title: Logic for checking in-between two numbers
body: <p>I would like to determine where the zone is given an arbitrary number. </p>
  
  <pre><code>zones = [0, 150, 300, 400, 600, 800]
  
  function checkZone(mouseX) {
      // if mouseX is 321, it should be index 2 of zones array
  }
  </code></pre>