proteinTranslation.js

/*
Translate RNA sequences into proteins.

RNA can be broken into three nucleotide sequences called codons, and then translated to a polypeptide like so:

RNA: "AUGUUUUCU" => translates to

Codons: "AUG", "UUU", "UCU" => which become a polypeptide with the following sequence =>

Protein: "Methionine", "Phenylalanine", "Serine"

There are 64 codons which in turn correspond to 20 amino acids; however, all of the codon sequences and resulting amino acids are not important in this exercise. 
If it works for one codon, the program should work for all of them. However, feel free to expand the list in the test suite to include them all.

There are also three terminating codons (also known as 'STOP' codons); 
if any of these codons are encountered (by the ribosome), all translation ends and the protein is terminated.

All subsequent codons after are ignored, like this:

RNA: "AUGUUUUCUUAAAUG" =>

Codons: "AUG", "UUU", "UCU", "UAA", "AUG" =>

Protein: "Methionine", "Phenylalanine", "Serine"

Note the stop codon "UAA" terminates the translation and the final methionine is not translated into the protein sequence.

Below are the codons and resulting Amino Acids needed for the exercise.

Codon	              Protein
AUG	                Methionine
UUU, UUC	          Phenylalanine
UUA, UUG	          Leucine
UCU, UCC, UCA, UCG	Serine
UAU, UAC	          Tyrosine
UGU, UGC	          Cysteine
UGG	                Tryptophan
UAA, UAG, UGA	      STOP

If an invalid character or codon is encountered during translation, it should throw an error with the message Invalid codon.
*/

const translate = (rna) => {
  if(rna === undefined) {
    return [];
  }
  let proteins = [];
  let codons = [];
  for(let i=0; i<rna.length; i+=3) {
    codons.push(rna.slice(i, i+3));
  }
  let stopCodonIndex = codons.length-1;
  //Position of first stop codon
  for(let i=0; i<codons.length; i++) {
    if(stopCodons.includes(codons[i])) {
      stopCodonIndex = i;
      break;
    }
  }
  //Getting all valid codons
  let validateCodons = stopCodons;
  for(let i=0; i<mapping.length; i++) {
    let mappingCodons = [...mapping[i][1]];
    validateCodons = validateCodons.concat(mappingCodons);
  }
  //Associate codons with proteins
  for(let i=0; i<stopCodonIndex+1; i++) {
    //Throw an error if codon is invalid
    if(!validateCodons.includes(codons[i])) {
      throw new Error('Invalid codon');
    }
    for(let j=0; j<mapping.length; j++) {
      if(mapping[j][1].includes(codons[i])) {
        proteins.push(mapping[j][0]);
        break;
      }
    }
  }
  return proteins;
};

const mapping = [
  ['Methionine', ['AUG']],
  ['Phenylalanine', ['UUU', 'UUC']],
  ['Leucine', ['UUA', 'UUG']],
  ['Serine', ['UCU', 'UCC', 'UCA', 'UCG']],
  ['Tyrosine', ['UAU', 'UAC']],
  ['Cysteine', ['UGU', 'UGC']],
  ['Tryptophan', ['UGG']],
];

const stopCodons = ['UAA', 'UAG', 'UGA'];