;
      if (error) {
        // we can log the error, to make sure we're not doing anything wrong
        // that needs to be fixed - some errors can be legit, though
        console.error(error);
      }
      
      // NOTE: we should probably actually check the response
      if(response) {
        exists = true;
      }
      
      resolve({
        name: element.name,
        phone: element.phone,
        email: element.email,
        document: element.document,
        weight: element.weight,
        tags: element.tags,
        exists
      })
    });
  })
}

We take any error to mean that the email isn't valid.

Also, if element only contains those 6 properties (name, phone, email...), then you can simplify the resolve further with something like this:

resolve(Object.assign({},element,{exists}))

This will make a shallow clone of the object and add the exists property to it

i did it but i still have the next error: UnhandledPromiseRejectionWarning: Error: queryMx ESERVFAIL hotmail.com — garikoitzz, Apr 29 '21 at 09:40
As in you see the error (because there's a `console.error()` call in there to print it out), or because your program is still crashing? — divillysausages, Apr 29 '21 at 09:44
For example, if `emailExistence.check` throws an error, do you still want a response back? Right now, because you're using `Promise.all()`, if one promise errors out, the whole thing fails — divillysausages, Apr 29 '21 at 09:45
I get an error the try catch of "checkEmails". I want the program to continue @divillysausages — garikoitzz, Apr 29 '21 at 10:16
@garikoitzz I've added an edit with what I think you're looking for — divillysausages, Apr 29 '21 at 16:08

UnhandledPromiseRejection undefined problem

1 Answers1