A. Sundararajan's Weblog

Self, JavaScript and JSAdapter

JavaScript is lot similar to Self Programming Language. Similarities include:

• Prototype based object orientation.
• delegation instead of inheritance.
• support for closures.
• prototype is dynamic and assignable (JavaScript's __proto__). (Yeah, Self supports multiple "parent" slots).

Self treats methods and fields uniformly. JavaScript follows traditional syntax for methods and fields. i.e., Syntax for field access (a.k.a properties) is different from that for method call. Caller "knows" whether a particular object member is a field or a method and accesses with different syntax. JavaScript 1.5 supports getters and setters. While browser JavaScript implementation supports this feature, this is not supported in Rhino JavaScript implementation. The following works in Firefox browser.

// 'b' property is accessed by get accessor method
var d = { get b() { return "hello"; } }

alert(d.b); // calls function defined above 

// explicit accessor method set for property "f"
d.__defineGetter__("f", function() { return 23 * 2} );

alert(d.f); // calls function set in __defineGetter__

But, Mustang includes Rhino JavaScript engine and therefore does not support getters and setters. But, we have added global constructor called JSAdapter. This is more flexible than the getters and setters. JSAdapter accepts an object with specially named methods: __get__, __has__, __put__, __delete__ and __getIds__

 var y = {
            __get__    : function (name) { ... }
            __has__    : function (name) { ... }
            __put__    : function (name, value) {...}
            __delete__ : function (name) { ... }
            __getIds__ : function () { ... }
          };

 var x = new JSAdapter(y);


 x.i;                        // calls y.__get__
 i in x;                     // calls y.__has__
 x.p = 10;                   // calls y.__put__
 delete x.p;                 // calls y.__delete__
 for (i in x) { print(i); }  // calls y.__getIds__

Note that JSAdapter may also be created using Java anonymous class-like syntax:

 var x = new JSAdapter() {
            __get__    : function (name) { ... }
            __has__    : function (name) { ... }
            __put__    : function (name, value) {...}
            __delete__ : function (name) { ... }
            __getIds__ : function () { ... }
          };

The caller/user of the adapter object is isolated from the fact that the property accesses/mutations/deletions are really calls to JavaScript methods on adaptee. Use cases include 'smart' properties, property access tracing/debugging, encaptulation with easy client access - in short JavaScript becomes more "Self" like. In fact, we have used JSAdapter feature in Object Query Language (OQL) implementation.

Note that Rhino already supports special properties like __proto__ (to set, get prototype), __parent__ (to set, get parent scope). We follow the same double underscore nameing convention here for special methods for JSAdapter.

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