Is it possible to create or exchange Javascript properties on access

Question

I would like to have some sort of lazy initialied object properties in javascript and would thus want to somehow overload the property read and write access i.e.:

var someval = myobj.lazyprop; // invokes myobj.get("lazyprop");
myobj.lazyprop = someval;     // invokes myobj.set("lazyprop",someval);

where myobj is some object I provide to the script.

Rationale: I want to use Javascript (Rhino) as scripting engine in an application and the datastructures that need to be accessible by the scripts can be very large and complex. So I don't want to wrap them all in advance to javascript objects, esp. since the average script in this application will only need a very small subset of them. On the other hand I want the scripts to be as simple and readable as possible, so I don't want to require the use of get or set methods with string arguments explicitly in the scripts.

Answer 1

You can do it using Rhino 1.6R6 or higher with javascript getters and setters.

function Field(val){
   var value = val;

   this.__defineGetter__("value", function(){
       return value;
   });

   this.__defineSetter__("value", function(val){
       value = val;
   });

}

Now, if we wanted to, instead, define getters and setters within the context of our object prototype (and where having "private" data is less of a concern) we can then use an alternative object syntax for that.

function Field(val){
   this.value = val;

}

Field.prototype = {
   get value(){
       return this._value;
   },
   set value(val){
       this._value = val;
   }

};

Answer 2

many js engines support getters and setters on javascript objects:

var obj = {
  get field() {alert('getting field');}
  set field(val) {alert('setting field to ' + val);}
}

var x = obj.field     // alert pops up
obj.field = 'hello'   // alert pops up

more details:

http://robertnyman.com/2009/05/28/getters-and-setters-with-javascript-code-samples-and-demos/

Answer 3

Rhino versions > 1.6R6 support getters and setters, you could write something like this:

var myobj = {
  // a getter and setter
  get lazyprop(){ return this.get('lazyprop'); },
  set lazyprop(val){ return this.set('lazyprop', val); },

  // the logic of your get and set methods:
  get: function(p) { return this['_'+p]; },
  set: function(p, val) { this['_'+p] = val; }
};

Answer 4

I think this is something you should do using the Rhino-specific API from the Java side; not from Javascript.