28º agosto 2020
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Focusing: focus/blur

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

Focusing on an element generally means: “prepare to accept the data here”, so that’s the moment when we can run the code to initialize the required functionality.

The moment of losing the focus (“blur”) can be even more important. That’s when a user clicks somewhere else or presses 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 further on.

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 has an email entered, and if not – shows an error.
  • The focus handler hides the error message (on blur it will be checked again):
  .invalid { border-color: red; }
  #error { color: red }

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

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

input.onblur = function() {
  if (!input.value.includes('@')) { // not email
    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
    error.innerHTML = "";

Modern HTML allows us 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 to 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:

  .error {
    background: red;

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

  input.onblur = function() {
    if (!this.value.includes('@')) { // not email
      // show the error
      // ...and put the focus back
    } else {

It works in all browsers except Firefox (bug).

If we enter something into the input and then try to use 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.

JavaScript-initiated focus loss

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 avoid causing it ourselves.

Allow focusing on any element: tabindex

By default many elements do not support focusing.

The list varies a bit 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.

On the other hand, elements that exist to format something, such as <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.

Any element becomes focusable if it has tabindex. The value of the attribute is the order number of the element when Tab (or something like that) 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 Tab while in the first element – moves the focus into the second one.

The switch order is: elements with tabindex from 1 and above go first (in the tabindex order), and then elements without tabindex (e.g. a regular <input>).

Elements with matching tabindex are switched in the document source order (the default order).

There are two special values:

  • tabindex="0" puts an element among those without tabindex. That is, when we switch elements, elements with tabindex=0 go after elements with tabindex ≥ 1.

    Usually it’s used to make an element focusable, but keep the default switching order. To make an element a part of the form on par with <input>.

  • tabindex="-1" allows only programmatic focusing on an element. The Tab key ignores such elements, but method elem.focus() works.

For instance, here’s a list. Click the first item and press 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 in the example.
  <li tabindex="1">One</li>
  <li tabindex="0">Zero</li>
  <li tabindex="2">Two</li>
  <li tabindex="-1">Minus one</li>

  li { cursor: pointer; }
  :focus { outline: 1px dashed green; }

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

The property 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:

<!-- 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">

<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">

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

  // 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);

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">

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

  form.addEventListener("focusin", () => form.classList.add('focused'));
  form.addEventListener("focusout", () => form.classList.remove('focused'));


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

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.


importancia: 5

Create a <div> that turns into <textarea> when clicked.

The textarea allows to edit the HTML in the <div>.

When the user presses Enter or it loses focus, the <textarea> turns back into <div>, and its content becomes HTML in <div>.

Demo en nueva ventana

Abrir un entorno controlado para la tarea.

importancia: 5

Make table cells editable on click.

  • On click – the cell should became “editable” (textarea appears inside), we can change HTML. There should be no resize, all geometry should remain the same.
  • Buttons OK and CANCEL appear below the cell to finish/cancel the editing.
  • Only one cell may be editable at a moment. While a <td> is in “edit mode”, clicks on other cells are ignored.
  • The table may have many cells. Use event delegation.

The demo:

Abrir un entorno controlado para la tarea.

  1. On click – replace innerHTML of the cell by <textarea> with same sizes and no border. Can use JavaScript or CSS to set the right size.
  2. Set textarea.value to td.innerHTML.
  3. Focus on the textarea.
  4. Show buttons OK/CANCEL under the cell, handle clicks on them.

Abrir la solución en un entorno controlado.

importancia: 4

Focus on the mouse. Then use arrow keys to move it:

Demo en nueva ventana

P.S. Don’t put event handlers anywhere except the #mouse element. P.P.S. Don’t modify HTML/CSS, the approach should be generic and work with any element.

Abrir un entorno controlado para la tarea.

We can use mouse.onclick to handle the click and make the mouse “moveable” with position:fixed, then mouse.onkeydown to handle arrow keys.

The only pitfall is that keydown only triggers on elements with focus. So we need to add tabindex to the element. As we’re forbidden to change HTML, we can use mouse.tabIndex property for that.

P.S. We also can replace mouse.onclick with mouse.onfocus.

Abrir la solución en un entorno controlado.

Mapa del Tutorial


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