Missing letters problem. Can somebody help me with that

JavaScript
1

Tell us what’s happening:
I try to make my own solution. I really cant figured out how to do that. I spend a whole day and dont understand how to solve this. Please help me or give me some advice. Where should i look. From where i can start.

Your code so far

function fearNotLetter(str) {
  var arr = str.split('');
  var alfab = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"];
 for(var i = 0; i < arr.length; i++){
   for( var x = 0 ; x < alfab.length; x++){
   
       if(alfab[x] != arr[i]){
   
     }
   }
 }
}
fearNotLetter("abce");

Your browser information:

Your Browser User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36.

Link to the challenge:
https://www.freecodecamp.org/challenges/missing-letters