Convert arrays from normal to cumulative and back

cumulative.js

// [2, 4, 3, 8] => [2, 6, 9, 17]
const convertToCumulative = points => 
    points.reduce((acc, p, i) => acc.concat([
        p + (acc.length > 0 ? acc[i - 1] : 0)
    ]), []);

// [2, 6, 9, 17] => [2, 4, 3, 8]
const convertToNormal = points => 
    points.reduce((acc, p, i, ps) => acc.concat([
        p - (i > 0 ? ps[i - 1] : 0)
    ]), []);