For a simple use of slice, use my extension to Array Class:

Array.prototype.subarray = function(start, end) {
    if (!end) { end = -1; } 
    return this.slice(start, this.length + 1 - (end * -1));
};

Then:

var bigArr = ["a", "b", "c", "fd", "ze"];

Test1:

bigArr.subarray(1, -1);

< ["b", "c", "fd", "ze"]

Test2:

bigArr.subarray(2, -2);

< ["c", "fd"]

Test3:

bigArr.subarray(2);

< ["c", "fd","ze"]

Might be easier for developers coming from another language (i.e. Groovy).

sort() the numbers, reduce() to arrays, and map() to take the numbers that occur a single time out of their array:

let arr =  [ 1, 1, 1, 1, 2, 2, 2, 4, 5, 10, 20, 20, 391, 392, 591 ]

let sort = arr.sort((a,b) => a - b)

let res = sort.reduce((acc,cur) => {
    if(acc[acc.length-1] && cur === acc[acc.length-1][0]){
        acc[acc.length-1].push(cur)
    }else{
        acc.push([cur])
    }
    return acc
},[]).map(el => el.length === 1 ? el[0] : el)

console.log(res)

The easiest way to do this is with a dictionary (which JS objects naturally act as).

var datesArray = [ "2016-12-16", "2016-12-16", "2016-12-17", "2016-12-17", "2016-12-17", "2016-12-17", "2016-12-18", "2016-12-18", "2016-12-19" ]; //I've corrected the syntax for this to make each date a string, rather than a sequence of subtractions

var datesCount = {};
datesArray.forEach(function(date){
    if(!datesCount[date])
        datesCount[date] = []
    datesCount[date].push(date);
});

At this point, you have a set of group arrays, each containing every instance of a particular date. You can iterate over it with Object.getOwnPropertyNames and put each array into the larger array OR you could just use it directly.