Discussion on: And then the interviewer asks, "Can you do this with less code?"

[Jochem Stoel]

You can do it with less code, the parenthesis can be omitted here. :P

const duplicateCheck = a => new Set(a).size !== a.length;

[Michiel Hendriks]

If you are going to assume a is an array, then you could even use !=.

[Michael Solati]

Haha, I thought so too. And I guess we could use let instead of const since it's one letter less.

[Jon Randy 🎖️]

const duplicateCheck = a => a.length - new Set(a).size;

Shorter and more useful since it returns the number of duplicates

[Michiel Hendriks]

const duplicateCount = a => a.length - new Set(a).size;

And now with a meaningful name. Because, what does duplicateCheck mean, that is has, or does not have duplicates. For the boolean version hasDuplicates would be the better name.