atIndex-naive.js

const arrToManipulate = [3, 5, 9, 2]

const atIndex = arr => {
  const retArr = []

  for(let i = 0; i < arr.length; i ++) {
    let total = 1
    for (let j = 0; j < arr.length; j++) {
      if(j !== i) {
        total *= arr[j]
      }
    }
    retArr.push(total)
  }

  return retArr
}

atIndex(arrToManipulate) // [90, 54, 30, 135]