article.md

Keyboard: keydown and keyup

Before we get to keyboard, please note that on modern devices there are other ways to "input something". For instance, people use speech recognition (especially on mobile devices) or copy/paste with the mouse.

So if we want to track any input into an <input> field, then keyboard events are not enough. There's another event named input to handle changes of an <input> field, by any means. And it may be a better choice for such task. We'll cover it later in the chapter info:events-change-input.

Keyboard events should be used when we want to handle keyboard actions (virtual keyboard also counts). For instance, to react on arrow keys key:Up and key:Down or hotkeys (including combinations of keys).

[cut]

Teststand [#keyboard-test-stand]

To better understand keyboard events, you can use the [teststand](sandbox:keyboard-dump).

To better understand keyboard events, you can use the teststand below.

Try different key combinations in the text field.

[codetabs src="keyboard-dump" height=480]

Keydown and keyup

The keydown events happens when a key is pressed down, and then keyup -- when it's released.

event.code and event.key

The key property of the event object allows to get the character, while the code property of the event object allows to get the "physical key code".

For instance, the same key key:Z can be pressed with or without Shift. That gives us two different characters: lowercase z and uppercase Z.

The event.key is exactly the character, and it will be different. But event.code is the same:

Key	event.key	event.code
key:Z	z (lowercase)	KeyZ
key:Shift+Z	Z (uppercase)	KeyZ

If a user works with different languages, then switching to another language would make a totally different character instead of "Z". That will become the value of event.key, while event.code is always the same: "KeyZ".

Every key has the code that depends on its location on the keyboard. Key codes described in the [UI Events code specification](https://www.w3.org/TR/uievents-code/).

For instance:
- Letter keys have codes `"Key<letter>"`: `"KeyA"`, `"KeyB"` etc.
- Digit keys have codes: `"Digit<number>"`: `"Digit0"`, `"Digit1"` etc.
- Special keys are coded by their names: `"Enter"`, `"Backspace"`, `"Tab"` etc.

There are several widespread keyboard layouts, and the specification gives key codes for each of them.

See [alphanumeric section of the spec](https://www.w3.org/TR/uievents-code/#key-alphanumeric-section) for more codes, or just try the [teststand](#keyboard-test-stand) above.

```warn header="Case matters: \"KeyZ\", not `"keyZ"`" Seems obvious, but people still make mistakes.

Please evade mistypes: it's KeyZ, not keyZ. The check like event.code=="keyZ" won't work: the first letter of "Key" must be uppercase.

What if a key does not give any character? For instance, `key:Shift` or `key:F1` or others. For those keys `event.key` is approximately the same as `event.code`:


| Key          | `event.key` | `event.code` |
|--------------|-------------|--------------|
| `key:F1`      |`F1`          |`F1`        |
| `key:Backspace`      |`Backspace`          |`Backspace`        |
| `key:Shift`|`Shift`          |`ShiftRight` or `ShiftLeft`        |

Please note that `event.code` specifies exactly which key is pressed. For instance, most keyboards have two `key:Shift` keys: on the left and on the right side. The `event.code` tells us exactly which one was pressed, and `event.key` is responsible for the "meaning" of the key: what it is (a "Shift").

Let's say, we want to handle a hotkey: `key:Ctrl+Z` (or `key:Cmd+Z` for Mac). Most text editors hook the "Undo" action on it. We can set a listener on `keydown` and check which key is pressed -- to detect when we have the hotkey.

Please answer the question -- in such a listener, should we check the value of `event.key` or `event.code`?

Please, pause and answer.

Made up your mind?

If you've got an understanding, then the answer is, of course, `event.code`, as we don't want `event.key` there. The value of `event.key` can change depending on the language or `CapsLock` enabled. The value of `event.code` is strictly bound to the key, so here we go:

```js run
document.addEventListener('keydown', function(event) {
  if (event.code == 'KeyZ' && (event.ctrlKey || event.metaKey)) {
    alert('Undo!')
  }
});

Auto-repeat

If a key is being pressed for a long enough time, it starts to repeat: the keydown triggers again and again, and then when it's released we finally get keyup. So it's kind of normal to have many keydown and a single keyup.

For all repeating keys the event object has event.return property set to true.

Default actions

Default actions vary, as there are many possible things that may be initiated by the keyboard.

For instance:

• A character appears on the screen (the most obvious outcome).
• A character is deleted (key:Delete key).
• The page is scrolled (key:PageDown key).
• The browser opens the "Save Page" dialog (key:Ctrl+S)
• ...and so on.

Preventing the default action on keydown can cancel most of them, with the exception of OS-based special keys. For instance, on Windows key:Alt+F4 closes the current browser window. And there's no way to stop it by preventing the default action in JavaScript.

For instance, the <input> below expects a phone number, so it does not accept keys except digits, +, () or -:

<script>
function checkPhoneKey(key) {
  return (key >= '0' && key <= '9') || key == '+' || key == '(' || key == ')' || key == '-';
}
</script>
<input *!*onkeydown="return checkPhoneKey(event.key)"*/!* placeholder="Phone, please" type="tel">

Please note that special keys like key:Backspace, key:Left, key:Right, key:Ctrl+V do not work in the input. That's a side-effect effect of the strict filter checkPhoneKey.

Let's relax it a little bit:

<script>
function checkPhoneKey(key) {
  return (key >= '0' && key <= '9') || key == '+' || key == '(' || key == ')' || key == '-' ||
    key == 'ArrowLeft' || key == 'ArrowRight' || key == 'Delete' || key == 'Backspace';
}
</script>
<input onkeydown="return checkPhoneKey(event.key)" placeholder="Phone, please" type="tel">

Now arrows and deletion works well.

...But we still can enter anything by using a mouse and right-click + Paste. So the filter is not 100% reliable. We can just let it be like that, because most of time it works. Or an alternative approach would be to track the input event -- it triggers after any modification. There we can check the new value and highlight/modify it when it's invalid.

Legacy

In the past, there was a keypress event, and also keyCode, charCode, which properties of the event object.

There were so many browser incompatibilities that developers of the specification decided to deprecate all of them. The old code still works, as the browser keep supporting them, but there's totally no need to use those any more.

There was time when this chapter included their detailed description. But as of now we can forget about those.

Summary

Pressing a key always generates a keyboard event, be it symbol keys or special keys like key:Shift or key:Ctrl and so on. The only exception is key:Fn key that sometimes presents on a laptop keyboard. There's no keyboard event for it, because it's often implemented on lower level than OS.

Keyboard events:

• keydown -- on pressing the key (auto-repeats if the key is pressed for long),
• keyup -- on releasing the key.

Main keyboard event properties:

• code -- the "key code" ("KeyA", "ArrowLeft" and so on), specific to the physical location of the key on keyboard.
• key -- the character ("A", "a" and so on), for non-character keys usually has the same value as code.

In the past, keyboard events were sometimes used to track user input in form fields. That's not reliable, because the input can come from various sources. We have input and change events to handle any input (covered later in the chapter info:events-change-input). They trigger after any input, including mouse or speech recognition.

We should use keyboard events when we really want keyboard. For example, to react on hotkeys or special keys.