Flatten

// Single-level arrays are unaffected.
flatten([1, 2, 3]); // [1, 2, 3]

// Inner arrays are flattened into a single level.
flatten([1, [2, 3]]); // [1, 2, 3]
flatten([
  [1, 2],
  [3, 4],
]); // [1, 2, 3, 4]

// Flattens recursively.
flatten([1, [2, [3, [4, [5]]]]]); // [1, 2, 3, 4, 5]