const maybeCow = cow =>
  new Promise( (success, failure) =>
    cow === 'cow' ? success('it is a cow')
    : failure(`it's not a cow, it is: ${cow}`));

You pass in a String, and if it doesn’t equal ‘cow’, it’ll throw. That’s bad because once you go to using the async/await syntax, you now are forcing users to use a try/catch.

Instead, you either create a Promise that always succeeds:

const maybeCow = cow =>
new Promise( success =>
  cow === 'cow' ? success({ok: true, data: 'it is a cow'})
  : success({ok: false, error: `it's not a cow, it is: ${cow}`}));

When used as a Promise, there is no need to ever use the .catch, because it always returns a resolved Promise in the .catch:

maybeCow('chicken')
.then(result => console.log("result:", result))

It’s more useful in the async/await, that’d look like:

const {ok, error, data} = await maybeCow('chicken');

Now your code looks more like Go or Lua in error handling.

The other way is to just create a simple wrapper so “Look homey, I don’t know where I got this Promise, but uh, can you make sure it doesn’t throw and just let me know if it worked or not?”

const sureThing = promise =>
  promise
  .then(result => ({ok: true, result}))
  .catch(error => Promise.resolve({ok: false, error}));

And to use it:

const go = async () => {
    const {ok, error, result} = await sureThing(maybeCow('chicken'));
    console.log("ok:", ok);
    console.log("error:", error);
    console.log("result:", result);
};

go();
// ok: false
// error: it's not a cow, it is: chicken
// result: undefined

“You can ensure this is less of a problem by ensure all Promises you create never fire catch, and the success method returns an Either.”

I was trying to google the example of solution with Either but all I see is examples in Scala, not any in RamdaJS. Can you please update the article with the code example of how to implement error handling in async/await with JavaScript Either?