https://en.wikipedia.org/wiki/Triangular_number

triangleNumber.js

const sum = ((a, b) => a + b)

const triangleNumber = (number) => {
  const list = []
  list.length = number
  list.fill()

  return list.map((_, index) => index + 1).reduce(sum, 0)
}

console.log(
  triangleNumber(1), // 1
  triangleNumber(2), // 3
  triangleNumber(3), // 6
  triangleNumber(4), // 10
  triangleNumber(5), // 15
  triangleNumber(6), // 21
  triangleNumber(10), // 45
)