arbitrarily array to nested

clarify.js

// initial array
const a = [1, [2, 3], ["4", [5, 6, 7, [8, 9]]]];

// recursive function wich takes result array and inserts non-array items to it
function rec(result, arr) {
    for (let i=0; i<arr.length; ++i) {
        if (Array.isArray(arr[i]))
            rec(result, arr[i]);
        else
            result.push(arr[i]);
  }
};

const result = [];
rec(result, a);
console.log(result);