reduce.js

"use strict";

var res = [4,5,6,8,9,10,11,12,15,17]
  .reduce(function(acc, el, i){
    if(i === 0)
      return [[el]];

    var lastArray = acc[acc.length - 1];
    var lastEl = lastArray[lastArray.length - 1];

    if(lastEl + 1 === el)
      lastArray.push(el); // append to current sequence
    else
      acc.push([el]) // create new sequence

    return acc;
  }, [])
  .reduce(function(best, curr){
    return best.length > curr.length ? best : curr;
  });


console.log(res);