Try as hard as you can to avoid needing to care about the difference between falsy values.

Bad:

const built =
  memoized === undefined
    ? await resolveArtifact(buildKey)
    : { ok: true as const, source: memoized };

Better:

const built = memoized ? { ok: true, source: memoized } : await resolveArtifact(buildKey);

In general, something is badly wrong if there's a meaningful semantic difference in the above case when memoized is null vs undefined vs false vs "" vs 0.

This can't be a deterministic rule, partly because there are of course exceptions (especially with 0 which is obviously a legitimate number value). But often things like memoized will be plain old js objects, and should never be empty string or 0. So in most cases we just need to ensure that we're not trying to encode important information in the distinction between null and undefined, unless we've got a really great reason for doing that.