SOURCE

function mutation(arr) {
  arr = [arr[0].toLowerCase(),arr[1].toLowerCase()];
  var flag = [];
  var count = 0;
  for (var i = 0;i< arr[1].length; i++) {
    flag[i] = arr[0].indexOf(arr[1][i]);
  }
  for(i=0;i<flag.length;i++){
  	if(flag[i]<0){
    	count++;
    }
  }
  if(count===0){
    return true;
  }else{
    return false;
  }
}

console.log(mutation(["zyxwvutsrqponmlkjihgfedcba", "qrstuz"]));