title: Turn lowercase letters of a string to uppercase and the inverse
body: <pre><code>let message = "heY, WHAt are you dOING?";
  let count_changes = 0;
  let isLetter = (letter) =&gt; {
      if (('a'&lt;=letter &amp;&amp; letter &gt;='z') || ('A'&lt;=letter &amp;&amp; letter &gt;='Z')) {
          return letter;
      } else {
          return -1;
      }
  }
  
  for(let i = 0; i &lt;= message.length; i++) {
      if (isLetter(i) &amp;&amp; message[i].toLowerCase()) {
          message[i].toUpperCase();
          count_changes++;
          console.log(message[i].toLowerCase());
      }
      else if (isLetter(i) &amp;&amp; message[i].toUpperCase()) {
          message[i].toLowerCase();
          count_changes++;
      }
      else {
          console.error('Bad stirng');
      }
  }
  </code></pre>
  
  <p>Hello, I want to use the function isLetter to check the string message every character and when i use isLetter in the for loop to check in the if statement whether i is a Letter or not and also if its Lowercase letter later to when there is a change to Uppercase i increment count_changes++. Again with the second if statement if also i is Letter and in this case Uppercase letter then if change to lowercase letter to increment the count_changes++ so the count_changes to be my final result
  thank you</p>