`isRouteErrorResponse`

This returns true if a route error is a route error response.

import { isRouteErrorResponse } from "react-router-dom";

function ErrorBoundary() {
  const error = useRouteError();
  if (isRouteErrorResponse(error)) {
    return (
      <div>
        <h1>Oops!</h1>
        <h2>{error.status}</h2>
        <p>{error.statusText}</p>
        {error.data?.message && <p>{error.data.message}</p>}
      </div>
    );
  } else {
    return <div>Oops</div>;
  }
}

When a response is thrown from an action or loader, it will be unwrapped into an ErrorResponse so that your component doesn't have to deal with the complexity of unwrapping it (which would require React state and effects to deal with the promise returned from res.json())

import { json } from "react-router-dom";

<Route
  errorElement={<ErrorBoundary />}
  action={() => {
    throw json(
      { message: "email is required" },
      { status: 400 }
    );
  }}
/>;

function ErrorBoundary() {
  const error = useRouteError();
  if (isRouteErrorResponse(error)) {
    error.status; // 400
    error.data; // { "message: "email is required" }
  }
}

If the user visits a route that does not match any routes in the app, React Router itself will throw a 404 response.