Four words: Set, add, delete, Array.from.

Please don't make a ( horribly underspecified ) task out of reimplementing Set, including insertion order even.

The kata seems to be during the state of being fixed of critical issues: sample tests are not using test framework; specs are missing (throwing an error requires a specific message that isn't mentioned at all, and nothing is mentioned about "empty tasks". And why mark_done also accepts a numeric index?), and no random tests.

Please only publish a kata once it's ready for beta process.