Question

Many events have a default action associated with them. If you click a link, you will be taken to the link’s target. If you press the down arrow, the browser will scroll the page down. If you right-click, you’ll get a context menu. And so on.

For most types of events, the JavaScript event handlers are called before the default behavior is performed. If the handler doesn’t want the normal behavior to happen, typically because it has already taken care of handling the event, it can call the preventDefault method on the event object.

This can be used to implement your own keyboard shortcuts or context menu. It can also be used to obnoxiously interfere with the behavior that users expect. For example, here is a link that cannot be followed:

<a href="https://developer.mozilla.org/">MDN</a>
<script>
  var link = document.querySelector("a");
  link.addEventListener("click", function(event) {
    console.log("Nope.");
    event.preventDefault();
  });
</script>

Try not to do such things unless you have a really good reason to. For people using your page, it can be unpleasant when the behavior they expect is broken.

Depending on the browser, some events can’t be intercepted. On Chrome, for example, keyboard shortcuts to close the current tab (Ctrl-W or Command-W) cannot be handled by JavaScript.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.