Custom Error Types

Extending Error

class ValidationError extends Error {
  constructor(public field: string, message: string) {
    super(message);
    this.name = "ValidationError";
  }
}

function assertPositive(n: number) {
  if (n < 0) throw new ValidationError("n", "must be positive");
}

Type Guard for custom errors

function isValidationError(e: unknown): e is ValidationError {
  return e instanceof ValidationError;
}

try {
  assertPositive(-5);
} catch (e) {
  if (isValidationError(e)) console.error(e.field, e.message);
}