throw

function check(n) {
  if (n < 0) throw new Error("must be non-negative");
  return n;
}
try {
  check(-1);
} catch (e) {
  console.log(e.message);
}

must be non-negative

try {
  throw "plain string";
} catch (e) {
  console.log(typeof e, e);
}

string plain string

class ValidationError extends Error {}
try {
  throw new ValidationError("bad input");
} catch (e) {
  console.log(e.name, e.message);
}

Error bad input