isOneOfMyValues.ts

const values = ['foo', 'bar'] as const;
type MyValues = typeof values[number];

function isOneOfMyValues(elem: unknown): elem is MyValues {
  const opts: string[] = [...values];
  return opts.includes(String(elem));
}

function lengthOfMyThing(x: MyValues | null) {
  if (isOneOfMyValues(x)) {
    return x.length;
  }
  return 0;
}