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const flatten = list => list.reduce(
    (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);? flattenDepth(v, depth - 1) : v), [])
: arr.reduce((a, v) => a.concat(v), []);
// flattenDepth([1,[2],[[[3],4],5]], 2) -> [1,2,[3],4,5]