find complement strand

dna.js

(function(export) {
  var symbolComplements = {
    "A": "T",
    "T": "A",
    "C": "G",
    "G": "C"
  };

  export.DNA = {
    complementStrand: function(strand) {
      var complement = '';
      if(strand) {
        for(index in strand) {
          var symbol = strand[index];
          complement += symbolComplements[symbol];
        }
      }
      return complement;
    }
  };
})(window);

console.log(DNA.complementStrand("ATTGC"));