Cannot able create a Record from another Record (With all optional keys)

As you can see in the playground, the generated JS code for the mapper function is the following:

function mapper(a) {
  return {
          t: a.t,
          l: intToString(a.l),
          p: arrayToString(a.p)
        };
}

What JS code would you write to achieve what you’re looking for?

By the way, what is the problem with having present and undefined fields instead of absent fields?