Find the missing letter in the passed letter range and return it. If all letters are present in the range, return undefined.

missing-letters.js

function fearNotLetter(str) {
  //in unicode A=65 and Z=90, a=97 and z=122
  //get the starting letter's unicode, and then add one to check the next letter
  var start = str.charCodeAt(0) + 1;
  var missing = ''; //store missing letters
  
  //loop over the string, get the letters unicode value and compare it to what the next letter's code should be:
  for(var i = 1; i < str.length; i++){
    //if the codes don't match append the letter to the variable
    if(str.charCodeAt(i) !== start){
      missing += String.fromCharCode(start);
      start++; //add an extra one to the counter for the missing letter
    }
    start ++;
  }
  //if there's no missing letters return undefined.
  if(missing == ''){
    missing = undefined;
  }
  
  return missing;
}

fearNotLetter("pqrt");

function fearNotLetter(str) {
let alphabet="abcdefghijklmnopqrstuvwxyz".split("");
let debut=0;
let fin=0;
let missingEl="";
for(let i=0;i<alphabet.length;i++){
if(str[0]==alphabet[i]){
debut=i;
console.log(debut);
}
if(str[str.length-1]==alphabet[i]){
fin=i;
console.log(fin);
}
}

for(let i=debut;i<=fin;i++){
if(str.indexOf(alphabet[i])==-1){
missingEl=alphabet[i];
console.log(missingEl);
}
}

if(str.length==alphabet.length){
return undefined;
}

return missingEl;
}

fearNotLetter("abce");

function fearNotLetter(str) {
//Create array of alphabet
let alphabetArrray = "abcdefghijklmnopqrstuvwxyz".split("");

//Get the start and end index of the str from the alphabet
let startIndex = alphabetArrray.indexOf(str[0])
//startIndex = 18

let endIndex = alphabetArrray.indexOf(str[str.length-1])
//endIndex = 23

//Get the complete string range from the alphabet array
let strArray = alphabetArrray.slice(startIndex,endIndex)
//strArr = ["s","t","u","v","w","x"]

//Find the missing letter from the string array we created
let missingLetter = strArray.filter(item => (!str.includes(item)))
//missingLetter = ["u"]

//Check if missing letter has value
if( missingLetter.length > 0){
//If missing letter has a value return it as letter by using join method
return missingLetter.join("")

}else{
//This returns when there is no missing letter from the given string
return undefined
}
}

fearNotLetter("stvwx");