Question

I’ve been looking over the GNOME shell javascript interface and noticed the following snippet (popupMenu.js for those who are interested) from the prototype of a class:

PopupSwitchMenuItem.prototype = {
    __proto__: PopupBaseMenuItem.prototype,

    _init: function(text, active, params) {
        ... (code)
    },

    ... function definitions

    get state() {
        return this._switch.state;
    },

    ... more functions
};

Could anyone explain to me what the get state() { ... } means? I thought everything had to be of the form name: value within a javascript object? (If I make one of these objects I can do obj.state which returns what I assume is this._switch.state).

It may help to note that GNOME say they use a flavour of javascript (gjs) similar to Mozilla’s Spidermonkey, so is this behaviour a non-standard javascript behaviour and a feature of spidermonkey/gjs?

Also, is there documentation pertaining to this?

Answer 1

Check out https://developer.mozilla.org/en/JavaScript/Reference/Operators/get and John Resig’s post at http://ejohn.org/blog/javascript-getters-and-setters/

The interesting thing is that you access the properties just like any other property, except they are dynamically created… in example

foo = {
  get blah() {
    return "bar";
  }
}

foo.blah == "bar"

Without native getters you would have to do foo.blah(). Frankly I’ve never used them simply because IE8 doesn’t support it, and the advantage is just so minor. Most commonly I can see them being useful in a situation where I started with a “dumb” property like foo.blah, and at some point later in the development cycle realized that I wanted foo.blah(), and had to do the conversion at every point, while with the getters and setters such a conversion is unneccessary, since you simply change the object definition.