How to reject in async/await syntax?

Question

How can I reject a promise that returned by an async/await function?

e.g. Originally:

Answer 1

I know this is an old question, but I just stumbled across the thread and there seems to be a conflation here between errors and rejection that runs afoul (in many cases, at least) of the oft-repeated advice not to use exception handling to deal with anticipated cases. To illustrate: if an async method is trying to authenticate a user and the authentication fails, that's a rejection (one of two anticipated cases) and not an error (e.g., if the authentication API was unavailable.)

To make sure I wasn't just splitting hairs, I ran a performance test of three different approaches to that, using this code:

const iterations = 100000;

function getSwitch() {
  return Math.round(Math.random()) === 1;
}

function doSomething(value) {
  return 'something done to ' + value.toString();
}

let processWithThrow = function () {
  if (getSwitch()) {
    throw new Error('foo');
  }
};

let processWithReturn = function () {
  if (getSwitch()) {
    return new Error('bar');
  } else {
    return {}
  }
};

let processWithCustomObject = function () {
  if (getSwitch()) {
    return {type: 'rejection', message: 'quux'};
  } else {
    return {type: 'usable response', value: 'fnord'};
  }
};

function testTryCatch(limit) {
  for (let i = 0; i < limit; i++) {
    try {
      processWithThrow();
    } catch (e) {
      const dummyValue = doSomething(e);
    }
  }
}

function testReturnError(limit) {
  for (let i = 0; i < limit; i++) {
    const returnValue = processWithReturn();
    if (returnValue instanceof Error) {
      const dummyValue = doSomething(returnValue);
    }
  }
}

function testCustomObject(limit) {
  for (let i = 0; i < limit; i++) {
    const returnValue = processWithCustomObject();
    if (returnValue.type === 'rejection') {
      const dummyValue = doSomething(returnValue);
    }
  }
}

let start, end;
start = new Date();
testTryCatch(iterations);
end = new Date();
const interval_1 = end - start;
start = new Date();
testReturnError(iterations);
end = new Date();
const interval_2 = end - start;
start = new Date();
testCustomObject(iterations);
end = new Date();
const interval_3 = end - start;

console.log(`with try/catch: ${interval_1}ms; with returned Error: ${interval_2}ms; with custom object: ${interval_3}ms`);

Some of the stuff that's in there is included because of my uncertainty regarding the Javascript interpreter (I only like to go down one rabbit hole at a time); for instance, I included the doSomething function and assigned its return to dummyValue to ensure that the conditional blocks wouldn't get optimized out.

My results were:

with try/catch: 507ms; with returned Error: 260ms; with custom object: 5ms

I know that there are plenty of cases where it's not worth the trouble to hunt down small optimizations, but in larger-scale systems these things can make a big cumulative difference, and that's a pretty stark comparison.

SO… while I think the accepted answer's approach is sound in cases where you're expecting to have to handle unpredictable errors within an async function, in cases where a rejection simply means "you're going to have to go with Plan B (or C, or D…)" I think my preference would be to reject using a custom response object.