title: Why can't a variable be set to a <number>.toString(), but you can can call toString() on a return value?
body: <pre><code>function s(e) {
      return e.toString();
  }
  
  var a = s(3); // works
  
  var b = 3.toString(); // error
  </code></pre>
  
  <p>For example, set var a to the return value of s() that returns the first argument.toString(), but you can't set var b to 3.toString()</p>