How to break out of a promise (.then) statement javascript

Question

I'm having a problem trying to break out of a promise statement when an error occurs in a catch statement.

I'm not sure if I can throw an error inside a catch statement.

The problem: The catch function isn't doing anything when I throw an error.

Expected result: For the catch statement to display an alert and break the promise chain.

The code:

        if (IsEmail(email)) {
        $('body').loadingModal({
              position: 'auto',
              text: 'Signing you in, please wait...',
              color: '#fff',
              opacity: '0.9',
              backgroundColor: 'rgb(0,0,0)',
              animation: 'doubleBounce'
        });

        var delay = function(ms){ return new Promise(function(r) { setTimeout(r, ms) }) };
        var time = 2000;

        delay(time)
        .then(function() { $('body').loadingModal('animation', 'foldingCube'); return delay(time); } )
        .then(function() { 
            firebase.auth().signInWithEmailAndPassword(email, password)
            .then(function () {
                var user = firebase.auth().currentUser;
                uid = user.uid;
                configure();
            })
            .catch(function(error) {
                throw error;
            });
        })
        .then(function() { $('body').loadingModal('color', 'white').loadingModal('text', 'Welcome to Dtt deliveries').loadingModal('backgroundColor', 'orange');  return delay(time); } )
        .then(function() { $('body').loadingModal('hide'); return delay(time); } )
        .then(function() { $('body').loadingModal('destroy') ;} )
        .catch(function(error) {
            alert("Database error: " + error);
        }); 
    }
    else {
        alert("Please enter a valid email");
        return;
    }

CertainPerformance · Accepted Answer · 2019-06-30T23:38:51.830

The second .then after the delay resolves immediately, because nothing is being returned from it. Return the signInWithEmailAndPassword call instead, because it returns a Promise that you need to chain together with the outer Promise chain:

.then(function() {
  return firebase.auth().signInWithEmailAndPassword(email, password)
  // ...

Also, catching and immediately throwing doesn't really do anything - unless you need to handle an error particular to signInWithEmailAndPassword there, feel free to omit that catch entirely:

delay(time)
  .then(function() { $('body').loadingModal('animation', 'foldingCube'); return delay(time); } )
  .then(function() { 
    return firebase.auth().signInWithEmailAndPassword(email, password)
  })
  .then(function () {
    var user = firebase.auth().currentUser;
    uid = user.uid;
    configure(); // if configure returns a Promise, return this call from the `.then`
  })
  .then(
  // ...
  .catch(function(error) {
    alert("Database error: " + error);
  });

If configure returns a Promise as well, then you need to return it too. (if it's synchronous, there's no need)

(you might also consider using a more user-friendly way of displaying the error, perhaps use a proper modal instead of alert)

Another option to consider is using await instead of all these .thens, the control flow may be clearer:

(async () => {
  if (!IsEmail(email)) {
    alert("Please enter a valid email");
    return;
  }
  $('body').loadingModal({
    position: 'auto',
    text: 'Signing you in, please wait...',
    color: '#fff',
    opacity: '0.9',
    backgroundColor: 'rgb(0,0,0)',
    animation: 'doubleBounce'
  });

  var delay = function(ms) {
    return new Promise(function(r) {
      setTimeout(r, ms)
    })
  };
  var time = 2000;

  try {
    await delay(time);
    $('body').loadingModal('animation', 'foldingCube');
    await delay(time);
    await firebase.auth().signInWithEmailAndPassword(email, password)
    var user = firebase.auth().currentUser;
    uid = user.uid;
    configure(); // if this returns a Promise, `await` it
    $('body').loadingModal('color', 'white').loadingModal('text', 'Welcome to Dtt deliveries').loadingModal('backgroundColor', 'orange');
    await delay(time);
    $('body').loadingModal('hide');
    await delay(time);
    $('body').loadingModal('destroy');
  } catch(error) {
    alert("Database error: " + error);
  }
})();

How to break out of a promise (.then) statement javascript

1 Answers1