title: Using indexOf() to compare characters in an Array
body: <pre><code>  function mutation(arr) {
  
    var tester = arr[1].split('');
  
    for (var i = 0; i &lt; tester.length; i ++) {
      if (!arr[0].indexOf(tester[i])) return false;
    }
    return true;
    }
  
    mutation(["hello", "hey"]);
  </code></pre>
  
  <p>Here I should return true if the string in the first element of the array contains all of the letters of the string in the second element of the array.</p>
  
  <p>I do not see any problems with this code but it passes like only 90% of the tests and I do not know why. And I can not see a pattern there — what exact conditions should I meet to fail the test.</p>