Typesafe access of 'unknown' type

I'm trying to access the error variables returned by a try-catch

7 Replies

domi?•2y ago

try {
    // ...
} catch (e) {
    if (e instanceof Object) {
        console.log(e.message); // Property 'message' does not exist on type 'Object'
    }
}

try {
    // ...
} catch (e) {
    if (e instanceof Object) {
        console.log(e.message); // Property 'message' does not exist on type 'Object'
    }
}

e is not an instance of Error as it's thrown by a third party library. How can I access the message key without casting to another type?

benten•2y ago

Since you don't know the type of an error you'll have to conditionally access it. Something like

try {
  // ...
} catch (e) {
  let message
  if (e instanceof Error){
    message = e.message
  } else {
     message = String(e) // or a custom message
  }
  console.log(message)
}

try {
  // ...
} catch (e) {
  let message
  if (e instanceof Error){
    message = e.message
  } else {
     message = String(e) // or a custom message
  }
  console.log(message)
}

I think Kent has an article about this lemme find it yep https://kentcdodds.com/blog/get-a-catch-block-error-message-with-typescript

domi?•2y ago

Thanks, that is a pretty good article. However, e is not an instance of Error as it's thrown by a third party library. It's just a normal object

ironnator•2y ago

Currently, I don't think there's a real nice way. You can hack around it by casting it to your own custom error object as you mentioned. But the type safe way to do it (for now) would be some sort of type guard function - found this thread which might help. https://stackoverflow.com/questions/70028907/narrowing-an-object-of-type-unknown

Unknown User•2y ago

Message Not Public

benten•2y ago

that's really nice Do you know the type being thrown?

domi?•2y ago

that's really nice It's just a normal object, no class

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Typesafe access of 'unknown' type