// Utils
const last = (arr) => arr[arr.length - 1]
const withoutLast = (arr) => arr.slice(0, -1)
const sumWithNext = (arr) => (val, index) => val + arr[index + 1]

// Sum the values from a row
const createRowValues = (arr) => withoutLast(arr).map(sumWithNext(arr))

// Create row padded by the start values
const createRow = (previous, start) => [
  start,
  ...createRowValues(previous),
  start,
]

// Appends a new row based on the last row
const appendRow = (start) => (rows) => [...rows, createRow(last(rows), start)]

// Takes a starting value and the number of rows and returns a nested array
const pascal = (rows, start = 1) =>
  [...Array(rows)].reduce(appendRow(start), [[start]])

pascal(10)

// [ [ 1 ],
//   [ 1, 1 ],
//   [ 1, 2, 1 ],
//   [ 1, 3, 3, 1 ],
//   [ 1, 4, 6, 4, 1 ],
//   [ 1, 5, 10, 10, 5, 1 ],
//   [ 1, 6, 15, 20, 15, 6, 1 ],
//   [ 1, 7, 21, 35, 35, 21, 7, 1 ],
//   [ 1, 8, 28, 56, 70, 56, 28, 8, 1 ],
//   [ 1, 9, 36, 84, 126, 126, 84, 36, 9, 1 ],
//   [ 1, 10, 45, 120, 210, 252, 210, 120, 45, 10, 1 ] ]