Here's another version of findMapForReduce as a one-liner that's still very readable:
findMapForReduce
const findMapForReduce = (data) => data.reduce( ([ seen, repeated ], element) => seen.has(element) ? [ seen, repeated.set(element, true) ] : [ seen.set(element, true), repeated ], [ new Map(), new Map() ] )[1].keys()
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Here's another version of
findMapForReduce
as a one-liner that's still very readable: