title: How is the event parameter passed when using addEventListener?
body: <p>I’m confused by the following code on <a href="https://www.w3schools.com/jsref/event_transition_propertyName.asp" rel="nofollow noreferrer">W3schools</a>:</p>
  
  <pre><code>document.getElementById("myDIV").addEventListener("transitionend", myFunction);
  
  function myFunction(event) {
    this.innerHTML = "Property name is: " + event.propertyName;
  }  
  </code></pre>
  
  <p>How is it that <code>myFunction()</code> can have a parameter <code>event</code> reference passed in when the preceding statement call merely passed in <code>myFunction</code> <em>without any arguments</em>?</p>
  
  <p>Doesn’t that constitute an undeclared variable usage in normal circumstances? </p>
  
  <p>Are there any other such occurences elsewhere in the DOM event I should be aware of?</p>