title: JavaScript isNaN function
body: <p>I am using isNan functionto check if entered value is a number:</p>
  
  <pre><code>if(isNaN(num)) {
      alert('I am not a number');
  }
  else {
  //
       }
  </code></pre>
  
  <p>However, if entered value is, for example, "5748hh" it is understandable like a number and return <code>false</code>. What is wrong?</p>