Function to flat arrays

flatArray

// const a = [1, [], [2, 3, [4],
//   [5],
//   [1, 2, 3, 4, [5, [6],
//     [4], [],
//   ]]
// ], 3, 6];
// const b = [1, 2, 3];

/**
 * Function to flat arrays
 */
function flatArray(array = []) {
  // We reduce the original array to a flat new array
  return array.reduce(
    (acu, curr) =>
      // If the current element is not array
      (!Array.isArray(curr) ?
        // Then just continue adding elements to our array
        [...acu, curr] :
        // If it's an array, we just flat this position of the array
        [...acu, ...flatArray(curr)])
    , []
  );
}

// console.log(a);
// console.log(flatArray(a));
// console.log('-');
// console.log(b);
// console.log(flatArray(b));