Laurent Dami
and 1 contributors


Alien::GvaScript::KeyMap - Manage maps of handlers for key events


  var rules = {
    // attach handlers to specific keys
    RETURN: function(event){doSomethingWith(event)},  
    C_DOWN: ctrlArrowDownHandler,       
    C_S_F7: ctrlShiftF7Handler,
    // special rules using regular expressions
    REGEX: [ ["",   /^[0-9]$/,      digitHandler  ],
             ["C_", /^[aeiou]$/i, ctrlVowelHandler] ], 
    // use Ctrl-X as a prefix for another set of rules
    C_X: KeyMap.Prefix({R: ctrlX_R_handler,
                        4: ctrlX_4_handler})
  // create a keymap object
  var aKeyMap = new KeyMap(rules);

  // attach the corresponding handler to the keydown event (on document)
  // other way to attach : manually insert handler
  document.onkeydown = aKeyMap.eventHandler({preventDefault: true,
                                             ignoreShift   : true});

  // dynamically change the map
  // idem, temporarily ignore all keys
  // back to previous handling state


Provides an abstraction layer for associating handlers with HTML key events, in a browser-independent way.

A keymap is a stack of collections of rules. Each rule has a key specification or a regexp specification, and a handler to be called whenever the specification is met. The keymap object as a whole can then be registered as a usual HTML event handler associated to some DOM element (most often the document element), and will dispatch key events to appropriate handlers.

Key specifications look like A (key 'A'), C_S_A (control-shift-A), A_DELETE (alt-Delete). They are formed from :


For printable characters, the keyname is just that character; for special editing keys such as backspace, arrow up, etc., names are taken from the following list of builtins :

  LEFT      UP         RIGHT  DOWN
  F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12
  CTRL      SHIFT      ALT

Modifiers are specified through prefixes C_, S_ and A_, corresponding to key modifiers control, shift and alt. Several prefixes may be combined, but must appear in the order just given (so for example S_C_A would be illegal).

Alternatively, key specifications may also formed from key codes instead of key names, so for example C_13 is equivalent to C_RETURN. For key codes 0-9, and additional '0' is required to avoid confusion with digits: so C_09 is equivalent to C_TAB, while C_9 means "control-numeric 9".

In addition, keymap objects can also manage regex rules that cover several possible key events; details are given below.


Following the W3C event model, handlers called from the keymap object receive an event object as argument. This is the usual HTML event object, augmented with two properties keyName and keyModifiers, computed according to the specifications given above. So for example a simple handler can be

  var myHandler = function (event) {
    alert(event.keyName + " was pressed with modifiers " + 

Further propagation of the event to other handlers is cancelled by default : W3C methods event.stopPropagation() and event.preventDefault() are called automatically by the keymap object (or, if running under Microsoft Internet Explorer, property cancelBubble is set to true and and property returnValue is set to false). This default behaviour can be disabled if necessary, as explained below.


Keymaps may be attached to HTML elements on the keydown, keypress or keyup event types. Choosing the proper event type is important, as it affects not only the time at which events are fired, but also the returned keycodes :

keydown and keyup

These are "low-level" event types that capture almost every key on the keyboard, including special keys like ESCAPE, F1, PAGE UP, etc. Returned key codes remain at a raw level, i.e. they are not translated into characters. This means that if Shift-1 is marked on your keyboard as an exclamation mark, a plus sign, or some other special character, you will not receive that keycode when capturing keydown events : rather, you will receive keycode 49 (ASCII character '1'). Similarly, all letters are received as uppercase.


By contrast, the keypress event type is higher-level in that it performs the translation from keys to characters, according to your specific keyboard. However, this event type only fires for printable characters, so you cannot observe keypress if you intend to capture special keys such as arrow keys, function keys, etc.

In theory, attributes such as onkeydown or onkeypress may be used with most HTML elements; but in practice, most of them will actually never fire the key events! So the most common and most sensible way for capturing key events is to attach to the document element.

Events keypress and keydown will repeat if the key is held down.

In order to attach the keymap to an element, you can either use the supplied "observe" method, or call the "eventHandler" method to get the keymap event handler, and then use your favorite technique to attach that handler to an element.



  var myKeyMap = new KeyMap(rules);

Constructor for a keymap object.

