how to get the sum in javascript code example

Example 1: total array with function

console.log(
  [1, 2, 3, 4].reduce((a, b) => a + b, 0)
)
console.log(
  [].reduce((a, b) => a + b, 0)
)

Example 2: summation js

const summation = (n) => (n * (n + 1)) / 2;