JScript and VBScript Arrays




I alluded to the fact that JScript arrays are objects but VBScript arrays are not.  What’s
up with that?


kind of strange.  Consider the properties
of a JScript array.  A Jscript array is


one dimensional.

associative; JScript arrays are indexed by strings.  Numeric
indices are actually converted to strings internally.

= 123; arr[1000000] = 456;
you a two-member array, not a million-member array.

an object with properties and methods.


a VBScript array is



indexed by integer tuples


not an object


is hard to come up with two things that could be more different and yet both called


you might expect, JScript and VBScript arrays have completely different implementations
behind the scenes.  A JScript array is
basically just a simple extension of the existing JScript expando object infrastructure,
which is implemented as a (rather complicated) hash table.  A
VBScript array is implemented using the SAFEARRAY data structure, which is pretty
much just a structure wrapped around a standard “chunk of memory” C-style array.


all the COM objects in the world expect VBScript-style arrays and not JScript-style
arrays, making JScript interoperate with COM is not always easy.  I
wrote an object called
VBArray into
the JScript runtime to translate VBScript-style arrays into JScript arrays, but it
is pretty kludgy.  And though I wrote
some code to go the other way — to turn JScript arrays into VBScript arrays — there
were just too many thorny issues involving object identity and preservation of information
for us to actually turn it on.  There
weren’t exactly a whole lot of users demanding the feature either, so that part of
the feature got cut.  (If I recall correctly,
my colleague Peter Torr wrote some COM code to do that before he came to work at Microsoft.  He
might still have it lying around.)


got even weirder when I wrote the code to interoperate between CLR arrays and JScript
.NET arrays, but that’s another story.


Comments (17)

  1. Why are there two types of multidimentional arrays? What is the difference between the (x)(y) and (x,y) notation?

    dim arrBar
    arrBar = Array(Array(1,2), Array(3,4))

    response.write arrBar(0)(0) & "<BR>"
    response.write arrBar(0)(1) & "<BR>"
    response.write arrBar(1)(0) & "<BR>"
    response.write arrBar(1)(1) & "<BR>"

    dim arrFoo (1,1)
    arrFoo (0,0) = 1
    arrFoo (0,1) = 2
    arrFoo (1,0) = 3
    arrFoo (1,1) = 4

    response.write arrFoo(0,0) & "<BR>"
    response.write arrFoo(0,1) & "<BR>"
    response.write arrFoo(1,0) & "<BR>"
    response.write arrFoo(1,1) & "<BR>"

    ‘these don’t work
    response.write arrFoo(0)(0)
    response.write arrBar(0,0)

  2. priya says:

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  3. Daren says:

    I understand the issues with converting an existing JScript array to a VBArray. But would it be possible to include within JScript an straight forward way to create a new VBArray?

    For example, I am trying to execute IADs::GetInfoEx( <vbarray>, 0 ); I have tried every which way to create a valid VBArray this method will actually accept, such as using the Scripting.Dictionary – Items() method. No such luck. In the end I had to convert the script to wsf and write a VB Function to handle this part, while the rest is in JScript.

    It would be nice to natively and easily create these types of objects..



  4. Eric Lippert says:

    Yes, it would be nice. Unfortunately, I never did get around to implementing that in JScript Classic. Sorry about that.

    I did add much better support for arrays in JScript .NET though. (Small consolation, I know.)

  5. David Karlton says:

    I’m one of those crazed users who’s trying to figure out how to pass a JScript array argument to C# code. If I cast the argument as an object, its type is System.__COMObject, so I assume I need to marshal it somehow.

    I currently am exposing JScript methods to my C# code by implementing a subclass of IDispatch, called ICustomMethods, and registering my C# application:

    public void GetExternal([MarshalAs(UnmanagedType.IDispatch)] out object ppDispatch)


    ppDispatch = this as ICustomMethods;


    From JScript, it just requires calling


    So, the problem is that one of these JScript methods I need to support has a JScript array argument, and I don’t know how to marshal it.

    Any ideas?

    Thanks much!


  6. EricLippert says:

    A JScript array is a dispatch object which when invoked on its indexes, gets/sets the results.

    In C++ what you’d do to simulate arr[123], for instance, is you’d get the dispid for "123", and invoke on that dispid.

    How you’re going to do that in C# beats the heck out of me. We designed JScript arrays years before C# was a gleam in Anders’ eye. If you figure something out, let me know.

