Number of Dice Rolls With Target Sum LeetCode solution in JavaScript

1
var numRollsToTarget = function (n, k, target) {
2
  if (n > target || n * k < target) return 0; //target is impossible to reach
3
  let arr = new Array(k).fill(1),
4
    depth = n; //start the first layer of Pascal's N-ary Triangle.
5
  while (depth > 1) {
6
    //more layers of Triangle to fill out
7
    tempArr = []; //next layer of triangle. not done in place as previous layer's array values are needed
8
    for (let i = 0; i < arr.length + k - 1 && i <= target - n; i++) {
9
      //looping is bounded by size of next layer AND how much data we actually need
10
      let val =
11
        ((tempArr[i - 1] || 0) + (arr[i] || 0) - (arr[i - k] || 0)) %
12
        1000000007; //current index value is the sum of K number of previous layer's values, once we hit K we add next and subtract last so we don't have to manually add all K values
13
      tempArr.push(val);
14
    }
15
    arr = tempArr;
16
    depth -= 1;
17
  }
18
  let ans = arr[target - n]; //answer will be in target - nth index
19
  return ans < 0 ? ans + 1000000007 : ans;
20
};