Why does this promise silently fail?

Question

db.collection.findOne is an async operation (MongoDB, but that doesn\'t really matter here), which is why I\'m wrapping it in a promise here.

var l         


        

Answer 1

It is not swallowing that error, but if a then or catch handler results in an error, then the current promise will be rejected with that error.

In your case, ReferenceError is thrown, but it rejects the promise. You can see the actual error being propagated, by attaching a catch handler, like this

new Promise(function (resolve, reject) {
    resolve(true);
  })
  .then(function (result) {
    console.log(result, bar);
  })
  .catch(function (er) {
    console.error('Inside Catch', er);
  });

Now you will see

Inside Catch [ReferenceError: bar is not defined]

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Further Reading:

1. Why cannot I throw inside a Promise.catch handler?