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;
   }

};