How do you flattening multi dimensional arrays?

Flattening bi-dimensional arrays is trivial with Spread operator.

      const biDimensionalArr = [11, [22, 33], [44, 55], [66, 77], 88, 99];
      const flattenArr = [].concat(...biDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
      

But you can make it work with multi-dimensional arrays by recursive calls,

      function flattenMultiArray(arr) {
        const flattened = [].concat(...arr);
        return flattened.some((item) => Array.isArray(item))
          ? flattenMultiArray(flattened)
          : flattened;
      }
      const multiDimensionalArr = [
        11,
        [22, 33],
        [44, [55, 66, [77, [88]], 99]],
      ];
      const flatArr = flattenMultiArray(multiDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
      

Also you can use the flat method of Array.

      const arr = [1, [2, 3], 4, 5, [6, 7]];
      const fllattenArr = arr.flat(); // [1, 2, 3, 4, 5, 6, 7]

      // And for multiDimensional arrays
      const multiDimensionalArr = [
        11,
        [22, 33],
        [44, [55, 66, [77, [88]], 99]],
      ];
      const oneStepFlat = multiDimensionalArr.flat(1); // [11, 22, 33, 44, [55, 66, [77, [88]], 99]]
      const towStep = multiDimensionalArr.flat(2); // [11, 22, 33, 44, 55, 66, [77, [88]], 99]
      const fullyFlatArray = multiDimensionalArr.flat(Infinity); // [11, 22, 33, 44, 55, 66, 77, 88, 99]