Adding together a sequence

sumSequence.js

// With a loop O(n)
const sumSequenceLoop = (seq) => seq.reduce((accum, current) => accum + current);
console.log(sumSequenceLoop([1, 2, 3, 4])) // 10
console.log(sumSequenceLoop([2, 4, 6])) // 12
console.log(sumSequenceLoop([3, 6, 9, 12])); // 30

// With Gauss's formula O(1)
const sumSequence = (seq) => seq.length * (seq[0] + seq[seq.length - 1]) / 2;
console.log(sumSequence([1, 2, 3, 4])) // 10
console.log(sumSequence([2, 4, 6])) // 12
console.log(sumSequence([3, 6, 9, 12])); // 30