Get range function

getRanges.js

const getRanges = arr =>
arr
.reduceRight(
    (prev, curr) => (
        prev.length
        ? (
            prev[0][0] === curr + 1
            ? prev[0].unshift(curr)
            : prev.unshift([curr])
        ) && prev
        : [[curr]]
    ),
    []
)
.map(
    a => a.join('-')
)
.join(',')