Not able to grab the second array element

var longestCommonPrefix = function(strs) {

  var word = "";
  for (var i = 0; i < strs.length; i++) {
    var firstWord = strs[i];
    var secondWord = strs[1 + i];
    if (strs[i].length >= secondWord.length) {
      for (var d = 0; d < strs[i].length; d++) {
        var store1 = firstWord[d];
        if (secondWord.indexOf(store1) != -1) {
          word += store1;
        } else {
          word = "";
        }
      }
    } else {
      for (var f = 0; f < strs[i + 1].length; f++) {
        var store2 = firstWord[f];
        if (firstWord.indexOf(store1) != -1) {
          word += store2;
        } else {
          word = "";
        }
      }
    }
  }
  return (word);
};