productPuzzle.js

function productPuzzle(data) {
  
  var len = data.length
  var left = new Array(len)
  var right = new Array(len)
  
  // prepare the left array
  left[0] = 1
  for(let i = 1; i < len; i++) 
    left[i] = left[i-1] * data[i-1]
  console.log(left) // [ 1, 1, 2, 6 ]

  // prepare the right array
  right[len-1] = 1
  for (let j = len-2; j >= 0; j-- ) 
    right[j] = right[j+1] * data[j+1]
  console.log(right) // [ 24, 12, 4, 1 ]
  
  return data.map((el, index) => {
    return left[index] * right[index]
  })

}

console.log(productPuzzle([1, 2, 3, 4])) // [ 24, 12, 8, 6 ]