KeyboardEvent.key

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 ealier.

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

   .

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",
  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,
);