title: Instantly detect an input being checked
body: <p>Say that I have an input with the type "checkbox"</p>
  
  <pre><code>&lt;input type='checkbox' id='checker'&gt;
  </code></pre>
  
  <p>I need to be able to detect instantly whether this box is checked, without a button click or function call. Checking and unchecking the box will lighten or darken text. How do I constantly check whether the box is unchecked or not IN VANILLA JAVASCRIPT? I need to be able to do this, preferably without an infinite for-loop that'll crash the page. I know the method for it</p>
  
  <pre><code>var value=document.getElementById('checker').checked;
  </code></pre>
  
  <p>but I need a way to constantly see whether it has been checked or not without reloading the page using vanilla javascript. Please help!</p>
  
  <p>EDIT: I am also considering using two radio buttons, one with value true and the other with value false, but I still need a way of seeing which one is checked.</p>