refactored-firstNonRepeatingLetter.js

// this solution is the most efficient because it walks the array only once and 
// it doesn't use any helper methods
const firstNonRepeatingLetterA = (str) => {
    let theObj = {};
    for(let i = 0; i< str.length; i++){
      if(theObj[str[i]]){
        theObj[str[i]]+=1;
      } else {
        theObj[str[i]] = 1;
      }
    }
    return theObj;
  }

  
// I looked up string methods and I learned that .lastIndexOf() is super cool because 
// returns the index inside the string of the very last occurrence of the character,
// and it searches backwards from the index. If it doesn't find the character, returns -1

  const firstNonRepeatingLetterB = (str) => {
    if(str.length === 1){
      return str;
    }
    if(!str){
      return "You passed the empty string" ;
    }
    str = str.toLowerCase();
    for(let i = 0; i < str.length; i++){
      if(str.indexOf(str[i]) === str.lastIndexOf(str[i])){
        return str[i];
      }
    }
    return "The passed string doesn\'t have non repetitive letters" ;
  }
  
  // I can't check the tests, I still might fail some of them.