function flatten(arr) { const result = [] arr.forEach((i) => { if (Array.isArray(i)) { result.push(...flatten(i)) } else { result.push(i) } }) return result } // Usage const nested = [1, 2, 3, [4, 5, [6, 7], 8, 9]] flatten(nested) // [1, 2, 3, 4, 5, 6, 7, 8, 9]