[рони]

"use strict"
function createIntervals(a) {
     let c;
     return a.sort((a, b) => a - b)
             .reduce((a, b) => (c && b === c[1] + 1 ? c[1] = b : a.push(c = [b, b]), a), []);
 }


let output =  createIntervals([1, 2, 3, 4, 5, 7, 8, 12]);
alert(JSON.stringify(output)); // output [ [ 1, 5 ], [ 7, 8 ], [ 12, 12 ] ]