[MFA-G]

Closes #175.

Problem

An option declared with type: [] but never passed on the command line ends up as [undefined] as soon as any unrelated option appears in argv:

const cli = cac()
  .option('--type <type>', 'Choose a project type', { default: 'node' })
  .option('--externals <external>', 'Add externals', { type: [] })

cli.parse(['node', 'bin'])
// { '--': [], type: 'node' }                      ✅ externals absent

cli.parse(['node', 'bin', '--type', 'node'])
// { '--': [], type: 'node', externals: [ undefined ] }   ❌

Consumers that check if (options.externals) or spread the options into a config object get a truthy one-element array of undefined instead of nothing.

Cause

In CAC#mri, setByType(options, transforms) is called once per parsed key. setByType then loops over all array-typed transforms, not just the key that was actually set:

for (const [key, transform] of Object.entries(transforms)) {
  if (transform.shouldTransform) {
    obj[key] = [obj[key]].flat()   // obj[key] may be undefined

With no parsed keys at all the loop body never runs, which is why the plain node bin case looks fine. Passing --type gives the outer loop one iteration, setByType runs, and [undefined].flat() is written to the untouched externals key.

Fix

Only transform keys that are actually present on the options object:

if (transform.shouldTransform && key in obj) {

key in obj (rather than a value check) keeps an explicitly-set falsy or undefined value transformable, and still lets a declared default be wrapped into an array as before.

Test

Added a regression test in tests/index.test.ts covering the three states: option omitted entirely, option omitted while an unrelated option is passed, and option actually passed. It fails on main ([undefined] vs absent) and passes with the fix.

Validation: pnpm vitest run → 18/18 passing, pnpm lint and pnpm typecheck clean.