nucleotides_solution.js

var DNAStringFactory = function(string){
    var DNA = {
        string: string,
        count: function(letter){
            if (!_.contains(['A', 'G', 'C', 'T', 'U'], letter)){
                return alert('This is not valid!');
            }

            var result = _.filter(this.string, function(nucleotide){
                return nucleotide == letter;
            })
            return result.length;
        },
        nucleotidesCount: function(){
            var nucleotideHash = {}
            _.each(this.string, function(nucleotide){
                nucleotideHash[nucleotide] = nucleotideHash[nucleotide] || 0;
                nucleotideHash[nucleotide]++;
            })
            return nucleotideHash;
        }
    }
    return DNA;
}

var dna = DNAStringFactory('AGGTGGU')