Nossos voluntários ainda não traduziram este artigo para o Português (do Brasil). Junte-se a nós e ajude-nos a fazer o trabalho!
Você também pode ler o artigo em English (US).

The KeyboardEvent interface's key read-only property returns the value of the key pressed by the user, taking into consideration the state of modifier keys such as Shift as well as the keyboard locale and layout. Its value is determined as follows:

Key values

See a full list of key values.

  • If the pressed key has a printed representation, the returned value is a non-empty Unicode character string containing the printable representation of the key.
  • If the pressed key is a control or special character, the returned value is one of the pre-defined key values.
  • If the KeyboardEvent represents the press of a dead key, the key value must be "Dead".
  • Some specialty keyboard keys (such as the extended keys for controlling media on multimedia keyboards) don't generate key codes on Windows; instead, they trigger WM_APPCOMMAND events. These events get mapped to DOM keyboard events, and are listed among the "Virtual key codes" for Windows, even though they aren't actually key codes.
  • If the key cannot be identified, the returned value is "Unidentified".

KeyboardEvent sequence

Every KeyboardEvent is fired in a pre-determined sequence. For a given key press, the sequence of KeyboardEvents fired is as follows assuming that Event.preventDefault is not called:

  1. keydown event is first fired. If the key is held down further and the key produces a character key, then the event continues to be emitted in a platform implementation dependent interval and the KeyboardEvent.repeat read only property  is set to true.
  2. If the key produces a character key that would result in a character being inserted into possibly an <input>, <textarea> or an element with HTMLElement.contentEditable set to true, the beforeinput and input event types are fired in that order. Note that some other implementations may fire keypress event if supported. The events will be fired repeatedly while the key is held down.
  3. keyup event is fired once the key is released. This completes the process.

In sequence 1 & 3, the KeyboardEvent.key attribute is defined and is set appropriately to a value according to the rules defined ealier.

KeyboardEvent sequence example

Consider the event sequence generated when we interact with the Shift and the 2 key using a U.S keyboard layout as compared to when we do so using a UK keyboard layout.

Try experimenting using the following two test cases:

  1. Press and hold the Shift key, then press 2 and release it. Next, release the Shift key.
  2. Press and hold the Shift key, then press and hold 2. Release the Shift key. Finally, release 2.

HTML

<div class="fx">
  <div>
    <textarea rows="5" name="test-target" id="test-target"></textarea>
    <button type="button" name="btn-clear-console" id="btn-clear-console">clear console</button>
  </div>
  <div class="flex">
    <pre id="console-log"></div>
  </div>
</div>

CSS

.fx {
  -webkit-display: flex;
  display: flex;
  margin-left: -20px;
  margin-right: -20px;
}

.fx > div {
  padding-left: 20px;
  padding-right: 20px;
}

.fx > div:first-child {
   width: 30%;
}

.flex {
  -webkit-flex: 1;
  flex: 1;
}

#test-target {
  display: block;
  width: 100%;
  margin-bottom: 10px; 
}

JavaScript

let textarea = document.getElementById('test-target'),
consoleLog = document.getElementById('console-log'),
btnClearConsole = document.getElementById('btn-clear-console');

function logMessage(message) {
  document.getElementById("console-log").innerHTML += message + "<br>";
}

textarea.addEventListener('keydown', (e) => {
  if (!e.repeat)
    logMessage(`Key "${e.key}" pressed  [event: keydown]`);
  else
    logMessage(`Key "${e.key}" repeating  [event: keydown]`);
});

textarea.addEventListener('beforeinput', (e) => {
  logMessage(`Key "${e.data}" about to be input  [event: beforeinput]`);
});

textarea.addEventListener('input', (e) => {
  logMessage(`Key "${e.data}" input  [event: input]`);
});

textarea.addEventListener('keyup', (e) => {
  logMessage(`Key "${e.key}" released  [event: keyup]`);
});

btnClearConsole.addEventListener('click', (e) => {
  let child = consoleLog.firstChild;
  while (child) {
   consoleLog.removeChild(child);
   child = consoleLog.firstChild;
  }
});

Result

Note: On browsers that don't fully implement the InputEvent interface which is used for the beforeinput and input events, you may get incorrect output on those lines of the log output.

Case 1

When the shift key is pressed, a keydown event is first fired, and the key property value is set to the string "Shift". As we keep holding this key, the keydown event does not continue to fire repeatedly because it does not produce a character key.

