title: Check if string contains only digits
body: <p>I want to check if a <code>string</code> contains only digits. I used this:</p>
  <pre><code>var isANumber = isNaN(theValue) === false;
  
  if (isANumber){
      ..
  }
  </code></pre>
  <p>But realized that it also allows <code>+</code> and <code>-</code>. Basically, I want to make sure an <code>input</code> contains ONLY digits and no other characters. Since <code>+100</code> and <code>-5</code> are both numbers, <code>isNaN()</code> is not the right way to go.
  Perhaps a regexp is what I need? Any tips?</p>