Yep. That's the sound of a man who just got dumped by his girlfriend, chewed by a bear and then shot 32 times. It's also the sound I make when working with JavaScript.

Lol, just kidding. It's one of the sounds I make when working with JavaScript. Aaaaaaargh.

This module gives you a simple way to selectively handle errors in JavaScript.

Supposing you have a function like this:

function fetchTweets (userId) {
    return twitterApi.fetch(userIdd).then(response => {
        if (response.error && response.error.message.includes("Too many requests")) {
            throw new RateLimitExceededError();
        }
        return response.data;
    })
}

This function is meant to throw a specific error when you exceed your Twitter API rate limits.

So you call it like this:

fetchTweets(userId)
  .catch(e => {
      console.log('Rate limit exceeded; initiating exponential backoff');
      // do backoff
  })
  .then(tweets => {
      // do stuff with them
  })

or...

try {
    let tweets = await fetchTweets(userId);
} catch(e) {
      console.log('Rate limit exceeded; initiating exponential backoff');
      // do backoff
}

See the problem? Different errors could be thrown in that function. For instance, if you look closely at the function, you'll see I made a typo on the first line (userIdd instead of userId) which will throw a ReferenceError when executed. However, the calling code will expect a RateLimitError only.

There's a simple fix: use an if-statement:

try {
    let tweets = await fetchTweets(userId);
} catch(e) {
    if (e instanceof RateLimitExceededError) {
      console.log('Rate limit exceeded; initiating exponential backoff');
      // do backoff
    } else if (e instanceof SomeOtherError || e instanceof SomeOtherAnnoyingError) {
        // handle it
    } else {
        // this is important, so unexpected errors 
        // don't get swallowed by our app
        throw e;
    }
}

Hey, if that works for you, well, good, but if you're like me, you hate writing this kind of if statement. Too messy. I prefer PHP's inbuilt selective error handling:

try {
    $tweets = fetchTweets($userId);
} catch(RateLimitExceededError $e) {
    echo 'Rate limit exceeded; initiating exponential backoff';
     // do backoff
} catch (SomeOtherError | SomeOtherAnnoyingError $e) {
    // handle it
}

// Any error that doesn't match the types you specified is not caught

So I decided to do something similar for JS. Here's how you use it:

try {
    let tweets = await fetchTweets(userId);
} catch(e) {
    return aargh(e)
        .type(RateLimitExceededError, (e) => {
            console.log('Rate limit exceeded; initiating exponential backoff');
            // do backoff
        })
        .type([SomeOtherError, SomeOtherAnnoyingError], (e) => {
            // handle it
        })
        .throw();
}

Works with Promise#catch() too:

fetchTweets(userId)
  .catch(e => {
      return aargh(e)
          .type(RateLimitExceededError, (e) =>  {
              console.log('Rate limit exceeded; initiating exponential backoff');
              // do backoff
          })
          .type([SomeOtherError, SomeOtherAnnoyingError], (e) => {
              // handle it
          })
          .throw();
  })
  .then(tweets => {
      // do stuff with them
  })

const aargh = require('aargh');

The entry point. You pass in the error object you want to handle. You should probably have this as the first statement in all your catch() blocks.

The first argument to the type function is the type or types (as an array) of errors you want to handle. The second is a callback containing the code you want to execute for that error. Aargh will call this callback with the error as the only parameter.

You can return stuff from this callback too. Aargh will return this value to the caller.

Calling the throw() function ends the chain and ensures any errors which weren't matched by your type() checks are thrown back to the caller.

Use the others() function to end the chain and specify a callback to be executed if the error wasn't matched by your type() checks. You should use either throw() or others(), and it should only be used after all type() calls.

try {
    return await fetchTweets(userId);
} catch(e) {
    return aargh(e)
        .type(RateLimitExceededError, (e) => {
            // do backoff
        })
        .others((e) => {
            // This will catch any other errors
            // Maybe log the error? Idk
        });
}

npm i aargh

I spent time thinking about the most intuitive/natural syntax/flow to use for this. If you're interested in this package, and you've got ideas, please hit me up on Twitter or open an issue.

A few tips for error handling in JS (whether you're using this package or if-statements):

• Create your custom errors. Don't throw strings or just Error. Why? So you can trace exactly what went wrong in your code.
• Make sure your errors extend from the Error class. Why? So you can leverage all the awesome debugging tools out there, plus inbuilt Error properties like .stack.
• Handle only errors you expect. Let the rest crash your app. Why? So you know when something goes wrong.