isError #

The isError function helps discriminate the state from any error that can occur.

When using the state property from any hook, whether useStatefulRequest or useStatelessRequest, you will get an enumeration that looks like that.

type State<Type> =
  | Type
  | CancelError
  | NetworkError
  | UnexpectedError
  | ExpectedError

You could argue that this is not very practical, but this implies something that is highly beneficial for any applications that aims to seek robustness and reliability: predictability in the behavior.

Predictability here means that you can be sure that any request has to be checked, whether manually, or through the use of helpers like the isError function exposed by the saint-bernard library.

Manual checking #

You don't really need the isError function in order to check whether the state is indeed the type you expected it to be. It can easily be done manually.

const state: State<number> = 123.4567890;

if (state instanceof CancelError) {
  console.error("Request canceled.");
} else if (state instanceof NetworkError) {
  console.error("Network error.");
} else if (state instanceof UnexpectedError) {
  console.error(`Unexpected error: ${state.message}`);
} else if (state instanceof ExpectedError) {
  console.error(`Error: ${state.message}`);
} else {
  console.log(state.toFixed(2));
}

If you only care about the data, and the error is not of importance for you, you can still benefit for a little bit of security in here, while be assured to get the data when necessary when using the isError function.

Let's say that you want to display the same message for all errors, it would be cumbersome to do so in this example.

const state: State<number> = 123.4567890;

if (state instanceof CancelError) {
  console.error("An error occurred"); 
} else if (state instanceof NetworkError) {
  console.error("An error occurred"); 
} else if (state instanceof UnexpectedError) {
  console.error("An error occurred"); 
} else if (state instanceof ExpectedError) {
  console.error("An error occurred"); 
} else {
  console.log(state.toFixed(2));
}

Instead, we can leverage the use of the isError function in order to properly discriminate the error from the data.

import { isError } from "saint-bernard";

const state: State<number> = 123.4567890;

if (isError(state)) {
  console.error("An error occurred");
} else {
  console.log(state.toFixed(2));
}

That's way easier this way than manually adding all logs necessary for our use case.

Any value that is not a valid saint-bernard error will return false.

isError(new Error); // false
isError(new AbortError); // false
isError("saint-bernard"); // false

While any valid saint-bernard error will return true.

isError(new CancelError); // true
isError(new NetworkError); // true
isError(new UnexpectedError("because reasons")); // true