Question

I am working on an ASP classic project where I have implemented the JScript JSON class found here. It is able to interop with both VBScript and JScript and is almost exactly the code provided at json.org. I am required to use VBScript for this project by the manager of my team.

It works very well on primitives and classes defined within ASP. But I have need for Dictionary objects which from my knowledge are only available through COM interop. (via Server.CreateObject("Scripting.Dictionary")) I have the following class which represents a product: (ProductInfo.class.asp)

<%
Class ProductInfo

    Public ID
    Public Category
    Public PriceUS
    Public PriceCA
    Public Name
    Public SKU
    Public Overview
    Public Features
    Public Specs

End Class
%>

The Specs property is a Dictionary of key:value pairs. Here’s how I’m serializing it: (product.asp)

<%
dim oProd
set oProd = new ProductInfo
' ... fill in properties
' ... output appropriate headers and stuff
Response.write( JSON.stringify( oProd ) )
%>

When I pass an instance of ProductInfo to JSON.Stringify (as seen above) I get something like the following:

{
    "id": "1547",
    "Category": {
        "id": 101,
        "Name": "Category Name",
        "AlternateName": "",
        "URL": "/category_name/",
        "ParentCategoryID": 21
    },
    "PriceUS": 9.99,
    "PriceCA": 11.99,
    "Name": "Product Name",
    "SKU": 3454536,
    "Overview": "Lorem Ipsum dolor sit amet..",
    "Features": "Lorem Ipsum dolor sit amet..",
    "Specs": {}
}

As you can see, the Specs property is an empty object. I believe that the JSON stringify method knows that the Specs property is an object, so it appends the {} to the JSON string around the stringified output. Which in this case is an empty string. What I expect it to show, however is not an empty object. See below:

"Specs": {
    "foo":"bar",
    "baz":1,
    "etc":"..."
}

I believe the problem area of the JSON library is here: (json2.asp)

// Otherwise, iterate through all of the keys in the object.

for (k in value) {
    if (Object.hasOwnProperty.call(value, k)) {
        v = str(k, value);
        if (v) {
            partial.push(quote(k) + (gap ? ': ' : ':') + v);
        }
    }
}

I postulate that the problem with the above code is that it assumes that all objects inherit from the Object class. (The one that provides hasOwnProperty) However I think that it’s likely that COM objects don’t inherit from the Object class — or at least the same Object class. Or at least don’t implement whatever interface is required to do for ... in on them.

Update: While I feel it is irrelevant for the question to be answered — I expect some sort of web client to request (via http) the JSON representation of this object or a collection of this object.

tl;dr The question: What should I do to make it so that the Scripting.Dictionary can be output properly as JSON instead of failing and returning just an empty string? Do I need to ‘reinvent the wheel’ and write my own Dictionary class in VBScript that does act as a normal object in ASP?

Answer 1

Javascript’s for...in construct (which is used in the JSON serializer you refer to) only works on native JS objects. To enumerate a Scripting.Dictionary’s keys, you need to use an Enumerator object, which will enumerate the keys of the Dictionary.

Now the JSON.stringify method has a nifty way of allowing custom serialization, by checking for the presence of a toJSON method on each property. Unfortunately, you can’t tack new methods on existing COM objects the way you can on native JS objects, so that’s a no-go.

Then there’s the custom stringifier function that can be passed as second argument to the stringify method call. That function will be called for each object that needs to be stringified, even for each nested object. I think that could be used here.

One problem is that (AFAIK) JScript is unable to differentiate VBScript types on its own. To JScript, any COM or VBScript object has typeof === 'object'. The only way I know of getting that information across, is defining a VBS function that will return the type name.

Since the execution order for classic ASP files is as follows:

• <script> blocks with non-default script languages (in your case, JScript)
• <script> blocks with the default script language (in your case, VBScript)
• <% ... %> blocks, using the default script language (in your case, VBScript)

The following could work — but only when the JSON.stringify call is done within <% ... %> brackets, since that’s the only time when both JScript and VBScript <script> sections would both have been parsed and executed.

The final function call would be this:

<%
    Response.Write JSON.stringify(oProd, vbsStringifier)
%>

In order to allow JScript to check the type of a COM object, we’d define a VBSTypeName function:

<script language="VBScript" runat="server">
    Function VBSTypeName(Obj)
        VBSTypeName = TypeName(Obj)
    End Function
</script>

And here we have the full implementation of the vbsStringifier that is passed along as second parameter to JSON.stringify:

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

    function vbsStringifier(holder, key, value) {
        if (VBSTypeName(value) === 'Dictionary') {
            var result = '{';
            for(var enr = new Enumerator(value); !enr.atEnd(); enr.moveNext()) {
                key = enr.item();
                result += '"' + key + '": ' + JSON.stringify(value.Item(key));
            }
            result += '}';
            return result;
        } else {
        // return the value to let it be processed in the usual way
            return value;
        }
    }

</script>

Of course, switching back and forth between scripting engines isn’t very efficient (i.e. calling a VBS function from JS and vice versa), so you probably want to try to keep that to a minimum.

---

Also note that I haven’t been able to test this, since I no longer have IIS on my machine. The basic principle should work, I’m not 100% certain of the possibility to pass a JScript function reference from VBScript. You might have to write a small custom wrapper function for the JSON.stringify call in JScript:

<script runat="server" language="JScript">
    function JSONStringify(object) {
        return JSON.stringify(object, vbsStringifier);
    }
</script>

after which you can simply adjust the VBScript call:

<%
    Response.Write JSONStringify(oProd)
%>