Add isErrorObject type guard / function

[fregante]

What do you think about exporting a type-checker function (like Array.isArray) that doubles as a type guard?

export function isErrorObject(error: unknown): error is ErrorObject {
  return error && typeof error === 'object' && 'message' in error;
}

[sindresorhus]

What would you use it for that is related to this package?

[fregante]

To detect whether is a serialized error or regular error. Example:

try {
…
} catch(error: unknown) {
  if (isErrorObject(error) && error.message === blah) {
    // ignore
  } else {
    throw error
  }
}

Typescript makes it a pain to check properties of unknown objects, even with ?., so a type guard would make it easier

[sindresorhus]

Yes, but why have it here and not in https://github.com/sindresorhus/is? And what properties should it enforce?

I think it should be named isErrorLike. This is consistent with TypeScript methods for duck-typing, like the PromiseLike type.

https://stackoverflow.com/questions/43712705/why-does-typescript-use-like-types

[fregante]

Here because {message: string} isn’t really an error on its own, but this package can turn it into one. I don’t think it would make sense in is

isErrorLike could also work but isErrorObject matches the name you’re using here.

In this package the type ErrorObject could also make a little more tight because currently it matches {}, but that’s another issue.

[sindresorhus]

Ah, I see. Ok, sure, let's add. isErrorObject is fine.