Complementary-DNA.js

/*
  Deoxyribonucleic acid (DNA) is a chemical found in the nucleus of cells and carries the "instructions" for the development and functioning of living organisms.

  If you want to know more: http://en.wikipedia.org/wiki/DNA

  In DNA strings, symbols "A" and "T" are complements of each other, as "C" and "G". Your function receives one side of the DNA (string, except for Haskell);
  you need to return the other complementary side. DNA strand is never empty or there is no DNA at all (again, except for Haskell).

  More similar exercise are found here: http://rosalind.info/problems/list-view/ (source)
*/

// Answer:

function DNAStrand(dna){
  const complements = {
    'A': 'T',
    'T': 'A',
    'C': 'G',
    'G': 'C',
  }
  let result = '';
  for (let char of dna) {
    result += complements[char]
  }
  return result;
}

// BigO: O(n)