Be Careful with Async Functions that Return Booleans

Here’s a fun bug I recently encountered… Let’s say we have this async JavaScript function:

const isBroken = async () => {
  return false;
}

if (isBroken()) {
  throw new Error("Oopsie woopsie");
}

I’ve kept the code simple for this post; in reality, you’ll probably fetch data asynchronously in the function (otherwise, there’s no need to mark it as async):

const isBroken = async () => {
  const x = await y();
  // ...
  // ... more stuff here maybe
  // ...
  return x === 42;
}

if (isBroken()) {
  throw new Error("Oopsie woopsie");
}

Either way, does the body of the if statement execute? Take a second to think this through. You can check if you’re right by copying the first code sample and executing it in your dev tools.

(You can probably already guess the answer, though, just based on the fact that I’m writing this post.)

Async Functions, Booleans, and Truthiness: A Slippery Bug

The if statement in this code will always execute its body, regardless of what the async function returns. ESLint won’t flag this as an error. TypeScript technically should, but it has not yet implemented this warning for un-awaited async functions.

To understand why the expression always evaluates to true, recall that async/await is just syntactic sugar for Promises. In JavaScript, an async function actually wraps its return value in a Promise object—even if it seems like the function is directly returning a value, and even if the function does not await anything.

We can verify this by logging the function call:

> console.log(isBroken())
Promise {<fulfilled>: false}

Our async function’s return value is not false itself but rather a Promise object that resolved with the value false. In other words, it’s the same as doing this:

const isBroken = () => {
  return Promise.resolve(false);
}

if (isBroken()) {
  throw new Error("Oopsie woopsie");
}

Spot the problem? This is an issue of truthiness: a Promise object is not one of the eight falsy values in JavaScript, so checking its truthiness is pointless: When coerced to a boolean, a Promise is always true.

For the code above to work as intended, you’ll need to await the result in another async function (or, equivalently, chain a .then call on the returned Promise object):

const isBroken = async () => {
  return false;
}

const foo = async () => {
  const somethingIsWrong = await isBroken();

  if (somethingIsWrong) {
    throw new Error("Oopsie woopsie");
  }
}

This is a pretty interesting bug that you may run into, though some basic tests will probably catch it before you need to go looking for it yourself. However, in the absence of tests, unless you remember that the function is async (and that async functions return Promises), this bug could easily slip right past you.