article.md

Focusing: focus/blur

An element receives a focus when the user either clicks on it or uses the key:Tab key on the keyboard. There's also an autofocus HTML attribute that puts the focus into an element by default when a page loads and other means of getting a focus.

Focusing generally means: "prepare to accept the data here", so that's the moment when we can run the code to initialize or load something.

The moment of losing the focus ("blur") can be even more important. That's when a user clicks somewhere else or presses key:Tab to go to the next form field, or there are other means as well.

Losing the focus generally means: "the data has been entered", so we can run the code to check it or even to save it to the server and so on.

There are important peculiarities when working with focus events. We'll do the best to cover them here.

Events focus/blur

The focus event is called on focusing, and blur -- when the element loses the focus.

Let's use them for validation of an input field.

In the example below:

• The blur handler checks if the field the email is entered, and if not -- shows an error.
• The focus handler hides the error message (on blur it will be checked again):

<style>
  .invalid { border-color: red; }
  #error { color: red }
</style>

Your email please: <input type="email" id="input">

<div id="error"></div>

<script>
*!*input.onblur*/!* = function() {
  if (!input.value.includes('@')) { // not email
    input.classList.add('invalid');
    error.innerHTML = 'Please enter a correct email.'
  }
};

*!*input.onfocus*/!* = function() {
  if (this.classList.contains('invalid')) {
    // remove the "error" indication, because the user wants to re-enter something
    this.classList.remove('invalid');
    error.innerHTML = "";
  }
};
</script>

Modern HTML allows to do many validations using input attributes: required, pattern and so on. And sometimes they are just what we need. JavaScript can be used when we want more flexibility. Also we could automatically send the changed value on the server if it's correct.

Methods focus/blur

Methods elem.focus() and elem.blur() set/unset the focus on the element.

For instance, let's make the visitor unable to leave the input if the value is invalid:

<style>
  .error {
    background: red;
  }
</style>

Your email please: <input type="email" id="input">
<input type="text" style="width:220px" placeholder="make email invalid and try to focus here">

<script>
  input.onblur = function() {
    if (!this.value.includes('@')) { // not email
      // show the error
      this.classList.add("error");
*!*
      // ...and put the focus back
      input.focus();
*/!*
    } else {
      this.classList.remove("error");
    }
  };
</script>

It works in all browsers except Firefox (bug).

If we enter something into the input and then try to use key:Tab or click away from the <input>, then onblur returns the focus back.

Please note that we can't "prevent losing focus" by calling event.preventDefault() in onblur, because onblur works after the element lost the focus.

A focus loss can occur for many reasons.

One of them is when the visitor clicks somewhere else. But also JavaScript itself may cause it, for instance:

- An `alert` moves focus to itself, so it causes the focus loss at the element (`blur` event), and when the `alert` is dismissed, the focus comes back (`focus` event).
- If an element is removed from DOM, then it also causes the focus loss. If it is reinserted later, then the focus doesn't return.

These features sometimes cause `focus/blur` handlers to misbehave -- to trigger when they are not needed.

The best recipe is to be careful when using these events. If we want to track user-initiated focus-loss, then we should evade causing it by ourselves.

Allow focusing on any element: tabindex

By default many elements do not support focusing.

The list varies between browsers, but one thing is always correct: focus/blur support is guaranteed for elements that a visitor can interact with: <button>, <input>, <select>, <a> and so on.

From the other hand, elements that exist to format something like <div>, <span>, <table> -- are unfocusable by default. The method elem.focus() doesn't work on them, and focus/blur events are never triggered.

This can be changed using HTML-attribute tabindex.

The purpose of this attribute is to specify the order number of the element when key:Tab is used to switch between them.

That is: if we have two elements, the first has tabindex="1", and the second has tabindex="2", then pressing key:Tab while in the first element -- moves us to the second one.

There are two special values:

• tabindex="0" makes the element the last one.
• tabindex="-1" means that key:Tab should ignore that element.

Any element supports focusing if it has tabindex.

For instance, here's a list. Click the first item and press key:Tab:

Click the first item and press Tab. Keep track of the order. Please note that many subsequent Tabs can move the focus out of the iframe with the example.
<ul>
  <li tabindex="1">One</li>
  <li tabindex="0">Zero</li>
  <li tabindex="2">Two</li>
  <li tabindex="-1">Minus one</li>
</ul>

<style>
  li { cursor: pointer; }
  :focus { outline: 1px dashed green; }
</style>

The order is like this: 1 - 2 - 0 (zero is always the last). Normally, <li> does not support focusing, but tabindex full enables it, along with events and styling with :focus.

```smart header="elem.tabIndex works too" We can add `tabindex` from JavaScript by using the `elem.tabIndex` property. That has the same effect.

## Delegation: focusin/focusout

Events `focus` and `blur` do not bubble.

For instance, we can't put `onfocus` on the `<form>` to highlight it, like this:

```html autorun height=80
<!-- on focusing in the form -- add the class -->
<form *!*onfocus="this.className='focused'"*/!*>
  <input type="text" name="name" value="Name">
  <input type="text" name="surname" value="Surname">
</form>

<style> .focused { outline: 1px solid red; } </style>

The example above doesn't work, because when user focuses on an <input>, the focus event triggers on that input only. It doesn't bubble up. So form.onfocus never triggers.

There are two solutions.

First, there's a funny historical feature: focus/blur do not bubble up, but propagate down on the capturing phase.

This will work:

<form id="form">
  <input type="text" name="name" value="Name">
  <input type="text" name="surname" value="Surname">
</form>

<style> .focused { outline: 1px solid red; } </style>

<script>
*!*
  // put the handler on capturing phase (last argument true)
  form.addEventListener("focus", () => form.classList.add('focused'), true);
  form.addEventListener("blur", () => form.classList.remove('focused'), true);
*/!*
</script>

Second, there are focusin and focusout events -- exactly the same as focus/blur, but they bubble.

Note that they must be assigned using elem.addEventListener, not on<event>.

So here's another working variant:

<form id="form">
  <input type="text" name="name" value="Name">
  <input type="text" name="surname" value="Surname">
</form>

<style> .focused { outline: 1px solid red; } </style>

<script>
*!*
  // put the handler on capturing phase (last argument true)
  form.addEventListener("focusin", () => form.classList.add('focused'));
  form.addEventListener("focusout", () => form.classList.remove('focused'));
*/!*
</script>

Summary

Events focus and blur trigger on focusing/losing focus on the element.

Their specials are:

• They do not bubble. Can use capturing state instead or focusin/focusout.
• Most elements do not support focus by default. Use tabindex to make anything focusable.

The current focused element is available as document.activeElement.