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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key below:

KeyboardEvent: key property - Web APIs

Baseline Widely available

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.

A string.

Its value is determined as follows:

• 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. For example: if the pressed key is the Space key, the returned value is a single space (" "). If the pressed key is the B key, the returned value is the string "b". However, if the Shift key is pressed at the same time (so shiftKey is true), the returned value is the string "B".
• 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.

See a full list of key values.

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. A 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. A 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 earlier.

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.

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

.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;
}

const textarea = document.getElementById("test-target");
const consoleLog = document.getElementById("console-log");
const btnReset = document.getElementById("btn-reset");

function logMessage(message) {
  consoleLog.innerText += `${message}\n`;
}

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]`);
});

btnReset.addEventListener("click", (e) => {
  let child = consoleLog.firstChild;
  while (child) {
    consoleLog.removeChild(child);
    child = consoleLog.firstChild;
  }
  textarea.value = "";
});

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.

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.

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.

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",
  (event) => {
    if (event.defaultPrevented) {
      return; // Do nothing if the event was already processed
    }

    switch (event.key) {
      case "ArrowDown":
        // Do something for "down arrow" key press.
        break;
      case "ArrowUp":
        // Do something for "up arrow" key press.
        break;
      case "ArrowLeft":
        // Do something for "left arrow" key press.
        break;
      case "ArrowRight":
        // Do something for "right arrow" key press.
        break;
      case "Enter":
        // Do something for "enter" or "return" key press.
        break;
      case " ":
        // Do something for "space" key press.
        break;
      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,
);

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