KeyboardEvent.keyCode

The KeyboardEvent.keyCode read-only property represents a system and implementation dependent numerical code identifying the unmodified value of the pressed key. This is usually the decimal ASCII (RFC 20) or Windows 1252 code corresponding to the key. If the key can't be identified, this value is 0.

The value of keypress event is different between browsers. IE and Google Chrome set the KeyboardEvent.charCode value. Gecko sets 0 if the pressed key is a printable key, otherwise it sets the same keyCode as a keydown or keyup event.

You should avoid using this if possible; it's been deprecated for some time. Instead, you should use KeyboardEvent.code, if it's implemented. Unfortunately, some browsers still don't have it, so you'll have to be careful to make sure you use one which is supported on all target browsers. Google Chrome and Safari have implemented KeyboardEvent.keyIdentifier, which was defined in a draft specification but not the final spec.

Example

window.addEventListener("keydown", function (event) {
  if (event.defaultPrevented) {
    return; // Should do nothing if the default action has been cancelled
  }

  var handled = false;
  if (event.key !== undefined) {
    // Handle the event with KeyboardEvent.key and set handled true.
  } else if (event.keyIdentifier !== undefined) {
    // Handle the event with KeyboardEvent.keyIdentifier and set handled true.
  } else if (event.keyCode !== undefined) {
    // Handle the event with KeyboardEvent.keyCode and set handled true.
  }

  if (handled) {
    // Suppress "double action" if event handled
    event.preventDefault();
  }
}, true);

Specifications

Browser compatibility

Value of keyCode

Printable keys in standard position

The value of key events which are caused by pressing or releasing printable keys in standard position is not compatible between browsers.

IE just exposes the native virtual keycode value as KeyboardEvent.keyCode.

Google Chrome, Chromium and Safari must decide the value from the input character. If the inputting character can be inputted with US keyboard layout, they use the keyCode value on US keyboard layout.

Starting in Gecko 15 (Firefox 15.0 / Thunderbird 15.0 / SeaMonkey 2.12), Gecko decides keyCode value from an ASCII character which is inputtable by the key even with shift modifier or ASCII capable keyboard layout.  See the following rules for details:

1. If the system is Windows and native keycode of pressed key indicates that the key is a-z or 0-9, use a keycode for it.
2. If the system is Mac and native keycode of pressed key indicates that the key is 0-9, use a keycode for it.
3. If pressed key inputs an ASCII alphabetic or numeric with no modifier key, use a keycode for it.
4. If pressed key inputs an ASCII alphabetic or numeric with Shift key, use a keycode for it.
5. If pressed key inputs another ASCII character with no modifier key, use a keycode for it.
6. If pressed key inputs another ASCII character with Shift key, use a keycode for it.
7. Otherwise, i.e., pressed key inputs a Unicode character:
   1. If the keyboard layout is ASCII capable keyboard layout (i.e., can input ASCII alphabets), use 0.
   2. Otherwise, i.e., the keyboard layout isn't ASCII capable, using ASCII capable keyboard layout which is installed on the environment and has highest priority:
      1. If pressed key on the alternative keyboard layout inputs an ASCII alphabetic or numeric, use a keycode for it.
      2. Otherwise, use 0.

*1 The value is input from JIS keyboard. When you use ANSI keyboard, the keyCode value and inputted character are what you select from the US keyboard layout.

*2 The key is a dead key. The value of keyup event is 0xBA (186).

*3 The key is a dead key. The value of keyup event is 0x10 (16).

*4 No key events are fired.

*5 The key isn't available with Greek keyboard layout (does not input any character). The value of keyup event is 0x00 (0).

Non-printable keys (function keys)

*1 On Windows, "AltGraph" key causes "ControlLeft" key event and "AltRight" key event.

*2 When Japanese keyboard layout is active, "CapsLock" key without Shift causes 0xF0 (240). The key works as "Alphanumeric" key whose label is "英数".

*3 When Japanese keyboard layout is active, "CapsLock" key without Shift causes 0x00 (0). The key works as "Alphanumeric" key whose label is "英数".

*1 keypress event is fired whose keyCode and charCode are 0x10 (16) but text isn't inputted into editor.

*2 On Mac, "Help" key is mapped to "Insert" key of PC keyboard. These keyCode values are the same as the "Insert" key's keyCode value.

*3 Tested on Fedora 20.

*4 Only keyup event is fired.

*5 PC's "PrintScreen", "ScrollLock" and "Pause" are mapped to Mac's "F13", "F14" and "F15". Chrome and Safari map same keyCode values with Mac's keys.

*6 "Pause" key with Control causes 0x03 (3).

*1 Tested on Fedora 20.

*2 PC's "PrintScreen", "ScrollLock" and "Pause" are mapped to Mac's "F13", "F14" and "F15". Firefox maps same keyCode values with PC's keys.

*3 Tested on Fedora 20. The keys don't cause GDK_Fxx keysyms. If the keys cause proper keysyms, these values must be same as IE.

*4 Tested on Fedora 20. The keys don't cause DOM key events on Chromium.

*5 keyUp event's keyCode value is 0x83 (131).

*6 Tested on Fedora 20. The keys don't cause DOM key events on Firefox.

*7 Only keydown event is fired.

*8 No DOM key events are fired on Firefox.

Numpad keys

*1 "NumLock" key works as "Clear" key on Mac.

* Recent Mac doesn't have "NumLock" key and state. Therefore, unlocked state isn't available.

Constants for keyCode value

Gecko defines a lot of keyCode values in KeyboardEvent for making the mapping table explicitly. These values are useful for add-on developers of Firefox, but not so useful in public web pages.

OEM specific keys on Windows

On Windows, some values of virtual keycode are defined (reserved) for OEM specific key. They are available for special keys on non-standard keyboard. In other words, some values are used for different meaning by two or more vendors (or hardware).

Starting Gecko 21 (and older than 15), OEM specific key values are available on the keyCode attribute only on Windows. So they are not useful for usual web applications. They are useful only for intranet applications or in similar situations.

See "Manufacturer-specific Virtual-Key Codes (Windows CE 5.0)" in MSDN for the detail.