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 track 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 Events: change, input, cut, copy, paste.

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

Teststand

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

Try different key combinations in the text field.

نتیجه

script.js

style.css

index.html

kinput.onkeydown = kinput.onkeyup = kinput.onkeypress = handle;

let lastTime = Date.now();

function handle(e) {
  if (form.elements[e.type + 'Ignore'].checked) return;

  area.scrollTop = 1e6;

  let text = e.type +
    ' key=' + e.key +
    ' code=' + e.code +
    (e.shiftKey ? ' shiftKey' : '') +
    (e.ctrlKey ? ' ctrlKey' : '') +
    (e.altKey ? ' altKey' : '') +
    (e.metaKey ? ' metaKey' : '') +
    (e.repeat ? ' (repeat)' : '') +
    "\n";

  if (area.value && Date.now() - lastTime > 250) {
    area.value += new Array(81).join('-') + '\n';
  }
  lastTime = Date.now();

  area.value += text;

  if (form.elements[e.type + 'Stop'].checked) {
    e.preventDefault();
  }
}

#kinput {
  font-size: 150%;
  box-sizing: border-box;
  width: 95%;
}

#area {
  width: 95%;
  box-sizing: border-box;
  height: 250px;
  border: 1px solid black;
  display: block;
}

form label {
  display: inline;
  white-space: nowrap;
}

<!DOCTYPE HTML>
<html>

<head>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <form id="form" onsubmit="return false">

    Prevent default for:
    <label>
      <input type="checkbox" name="keydownStop" value="1"> keydown</label>&nbsp;&nbsp;&nbsp;
    <label>
      <input type="checkbox" name="keyupStop" value="1"> keyup</label>

    <p>
      Ignore:
      <label>
        <input type="checkbox" name="keydownIgnore" value="1"> keydown</label>&nbsp;&nbsp;&nbsp;
      <label>
        <input type="checkbox" name="keyupIgnore" value="1"> keyup</label>
    </p>

    <p>Focus on the input field and press a key.</p>

    <input type="text" placeholder="Press keys here" id="kinput">

    <textarea id="area" readonly></textarea>
    <input type="button" value="Clear" onclick="area.value = ''" />
  </form>
  <script src="script.js"></script>


</body>
</html>

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 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`
`Z`	`z` (lowercase)	`KeyZ`
`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.

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.

Read the alphanumeric section of the spec for more codes, or just press a key in the teststand above.

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, Shift or F1 or others. For those keys, event.key is approximately the same as event.code:

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

Please note that event.code specifies exactly which key is pressed. For instance, most keyboards have two 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: Ctrl+Z (or 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.

There’s a dilemma here: in such a listener, should we check the value of event.key or event.code?

On one hand, the value of event.key is a character, it changes depending on the language. If the visitor has several languages in OS and switches between them, the same key gives different characters. So it makes sense to check event.code, it’s always the same.

Like this:

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

On the other hand, there’s a problem with event.code. For different keyboard layouts, the same key may have different characters.

For example, here are US layout (“QWERTY”) and German layout (“QWERTZ”) under it (from Wikipedia):

For the same key, US layout has “Z”, while German layout has “Y” (letters are swapped).

Literally, event.code will equal KeyZ for people with German layout when they press Y.

If we check event.code == 'KeyZ' in our code, then for people with German layout such test will pass when they press Y.

That sounds really odd, but so it is. The specification explicitly mentions such behavior.

So, event.code may match a wrong character for unexpected layout. Same letters in different layouts may map to different physical keys, leading to different codes. Luckily, that happens only with several codes, e.g. keyA, keyQ, keyZ (as we’ve seen), and doesn’t happen with special keys such as Shift. You can find the list in the specification.

To reliably track layout-dependent characters, event.key may be a better way.

On the other hand, event.code has the benefit of staying always the same, bound to the physical key location. So hotkeys that rely on it work well even in case of a language switch.

Do we want to handle layout-dependant keys? Then event.key is the way to go.

Or we want a hotkey to work even after a language switch? Then event.code may be better.

Auto-repeat

If a key is being pressed for a long enough time, it starts to “auto-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 events triggered by auto-repeat, the event object has event.repeat 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 (Delete key).
The page is scrolled (PageDown key).
The browser opens the “Save Page” dialog (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 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') || ['+','(',')','-'].includes(key);
}
</script>
<input onkeydown="return checkPhoneKey(event.key)" placeholder="Phone, please" type="tel">

The onkeydown handler here uses checkPhoneKey to check for the key pressed. If it’s valid (from 0..9 or one of +-()), then it returns true, otherwise false.

As we know, the false value returned from the event handler, assigned using a DOM property or an attribute, such as above, prevents the default action, so nothing appears in the <input> for keys that don’t pass the test. (The true value returned doesn’t affect anything, only returning false matters)

Please note that special keys, such as Backspace, Left, Right, do not work in the input. That’s a side effect of the strict filter checkPhoneKey. These keys make it return false.

Let’s relax the filter a little bit by allowing arrow keys Left, Right and Delete, Backspace:

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

Now arrows and deletion works well.

Even though we have the key filter, one still can enter anything using a mouse and right-click + Paste. Mobile devices provide other means to enter values. So the filter is not 100% reliable.

The alternative approach would be to track the oninput event – it triggers after any modification. There we can check the new input.value and modify it/highlight the <input> when it’s invalid. Or we can use both event handlers together.

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 while working with them, that developers of the specification had no way, other than deprecating all of them and creating new, modern events (described above in this chapter). The old code still works, as browsers keep supporting them, but there’s totally no need to use those any more.

Mobile Keyboards

When using virtual/mobile keyboards, formally known as IME (Input-Method Editor), the W3C standard states that a KeyboardEvent’s e.keyCode should be 229 and e.key should be "Unidentified".

While some of these keyboards might still use the right values for e.key, e.code, e.keyCode… when pressing certain keys such as arrows or backspace, there’s no guarantee, so your keyboard logic might not always work on mobile devices.

Summary

Pressing a key always generates a keyboard event, be it symbol keys or special keys like Shift or Ctrl and so on. The only exception is 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, such as Esc, 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 Events: change, input, cut, copy, paste). They trigger after any kind of input, including copy-pasting or speech recognition.

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

Keyboard: keydown and keyup

Teststand

Keydown and keyup

event.code and event.key

Auto-repeat

Default actions

Legacy

Mobile Keyboards

Summary

تمارین

Extended hotkeys

نظرات

فصل

مطالب درس