Promise doesn't reject when throwing an error

Question

According to this Stack Overflow answer here, there is no difference between using reject and throw within a promise to trigger a rejection.

However, I have found that, depending on the structure of an external module, throwing an error doesn't always trigger the catch block.

This is my code in index.js:

try {
  const service = require('./services/patreon.js');
  await service.start();
} catch(err) {
  console.log('Caught!');
}

And here is my code in patreon.js:

function start() {
  return new Promise(async (resolve, reject) => {
    throw new Error('Oh no!');
  });
}

module.exports = {
  start: start
}

My expected behavior is that my try/catch block would handle the thrown error and I would see "Caught!" in my console ouput. Instead, my try/catch does not handle the error and I end up with an unhandledRejection.

Interestingly, if I re-structure my module.exports in patreon.js I can fix this and my try/catch block will handle the error:

Update index.js:

try {
  const service = require('./services/patreon.js');
  await service();
} catch(err) {
  console.log('Caught!');
}

Update patreon.js:

module.exports = () => {
  return new Promise(async (resolve, reject) => {
    throw new Error('Oh no!');
  });
}

With the above updates, my try/catch works perfectly but I no longer have the ability to export more than just a function.

The other solution is to simply reject() the promise instead of throw - and this works fine in both examples but for personal preference and code cleanliness I would rather be able to throw.

So my question is, why can't I use throw when my module is structured in the way shown in my first example? In both cases, I end up with the exact same thing: a function that returns a promise.

Promise doesn't reject when throwing an error

0 Answers0