Returns a 'flattened' copy of array, un-nesting arrays one level
flatten([1, [2, 3], [[4]] ]);// Yields: [ 1, 2, 3, [4]]; Copy
flatten([1, [2, 3], [[4]] ]);// Yields: [ 1, 2, 3, [4]];
Returns a 'flattened' copy of array, un-nesting arrays one level