Shouldn't we keep this check? Without this, the error can be implemented without passing in actual too, so no matter what the user passes as the actual argument, the error message will say undefined

I just moved the check so it will be executed for all errors. That is actually what this PR is all about.

Can you explain this and the change to fs? I didn't expect to see those here.

The length was never used in this case. It was a weird API that I fixed.

Originally the implementation was like:

function a (a, b) {
  if (b === true) {
    return 'foo'
  }
  return `${a} foo`
}

Now it is:

function a(a = undefined) {
  if (a === undefined) {
    return 'foo'
  }
  return `${a} foo`
}

I don't think this is truly needed. Or is it?

Sadly that is necessary. Default arguments do not count towards function.length as in:

function a(a) {}
function b(a = undefined) {}
assert.strictEqual(a.length, 1);
assert.strictEqual(b.length, 0);

Otherwise it is difficult to validate a couple of functions, but I think it is worth it because it makes sure we make less errors while implementing the errors.

Can you add a comment before the function then? Otherwise it won't be clear to the reader.

Done.

I don't think the default argument is truly needed.

See above.