Shorter version of 2, using reduce:
reduce
const findDupes = a=>a.reduce(([u,d],x)=>u.includes(x)?[u,[...d,x]]:[[...u,x],d],[[],[]])[1]
Or a vastly faster Set based version:
Set
const findDupes = a=>[...a.reduce(([u,d],x)=>u.has(x)?[u,d.add(x)]:[u.add(x),d],[new Set(),new Set()])[1]]
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Shorter version of 2, using
reduce
:Or a vastly faster
Set
based version: