An adventure in sparse arrays

I offered to coach JavaScript recently, and an exercise I dreamt up was to implement every array (prototype) method, and write tests that they work.
Since I’d come down with tonsillitis which apparently comes with a full on bedridden fever, I thought I’d have a go myself. And I learnt a few things along the way. Today, I wanted to share the swiss cheese wonder of sparse arrays!
(see what I did there? 🧀)