  7. Bim Jeam says:

    Something like this works for me

    In Html file’s Javascript

    window.external.ArrayReceivingFunction([‘Arg1′, ‘Arg2′, ‘Arg3′,etc.]);

    My C# "window.external" class

    public class WindowExternal


    public void ArrayReceivingFunction(object argArray)


    object[] objArr = DispatchHelpers.GetObjectArrayFrom__COMObjectArray(argArray);



    My C# Dispatch Helpers class

    internal class DispatchHelpers


    internal static object[] GetObjectArrayFrom__COMObjectArray(object comObject)


               int length = Convert.ToInt32(GetPropertyValue(comObject, "length"));

               object[] objArr = new object[length];

               for (int idx = 0; idx < length; idx++)


                   objArr[idx] = GetPropertyValue(comObject, idx.ToString());


               return objArr;


    internal static object GetPropertyValue(object dispObject, string property)


               object propValueRef = null;



                   Type type = dispObject.GetType();

                   propValueRef = type.InvokeMember(property, BindingFlags.GetProperty,

                                                   null, dispObject, null);


               catch (COMException) { /* Throws COMException on invalid property */ }

               return propValueRef;



  8. Bim Jim says:

    By the way Eric your info "A JScript array is a dispatch object which when invoked on its indexes, gets/sets the results." was invaluable assistance

  9. Jim d says:

    Hey, so I think I managed to hack together an okay solution.  It requires a bit of juggling but it manages to preserve elements’ datatypes (for primitives only).  Basically you create vbscript functions for manipulating the VBScript array, and then a JScript function which calls these manipulation functions..

    Here’s the vbscript code:

     ‘  return a blank dynamic array

     function getBlankArray()

       getBlankArray = Array()

     end function

    ‘  append  arr with the specified val

     function appendArray(arr, val)

       redim preserve arr(ubound(arr) + 1)

       arr(ubound(arr)) = val

       appendArray = arr

     end function

    And here’s the JScript portion.  I extended the Array class, but you could have a seperate function.

     Array.prototype.toVBArray = function() {

       //call our vbscript function to give us a blank VBArray

       var vbarray = getBlankArray();

       for(var i = 0; i < this.length; i++)  {

         //use our vbscript manipulation function to append the ith item

         vbarray = (appendArray(vbarray, this[i]));




    With that in place,  you should be able to convert simple jscript arrays  to VBArrays, a la

     var tmp = [1, 2, 3.14, ‘hi mom’];


  10. eric neville says:

    Playing with jscript here, and wondering if there’s a way to get at the KEYS of a jscript associative array from vbscript.  MSE7 seems to know what the keys are, and using For Each, i seem to be able to iterate the VALUES in the array, but I can’t seem to hit the keys, though the IDE seems to know they’re there.

    Am I out of luck with this one?  I know it’s probably a long shot…

  11. EricLippert says:

    You are out of luck.  There is no equivalent to the JScript for-in in VBScript.  Sorry!

  12. Jaiprakash says:

    Recently I investigated a bug in Jscript which was reported on Vista. The scenario was working perfectly

  13. nAty says:

    I have the same problem that David Karlton, but the only difference is that my array has two different dimensions.

    Here is the example:

    <script language="javascript" type="text/javascript">

         function Array2D(dim1, dim2)


           for (var i = 0; i < dim1; ++i)


               this[i] = new Array(dim2);


           this.length = new Array(dim1, dim2);


    function LoadPage()


          var mat= null;

          mat=new Array(5,2);

          var name = ’30′;

    var ar = new Array2D(5,2);

           ar[0][1] = "Dato1";

           ar[0,1]= 10;

           ar[1,0]= "Dato2";

           ar[1,1]= 20;

           ar[2,0]= "Dato3";

           ar[2,1]= 30;

           ar[3,0]= "Dato4";

           ar[3,1]= 40;

           ar[4,0]= "Dato5";

           ar[4,1]= 50;

           for (i=0;i<=4;i++)


            ar[i,0]= ‘dato’+i;

            ar[i,1]= (i+1)*10;





           mat = obj.LoadPage(’30’,mat); //Here is the problem



    I need to know if is possible to cast the mat array and bidimensional array that that was sent to C# like parameter in LoadPage Function, and How can I do this? I receive an object of type System.__COMObject but I need a type object [,] in C#.

  14. agent009 says:

    Here’s a simple VBScript function that converts single-dimension JScript array to VBArray:

    <script language="VBScript">

    Function toVBArray (ByRef jsa)

      Dim e, i, vba()

      i = 0

      For Each e In jsa    

         ReDim Preserve vba(i)

         vba(i) = e

         i = i + 1


      toVBArray = vba

    End Function


    In fact it can be used on any object, but you will lose property names. It will also skip "gaps" in sparse JScript arrays.

    You can call it from either JScript or VBscript functions. You can create similar functions for converting more dimensions into arr(1,2,3,…) style arrays by nesting For Each loops – it will require separate function for each given number of dimensions though.

    arr(1)(2)(3) style VBArrays can be created by modifying this function to call itself recursively (number of dimensions of JScript array would have to be provided as additional argument since there’s no way to reliably determine it programmaticaly).

  15. Pascal Lev92 says:

    The method from Bim Jim seems good, however I received objects System.__COMObject by reference and I don’t know how to get their value (as string for example).

    Is someone has an idea ?

  16. Rex Henderson says:

    Adding to agent009 JScript array to VBArray function, I just access JScript arrays using dot notation.

    x.[0]  ‘Access the first element of JScript array within VBScript.

    Works on XP SP3/IE 8 anyways.