Should an async API ever throw synchronously?

Question

I'm writing a JavaScript function that makes an HTTP request and returns a promise for the result (but this question applies equally for a callback-based implementation).

If I know immediately that the arguments supplied for the function are invalid, should the function throw synchronously, or should it return a rejected promise (or, if you prefer, invoke callback with an Error instance)?

How important is it that an async function should always behave in an async manner, particularly for error conditions? Is it OK to throw if you know that the program is not in a suitable state for the async operation to proceed?

e.g:

function getUserById(userId, cb) {
  if (userId !== parseInt(userId)) {
    throw new Error('userId is not valid')
  }

  // make async call
}

// OR...

function getUserById(userId, cb) {
  if (userId !== parseInt(userId)) {
    return cb(new Error('userId is not valid'))
  }

  // make async call
}

Answer 1

Ultimately the decision to synchronously throw or not is up to you, and you will likely find people who argue either side. The important thing is to document the behavior and maintain consistency in the behavior.

My opinion on the matter is that your second option - passing the error into the callback - seems more elegant. Otherwise you end up with code that looks like this:

try {
    getUserById(7, function (response) {
       if (response.isSuccess) {
           //Success case
       } else {
           //Failure case
       }
    });
} catch (error) {
    //Other failure case
}

The control flow here is slightly confusing.

It seems like it would be better to have a single if / else if / else structure in the callback and forgo the surrounding try / catch.