Should I throw an error or return a rejected promise inside an async function? [duplicate]

Answer 1

They are correct.

The call to myCustomFunction assumes that a promise is returned at all times (.then and .catch deal with resolved and rejected promises, respectively). When you throw an error, the function doesn't return a promise.

You could use this to catch the error:

try {
  myModule.myCustomFunction(someInput).then(result => {
    // carry on
  })
  .catch(err => {
    // do something with the error
  })
} catch(err) {
  ...
}

But as you can see, this results in two error handlers: try/catch for the synchronously thrown error, and .catch for any rejected promises that sns.createTopic(someParams) may return.

That's why it's better to use Promise.reject():

module.exports.myCustomFunction = input => {

    if (badInput) {
        return Promise.reject('failed');
    }

    return sns.createTopic(someParams).promise()
}

Then, the .catch will catch both types of errors/rejections.

NB: for newer versions of Node.js (v7.6 and up, I believe), the following will also work:

module.exports.myCustomFunction = async input => {

    if (badInput) {
        throw new Error('failed');
    }

    return sns.createTopic(someParams).promise()
}

The key here is the async keyword. By using this keyword, the function results are wrapped by a promise automatically (similar to what @peteb's answer is showing).