When key 2 is pressed, another keydown event is fired for this new key press, and the key property value for the event is set to the string "@" for the U.S keyboard type and """ for the UK keyboard type, because of the active modifier shift key. The beforeinput and input events are fired next because a character key has been produced.

As we release the key 2, a keyup event is fired and the key property will maintain the string values "@" and """ for the different keyboard layouts respectively.

As we finally release the shift key, another keyup event is fired for it, and the key attribute value remains "Shift".

Case 2

When the shift key is pressed, a keydown event is first fired, and the key property value is set to be the string "Shift". As we keep holding this key, the keydown event does not continue to fire repeatedly because it produced no character key.

When key 2 is pressed, another keydown event is fired for this new key press, and the key property value for the event is set to be the string "@" for the U.S keyboard type and """ for the UK keyboard type, because of the active modifier shift key. The beforeinput and input events are fired next because a character key has been produced. As we keep holding the key, the keydown event continues to fire repeatedly and the KeyboardEvent.repeat  property is set to true. The beforeinput and input events are fired repeatedly as well.

As we release the shift key, a keyup event is fired for it, and the key attribute value remains "Shift". At this point, notice that the key property value for the repeating keydown event of the key 2 key press is now "2" because the modifier shift key is no longer active. The same goes for the InputEvent.data property of the beforeinput and input events.

As we finally release the key 2, a keyup event is fired but the key property will be set to the string value "2" for both keyboard layouts because the modifier shift key is no longer active.

Example

This example uses EventTarget.addEventListener() to listen for keydown events. When they occur,  the key's value is checked to see if it's one of the keys the code is interested in, and if it is, it gets processed in some way (possibly by steering a spacecraft, perhaps by changing the selected cell in a spreadsheet).

window.addEventListener("keydown", function (event) {
  if (event.defaultPrevented) {
    return; // Do nothing if the event was already processed
  }

  switch (event.key) {
    case "Down": // IE/Edge specific value
    case "ArrowDown":
      // Do something for "down arrow" key press.
      break;
    case "Up": // IE/Edge specific value
    case "ArrowUp":
      // Do something for "up arrow" key press.
      break;
    case "Left": // IE/Edge specific value
    case "ArrowLeft":
      // Do something for "left arrow" key press.
      break;
    case "Right": // IE/Edge specific value
    case "ArrowRight":
      // Do something for "right arrow" key press.
      break;
    case "Enter":
      // Do something for "enter" or "return" key press.
      break;
    case "Esc": // IE/Edge specific value
    case "Escape":
      // Do something for "esc" key press.
      break;
    default:
      return; // Quit when this doesn't handle the key event.
  }

  // Cancel the default action to avoid it being handled twice
  event.preventDefault();
}, true);

Specification

Specification Status Comment
Document Object Model (DOM) Level 3 Events Specification
The definition of 'KeyboardEvent.key' in that specification.
Obsolete Initial definition, included key values.
UI Events
The definition of 'KeyboardEvent.key' in that specification.
Working Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
keyChrome Full support 51Edge Full support 12Firefox Full support 23IE Full support 9
Notes
Full support 9
Notes
Notes IE's implementation does not completely match the current spec because it is based on an older version of the spec.
Opera Full support 38Safari Full support YesWebView Android Full support 51Chrome Android Full support 51Firefox Android Full support 23Opera Android Full support 41Safari iOS Full support YesSamsung Internet Android Full support Yes
Non-printable keysChrome Full support 51Edge ? Firefox Full support 23IE Full support 9
Notes
Full support 9
Notes
Notes IE's implementation does not completely match the current spec because it is based on an older version of the spec.
Opera Full support 38Safari ? WebView Android Full support 51Chrome Android Full support 51Firefox Android Full support 23Opera Android Full support 41Safari iOS ? Samsung Internet Android ?
Printable keysChrome Full support 51Edge ? Firefox Full support 29IE Full support 9
Notes
Full support 9
Notes
Notes IE's implementation does not completely match the current spec because it is based on an older version of the spec.
Opera Full support 38Safari ? WebView Android Full support 51Chrome Android Full support 51Firefox Android Full support 29Opera Android Full support 41Safari iOS ? Samsung Internet Android ?
Dead keyChrome Full support 51Edge ? Firefox No support NoIE No support NoOpera Full support 38Safari ? WebView Android Full support 51Chrome Android Full support 51Firefox Android No support NoOpera Android Full support 41Safari iOS ? Samsung Internet Android ?

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
See implementation notes.
See implementation notes.