freecodecamp : DNA Pairing

dna_pair.js

function pair(str) {
  var dna_pair = { "A" : "T",
                   "T" : "A",
                   "C" : "G",
                   "G" : "C"};
  
  str = str.split("");
  
  for(var i = 0; i < str.length; i++){
    var arr = [str[i]];
    for(var key in dna_pair){
      if(key === str[i]){
        arr.push(dna_pair[key]);
      }
    }
    str[i] = arr;
  }
  return str;
}

pair("GCG");