Hackerrank: Sam and Substrings Solution

1function substrings(n) {
2  const arr = []
3  n.split("").forEach((d, i) => {
4    if (i === 0) arr[i] = Number(d)
5    else {
6      arr[i] = ((i + 1) * Number(d) + 10 * arr[i - 1]) % (Math.pow(10, 9) + 7)
7    }
8  })
9  return arr.reduce((a, b) => a + b, 0) % (Math.pow(10, 9) + 7)
10}