Why doesn't comparing array elements work?

Question

JavaScript

Good afternoon!
Please help me figure it out. I wrote a function, it compares 2 array elements (strings), determines whether all the letters from the second array are in the first array, if all - returns true. If there are other letters in the second array, it returns false.
Everything works, but, as they say, there is one point: comparing the arguments ["hello", "neo"] returns true. Against all common sense and against direct validation (see comments in code). What is the focus?

function mutation(arr) {
var arr1 = arr.splice(1,1);
var arr1 = arr1.join('').toLowerCase().split('');
var arr = arr.join('').toLowerCase().split('');
var x;
//console.log(arr.indexOf(arr1[i]));//проверка выдает -1, т.е. ключ, на который должен возвращаться false/
for(var i = 0; i < arr1.length; i++){	
  if(arr.indexOf(arr1[i]) === -1){
     x = false;
  }else if(arr.indexOf(arr1[i]) > -1){
     x = true;
    
  }
}
return x;
}

mutation(["hello", "neo"], "");

Answer 1

When the last letter, 'o', is checked, x becomes true, because there is such a letter in 'hello'. The minimum fix is ​​to do this:

for(var i = 0; i < arr1.length; i++){	
  if(arr.indexOf(arr1[i]) === -1){
     return false;
  }
}
return true;

In general, you can make it prettier:

function mutation(arr) {
    return Array.prototype.every.call(arr[1], (e) => ~arr[0].indexOf(e));
}