var numbers = [1,2,3,4,5,6,7,8,9,20,21,22,98,99,100,220,221];

function ranges(numbers){
    var sorted = numbers.sort(function(a,b){return a-b;});
    var first = sorted.shift();
    return sorted.reduce(function(ranges, num){
        if(num - ranges[0][1] <= 1){
            ranges[0][1] = num;
        } else {
            ranges.unshift([num,num]);
        }
        return ranges;
    },[[first,first]]).map(function(ranges){
        return ranges[0] === ranges[1] ?
            ranges[0].toString() : ranges.join('-');
    }).reverse();
}

alert(ranges(numbers));