const flattenTco = ([first, ...rest], accumulator) =>
(first === undefined)
? accumulator
: (Array.isArray(first))
? flattenTco([...first, ...rest])
: flattenTco(rest, accumulator.concat(first))
const flatten = (n) => flattenTco(n, []);
console.log(flatten([[1,[2,[[3]]]],4,[5,[[[6]]]]]))