Browser default actions

Many events automatically lead to browser actions.

For instance:

• A click on a link – initiates going to its URL.
• A click on submit button inside a form – initiates its submission to the server.
• Pressing a mouse button over a text and moving it – selects the text.

If we handle an event in JavaScript, often we don’t want browser actions. Fortunately, it can be prevented.

Preventing browser actions

There are two ways to tell the browser we don’t want it to act:

• The main way is to use the event object. There’s a method event.preventDefault().
• If the handler is assigned using on<event> (not by addEventListener), then we can just return false from it.

In the example below a click to links doesn’t lead to URL change:

<a href="/" onclick="return false">Click here</a>
or
<a href="/" onclick="event.preventDefault()">here</a>

Example: the menu

Consider a site menu, like this:

<ul id="menu" class="menu">
  <li><a href="/html">HTML</a></li>
  <li><a href="/javascript">JavaScript</a></li>
  <li><a href="/css">CSS</a></li>
</ul>

Here’s how it looks with some CSS:

Menu items are links <a>, not buttons. There are several benefits, for instance:

• Many people like to use “right click” – “open in a new window”. If we use <button> or <span>, that doesn’t work.
• Search engines follow <a href="..."> links while indexing.

So we use <a> in the markup. But normally we intend to handle clicks in JavaScript. So we should prevent the default browser action.

Like here:

menu.onclick = function(event) {
  if (event.target.nodeName != 'A') return;

  let href = event.target.getAttribute('href');
  alert( href ); // ...can be loading from the server, UI generation etc

  return false; // prevent browser action (don't go to the URL)
};

If we omit return false, then after our code executes the browser will do its “default action” – following to the URL in href.

By the way, using event delegation here makes our menu flexible. We can add nested lists and style them using CSS to “slide down”.

Prevent further events

Certain events flow one into another. If we prevent the first event, there will be no second.

For instance, mousedown on an <input> field leads to focusing in it, and the focus event. If we prevent the mousedown event, there’s no focus.

Try to click on the first <input> below – the focus event happens. That’s normal.

But if you click the second one, there’s no focus.

<input value="Focus works" onfocus="this.value=''">
<input onmousedown="return false" onfocus="this.value=''" value="Click me">

That’s because the browser action is canceled on mousedown. The focusing is still possible if we use another way to enter the input. For instance, the Tab key to switch from the 1st input into the 2nd. But not with the mouse click any more.

The “passive” handler option

The optional passive: true option of addEventListener signals the browser that the handler is not going to call preventDefault().

Why that may be needed?

There are some events like touchmove on mobile devices (when the user moves their finger across the screen), that cause scrolling by default, but that scrolling can be prevented using preventDefault() in the handler.

So when the browser detects such event, it has first to process all handlers, and then if preventDefault is not called anywhere, it can proceed with scrolling. That may cause unnecessary delays and “jitters” in the UI.

The passive: true options tells the browser that the handler is not going to cancel scrolling. Then browser scrolls immediately providing a maximally fluent experience, and the event is handled by the way.

For some browsers (Firefox, Chrome), passive is true by default for touchstart and touchmove events.

event.defaultPrevented

The property event.defaultPrevented is true if the default action was prevented, and false otherwise.

There’s an interesting use case for it.

You remember in the chapter Bubbling and capturing we talked about event.stopPropagation() and why stopping bubbling is bad?

Sometimes we can use event.defaultPrevented instead.

Let’s see a practical example where stopping the bubbling looks necessary, but actually we can do well without it.

By default the browser on contextmenu event (right mouse click) shows a context menu with standard options. We can prevent it and show our own, like this:

<button>Right-click for browser context menu</button>

<button oncontextmenu="alert('Draw our menu'); return false">
  Right-click for our context menu
</button>

Now let’s say we want to implement our own document-wide context menu, with our options. And inside the document we may have other elements with their own context menus:

<p>Right-click here for the document context menu</p>
<button id="elem">Right-click here for the button context menu</button>

<script>
  elem.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Button context menu");
  };

  document.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Document context menu");
  };
</script>

The problem is that when we click on elem, we get two menus: the button-level and (the event bubbles up) the document-level menu.

How to fix it? One of solutions is to think like: “We fully handle the event in the button handler, let’s stop it” and use event.stopPropagation():

<p>Right-click for the document menu</p>
<button id="elem">Right-click for the button menu (fixed with event.stopPropagation)</button>

<script>
  elem.oncontextmenu = function(event) {
    event.preventDefault();
    event.stopPropagation();
    alert("Button context menu");
  };

  document.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Document context menu");
  };
</script>

Now the button-level menu works as intended. But the price is high. We forever deny access to information about right-clicks for any outer code, including counters that gather statistics and so on. That’s quite unwise.

An alternative solution would be to check in the document handler if the default action was prevented? If it is so, then the event was handled, and we don’t need to react on it.

<p>Right-click for the document menu (fixed with event.defaultPrevented)</p>
<button id="elem">Right-click for the button menu</button>

<script>
  elem.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Button context menu");
  };

  document.oncontextmenu = function(event) {
    if (event.defaultPrevented) return;

    event.preventDefault();
    alert("Document context menu");
  };
</script>

Now everything also works correctly. If we have nested elements, and each of them has a context menu of its own, that would also work. Just make sure to check for event.defaultPrevented in each contextmenu handler.

Summary

There are many default browser actions:

• mousedown – starts the selection (move the mouse to select).
• click on <input type="checkbox"> – checks/unchecks the input.
• submit – clicking an <input type="submit"> or hitting Enter inside a form field causes this event to happen, and the browser submits the form after it.
• wheel – rolling a mouse wheel event has scrolling as the default action.
• keydown – pressing a key may lead to adding a character into a field, or other actions.
• contextmenu – the event happens on a right-click, the action is to show the browser context menu.
• …there are more…

All the default actions can be prevented if we want to handle the event exclusively by JavaScript.

To prevent a default action – use either event.preventDefault() or return false. The second method works only for handlers assigned with on<event>.

The passive: true option of addEventListener tells the browser that the action is not going to be prevented. That’s useful for some mobile events, like touchstart and touchmove, to tell the browser that it should not wait for all handlers to finish before scrolling.

If the default action was prevented, the value of event.defaultPrevented becomes true, otherwise it’s false.