assert-error

When using strict errors in TS the type of the error in the catch clause is unknown. This is a good thing since it forces you to properly handle the error.

This package provides two functions to make this error handling easier.

Install

The usual npm install @poppanator/assert-error, yarn add @poppanator/assert-error, etc...

Example

Ponder the following

try {
  // Do some funky salsa
} catch (err: unknown) {
  console.error('Error message:', err.message)
  // .................................^^^^^^^
  // It's a nono
}

Since TS doesn't know wether err is an error object or not you can't index on it.

But using assertError() you can:

import { assertError } from '@poppanator/assert-error'

try {
  // Do some funky chimichurri
} catch (err: unknown) {
  assertError(err)
  console.error('Error message:', err.message)
  // It's a Yay-Yay
}

Now TS known ist's an Error object and indexing on it is fine. If err is not an instance of (at least) Error, assertError() will throw.

To assert err is some other error than Error, you can provide the class constructor as second argument to assertError():

class MyError extends Error {
  public code = 12
}

try {
  // Do some funky pesto
} catch (err: unknown) {
  assertError(err, MyError)
  console.error('Error:', err.code)
}

Now, you may not always want something to throw in you catch clause. If that's the case you can use assumeError(), which is just a typeguard working in the same way as assertError().

assertError() is to prefer over assumeError(), but there might be situations where you just want to type-guard and not risking ending up with something throwing an error.

NOTE! assumeError() has no runtime-effect. It does nothing.

import { assumeError } from '@poppanator/assert-error'

try {
  // Do some funky gaspacho
} catch (err: unknown) {
  assumeError(err)
  console.error('Error:', err.message)

  // These two are the same thing
  assumeError<MyError>(err)
  assumeError(err, MyError)

  if (err.code) {
    console.error('Error code:', err.code)
  }
}