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PatrickMcSweeny's solution

to Nucleotide Count in the JavaScript Track

Published at Aug 26 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Given a single stranded DNA string, compute how many times each nucleotide occurs in the string.

The genetic language of every living thing on the planet is DNA. DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides. 4 types exist in DNA and these differ only slightly and can be represented as the following symbols: 'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' thymine.

Here is an analogy:

  • twigs are to birds nests as
  • nucleotides are to DNA as
  • legos are to lego houses as
  • words are to sentences as...


Go through the setup instructions for Javascript to install the necessary dependencies:



Install assignment dependencies:

$ npm install

Making the test suite pass

Execute the tests with:

$ npm test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing xtest to test.


The Calculating DNA Nucleotides_problem at Rosalind http://rosalind.info/problems/dna/

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import { NucleotideCounts } from './nucleotide-count';

describe('count all nucleotides in a strand', () => {
  test('empty strand', () => {
    expect(NucleotideCounts.parse('')).toEqual('0 0 0 0');

  xtest('can count one nucleotide in single-character input', () => {
    expect(NucleotideCounts.parse('G')).toEqual('0 0 1 0');

  xtest('strand with repeated nucleotide', () => {
    expect(NucleotideCounts.parse('GGGGGGG')).toEqual('0 0 7 0');

  xtest('strand with multiple nucleotides', () => {

  xtest('strand with invalid nucleotides', () => {
    expect(() => NucleotideCounts.parse('AGXXACT')).toThrow(new Error('Invalid nucleotide in strand'));
export class NucleotideCounts {
  static parse(strand) {
    let strandChars = strand.split("");
    let dnaTypes = ["A", "C", "G", "T"];

    if(strandChars.some(char => !dnaTypes.includes(char))){
      throw "Invalid nucleotide in strand";

    return dnaTypes.map(type => {
      return strandChars.filter(char => char === type).length;
    .map(num => num.toString())
    .join(" ");

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