Single-key rules

The rules argument is a map from key specifications to handlers, like for example

  { A:     function() {alert("pressed 'A'");},
    S_TAB: function() {alert("pressed 'Shift-Tab'");},
    CTRL:  function() {alert("pressed the 'Ctrl' key");},
    10:    function() {alert("pressed 'Linefeed' or maybe 'Ctrl-Return'");}

Each key specification in the map corresponds to exacly one key combination, so for example S_TAB will not fire if the user pressed Ctrl-Shift-Tab.

Regex rules

For situations where several key combination will fire the same handler, you can insert a REGEX entry in the map. This should be an array of triplets, where each triplet is of shape [modifiers, regex, handler], like for example

  var regexRules = [["C_",   "[0-9]",             myCtrlDigitHandler],
                    ["C_S_", /^[AEIOU]$/,         myCtrlShiftVowelHandler],
                    [null,   "RETURN|TAB|ESCAPE", someOtherHandler]   ];

Whenever a key event is received, it is converted into a keyname, and then that keynames is compared against the regex rules, in order : the first rule that matches calls the corresponding handler and terminates the event handling process.

More specifically, the members of rule triplets are :


A string specifiying the key modifiers for which the rule will fire; the string a concatenation of C_, S_ and A_, as explained above. An empty string means that the rule only fires when no modifiers are pressed. By contrast, a null value specifies that modifiers are ignored (the rule fires in any case).


Either a string containing a regular expression, or an already built Javascript RegExp object. Strings will be automatically converted to regular expressions, with start anchor ^ and end anchor $ automatically added. If you supply an already built RegExp object, make sure to deal properly with the anchors; otherwise the rule might fire in unexpected cases (for example the plain regex /[AEIOU]/ would match any builtin keyname like RETURN or ESCAPE, which is probably not the intended meaning of the rule).


The function to be called when the rule succeeds.

Antiregex rules

An ANTIREGEX entry in the map works exactly like a REGEX, except that the handler is called when the regex does not match. This is useful if you want to catch most key events, except a given set.


  document.onkeydown = aKeyMap.eventHandler(options);

Generates an event handler that can be attached to an HTML element. This method is called internally by the "observe" method. Use eventHandler directly if you need fine control on how the handler is attached to the dynamic HTML model.

The options argument is optional. If present, it should be an inline object containing truth values for the following keys :


ignore the Ctrl keyboard modifier


ignore the Shift keyboard modifier


ignore the Alt keyboard modifier


stop propagation of the event


prevent default navigator behaviour on that event

For example if ignoreCtrl is true, then the key specification "C_S_TAB" would never fire, because Ctrl-Shift-TAB key events would be encoded merely as "S_TAB".


  aKeyMap.observe(eventType, htmlElement, options);

This is the preferred way for attaching the keymap object to an HTML element, on a given event type (keydown, keypress or keyup). Arguments are optional. The default event type is "keydown", and the default element is document.

Options are passed to the "eventHandler" method. If not explicitly given, the options default to undefined except for event type keypress, where ignoreShift defaults to true. The reason is that the Shift modifier heavily depends on which keyboard the user is using, and often the user really has no choice on pressing or not the Shift key (this would generate a different keycode). So it makes sense to just stop paying attention to the Shift key for keypress events.



A DHTML application may need to temporarily change the key handlers (for example when switching from navigation mode to editing mode). Therefore, a keymap object actually holds a stack of rules and publishes this stack in its rules property. Rules pushed on top of that stack will take precedence over pre-existing rules; conversely, popping from the stack restores the keymap to its previous state.


  // grab all keys
  // ignore all keys
  aKeyMap.rules.push(KeyMap.MapAllKeys(function (){}));

Convenience function to build a regex rule that matches all keys.


  main_rules = {C_X: KeyMap.Prefix({R: ctrlX_R_handler,
                                    4: ctrlX_4_handler})};

Specifies that a key (here Ctrl-X) is a prefix to another set of rules : the next key event will be passed to these rules, and after that the main rules resumes normal behaviour. Hence you can attach handlers to sequences of keys, like for example in Emacs.



This method will remove the keymap handler attached the element/document. Call this method when the concerned element is removed from the DOM or to deactivate the keymap handler.