nucleotide.js

var DNA_NUCLEOTIDES = ['A', 'T', 'C', 'G'];
var ALL_NUCLEOTIDES = DNA_NUCLEOTIDES.concat('U');

function initialCounts() {
  return DNA_NUCLEOTIDES.reduce(function(counts, nucleotide) {
    counts[nucleotide] = 0;
    return counts;
  }, {});
}

function countNucleotides(sequence) {
  return sequence.split("").reduce(function(nucleotideCounts, nucleotide){
    if (nucleotideCounts[nucleotide] >= 0) {
      nucleotideCounts[nucleotide]++;
    }
    return nucleotideCounts;
  }, initialCounts());
}

function validateNucleotide(nucleotide) {
  if (ALL_NUCLEOTIDES.indexOf(nucleotide) === -1) {
    throw new Error("Invalid Nucleotide");
  }
}

function DNA(sequence) {
  this.nucleotideCounts = countNucleotides(sequence);
}

DNA.prototype.count = function(nucleotide) {
  validateNucleotide(nucleotide);
  return this.nucleotideCounts[nucleotide] || 0;
};

module.exports = DNA;