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rootulp's solution

to RNA Transcription in the JavaScript Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given a DNA strand, return its RNA complement (per RNA transcription).

Both DNA and RNA strands are a sequence of nucleotides.

The four nucleotides found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T).

The four nucleotides found in RNA are adenine (A), cytosine (C), guanine (G) and uracil (U).

Given a DNA strand, its transcribed RNA strand is formed by replacing each nucleotide with its complement:

  • G -> C
  • C -> G
  • T -> A
  • A -> U


Go through the setup instructions for ECMAScript 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.


Hyperphysics http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html

Submitting Incomplete Solutions

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


import Transcriptor from './rna-transcription';

describe('Transcriptor', () => {
  const transcriptor = new Transcriptor();

  test('transcribes cytosine to guanine', () => {

  xtest('transcribes guanine to cytosine', () => {

  xtest('transcribes adenine to uracil', () => {

  xtest('transcribes thymine to adenine', () => {

  xtest('transcribes all dna nucleotides to their rna complements', () => {

  xtest('correctly handles invalid input', () => {
    expect(() => transcriptor.toRna('U')).toThrow(
      new Error('Invalid input DNA.'),

  xtest('correctly handles completely invalid input', () => {
    expect(() => transcriptor.toRna('XXX')).toThrow(
      new Error('Invalid input DNA.'),

  xtest('correctly handles partially invalid input', () => {
    expect(() => transcriptor.toRna('ACGTXXXCTTAA')).toThrow(
      new Error('Invalid input DNA.'),
const COMPLEMENTS = new Map([
  ["G", "C"],
  ["C", "G"],
  ["T", "A"],
  ["A", "U"]

class Transcriptor {
  toRna(strand) {
    if (!this.validInput(strand)) {
      throw new Error("Invalid input DNA.");
    return [...strand].map(nucleotide => COMPLEMENTS.get(nucleotide)).join("");

  validInput(strand) {
    return strand.match(/^[GCTA]+$/);

export default Transcriptor;

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