Q.

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". You have function with one side of the DNA (string, except for Haskell); you need to get 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)

Example: (input --> output)

"ATTGC" --> "TAACG"
"GTAT" --> "CATA"
dnaStrand [] shouldBe []
dnaStrand [A,T,G,C] shouldBe [T,A,C,G]
dnaStrand [G,T,A,T] shouldBe [C,A,T,A]
dnaStrand [A,A,A,A] shouldBe [T,T,T,T]

A)

function DNAStrand(dna){
  let result = "";
  for (let el of dna) {
    if (el === 'A')
      result += 'T';
    else if (el === 'T')
      result += 'A';
    else if (el === 'G')
      result += 'C';
    else
      result += 'G';
  }
  return result;
}

other

헉 이렇게 구조체로 해당하는 알파벳을 바꿔주는 솔루션도 있다!

var pairs = {'A':'T','T':'A','C':'G','G':'C'};

function DNAStrand(dna){
  return dna.split('').map(function(v){ return pairs[v] }).join('');
}