task2.js

const arr = [[[1, 2], 3], [4], [5, 6], [7, [8, 9]]];

const convertFlat = (arr = []) => arr.flat(Infinity);

const convertFlatSome = (arr = []) =>
  arr.some(Array.isArray) ? convertFlatSome(arr.flat(1)) : arr;

const convertFlatToStringMap = (arr = []) =>
  arr.toString("").split(",").map(Number);

const flattenedJsonParse = (arr) => JSON.parse("[" + JSON.stringify(arr).replace(/\[|\]/g,'').replace(/,/g,',') + "]");

console.log(flattenedJsonParse(arr))
console.log(convertFlat(arr));
console.log(convertFlatSome(arr));
console.log(convertFlatToStringMap(arr));

update use json