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function isIsogram(str = null){
    if((str == null) || (typeof str !== 'string') || (str.length <= 0)) {
      return false;
    } else {
      // Convert word to array and split word
      let words = str.split('')
      // Return true if word is an isogram
      return words.length === words.filter(
          (string, index, arr) => arr.indexOf(string) == index
      ).length
    }
}