Missing letters problem. Can somebody help me with that

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]){

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:

If you were given this problem how would you manually solve this problem (without a computer)? What would your actually steps be to solve it? See if you can write down in plain language(no code on paper or typed out) those steps which will form the logic of your function. Once you have tested your written steps on a couple of the test cases, then think about any JavaScript you have already learned which might assist you in applying the steps (your algorithm). If you know what you want to do, but do not think you have learned how to do it yet, then search the internet for JavaScript functions or methods and try to implement them. If all else fails, come back to this thread and post your written algorithm and any code you have tried to implement your algorithm.