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                    function isIsomorphic(firstString, secondString) {

      // Check if the same lenght. If not, they cannot be isomorphic
      if (firstString.length !== secondString.length) return false

      var letterMap = {};

      for (var i = 0; i < firstString.length; i++) {
        var letterA = firstString[i],
            letterB = secondString[i];

        // If the letter does not exist, create a map and map it to the value
        // of the second letter
        if (letterMap[letterA] === undefined) {
          letterMap[letterA] = letterB;
        } else if (letterMap[letterA] !== letterB) {
          // Eles if letterA already exists in the map, but it does not map to
          // letterB, that means that A is mapping to more than one letter.
          return false;
      // If after iterating through and conditions are satisfied, return true.
      // They are isomorphic
      return true;