How to handle exceptions in Typescript

In Typescript, exceptions can be a bit weird. This is especially true if you’re writing library code or a VSCode plugin. Because almost surely you’ll want to output nice and custom error messages to the end user.

Typescript doesn’t allow for a try/catch block to catch type errors, like it would for runtime errors.

So the question becomes;

How do you handle exceptions in a typesafe way?

Let’s see if I can’t answer that today!

Type Predicate functions

When working on large code-bases you will almost surely be faced with working with:

• Errors
• Json decoded data (any by default)
• Third party APIs (unknown by default)

Type predicate functions are a way to make working with the above slightly more convenient.

Let’s imagine the following predicate function:

const isErrorWithCode = (err: unknown): err is Error & { code: unknown } => {
  return err instanceof Error && 'code' in err;
}

This method, isErrorWithCode takes an input and ensures that it is of type Error and it has the code attribute.

This method, we can actually use in a try/catch block! So we could end up having a try/catch looking as so:

try {
  // do thing
} catch (err) {
  if (isErrorWithCode(err) && err.code === 'ENOENT') {
    // not found
  }
}

Using the function as such will actually bubble up the error providing good user feedback while ensuring a consistent reliable behaviour.

There it is! 🎯

Today was just a brief little helper that I hope can help you deal with some of those pesky Type errors that we see from exceptions .