Generating JSON in ASP/VBScript

JSON is damn cool - we've been using it for a while now at work. Unfortunately though, VBScript lacks the OO functionality to properly handle converting JSON to and from object structures, so we've been painfully generating JSON responses manually from the server via string building.

However, after mixing JScript with VBScript (the other day), I got thinking, had a little play around, and realised that wee bit of server-side JS can make JSON production super easy:

<%@ LANGUAGE=VBScript LCID=5129 %>

<%

Dim sJSON, package, book, author

Set package          = JsInterop.Object("VbInterop.Json.RpcResponse")

Set package.result      = JsInterop.Object("VbInterop.Json.BooksCollection")

Set package.result.books  = JsInterop.NewArray()

For i = 1 to 3

  Set book      = JsInterop.Object("VbInterop.Json.Book")

  book.title      = "Slack"

  book.year      = 2002

  book.available    = True

  book.publisher    = "BroadwayBooks"

    Set book.authors  = JsInterop.NewArray()

    Set author      = JsInterop.Object("VbInterop.Json.Author")

    author.firstName  = "Tom"

    author.lastName    = "DeMarco"

    book.authors.push(author)

  package.result.books.push(book)

Next

Response.Write package.toJSON()

%>

<script language="jscript" runat="server" src="vb-interop.js"></script>

<script language="jscript" runat="server">

// Books collection

VbInterop.Json.BooksCollection = function () {

  this.books      = OBJ;

};

// Book class

VbInterop.Json.Book = function () {

  this.title      = STR;

  this.year      = NUM;

  this.available    = BOOL;

  this.authors    = RG;

  this.publisher    = STR;

};

// Author class

VbInterop.Json.Author = function () {

  this.firstName    = STR;

  this.lastName    = STR;

};

</script>

I don't really understand the workings of JS and VBs interop, but in a very rough sense it seems that:
a) Any JS features implemented via the IDispatchEx interface (see http://blogs.msdn.com/ericlippert/archive/2004/10/07/239289.aspx) cannot be used in VB.
b) JS/VB arrays do not play dice (see http://blogs.msdn.com/ericlippert/archive/2003/09/22/53061.aspx).

So, the two key features needed for JSON generation in a 'nice' fashion are:
1. New object creation (which requires a constructor) and
2. Arrays

#1 can easily be achieved by using an intermediary JS function to instantiate new objects from JS, then return them to VB:

JsInterop.Object = function ( sClass ) {

  var obj      = eval('new '+ sClass + '()');

  obj.toJson    = Object.prototype.toJSONString;

  return obj;

}

#2 is slightly more fiddly. Instead of an actual array, create an object with a public .push() method and a private array variable:

JsInterop.NewArray: function () {

  return new VbInterop.Array();

}

VbInterop.Array = function () {

  var rg = [];

  this.push = function( variant ) {

    rg.push(variant);

  }

  this.toJson = function () {

    return rg.toJSONString();

  }

}

Pop() and Get/Set methods could also be added here if desired.

Note the augumentation of a .toJson() method that utilizes Douglas Crockford's json.js jsonification. This is where the disco happens.

Suffice to say it's all pretty simple, and the code is self-explanatory, so take a look if you're interested:

VB-INTEROP.JS

//  vb-interop.js

//  Utility code to build an object structure in VBScript that can be serialized into JSON

//  VBScript-facing static class

//  An intermediary for new object creation, required as we cannot directly instantiate JS classes from VBScript

var JsInterop = {

  NewObject: function () {

    var obj      = {};

    obj.toJson    = Object.prototype.toJSONString;

    return obj;

  }

  , NewArray: function () {

    return new VbInterop.Array();

  }

  , Object: function ( sClass ) {

    var obj      = eval('new '+ sClass + '()');

    obj.toJson    = Object.prototype.toJSONString;

    return obj;

  }

};

//  JScript-facing static class

//  Interop namespace

var VbInterop = {};

//  Seperate namespace for JSON class definitions

VbInterop.Json = {};

//  VBScript-compatible Pseudo-array class

//  Note that the actual array variable *must* be private, if it is public then VBScript will setup-the-bomb on it and launch all ships..

VbInterop.Array = function () {

  var rg = [];

  this.push = function( variant ) {

    rg.push(variant);

  }

  this.toJson = function () {

    return rg.toJSONString();

  }

};

//  Simple constants for clean data-typed class definitions

var STR   = '';

var OBJ   = {};

var BOOL   = false;

var RG     = {};     // We are using objects to emulate arrays in VBs

var NUM   = -1;

var DT    = '';     // No literal syntax exists for dates, so we will treat them as strings.

// Example JSON-RPC Response class

VbInterop.Json.RpcResponse = function () {

  this.id        = NUM;

  this.result      = OBJ;

  this.error      = OBJ;

};

JSON.JS MODIFICATIONS

            object: function (x) {

                if (x) {

                    if (x instanceof Array) {

                        return s.array(x);

                    }

                    /* begin vb-interop.js addition */

                    if (VbInterop && x instanceof VbInterop.Array) {

            return x.toJson();

          }

          /* end vb-interop.js addition */

INTEROP-TESTS.ASP

<%@ LANGUAGE=VBScript LCID=5129 %>

<%

' VALID

Response.Write    test1.property

' INVALID (Class not defined)

'Set foo = New test2

' VALID

Set foo = test2a()

Response.Write foo.property

Response.Write test3.property.[0]

Response.Write test3.property.[1]

' INVALID (Property/method not supported)

'i = 0

'Response.Write test3.property.[i]

' VALID

Set foo = test4()

Response.Write foo.get(0)

' VALID

Set foo = test5()

Response.Write foo.get(0).bar

%>

<script language="jscript" runat="server">

var test1 = {

  property: 'test1'

}

var test2 = function () {

  this.property = 'test2';

}

var test2a = function () {

  return new test2();

}

var test3 = {

  property: ['test3', 'test3b']

}

var test4 = function () {

  return new test4a();

}

var test4a = function () {

  var property = ['test4'];

  this.get = function (ix) {

    return property[ix];

  }

}

var test5 = function () {

  return new test5a();

}

var test5a = function () {

  var property = [

    { bar: "test5" }

  ];

  this.get = function (ix) {

    return property[ix];

  }

}

</script>

Finally, muchos graci to Eric Lippert for helpful responses to ignorant queries I had in on all this.