internal-error-example.ts

class InternalError extends Error {}

type Predicate<T> = (item: T) => boolean;

function single<T>(array: T[], predicate?: Predicate<T>) {
  const items = predicate ? array.filter(predicate) : array;
  
  if (items.length !== 1) {
    throw new InternalError("Array does not contain exactly one item");
  }
  
  return items[0];
}