Javascript Array forEach() flatten array

function flatten(arr) {
  const result = []/*from  www .  ja  v  a2 s . c om*/

  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]]
const a = flatten(nested);
console.log(a);