Return multiple variables to JavaScript from COM

Question

I have COM function in a disp interface as shown below,

[id(1)] HRESULT MultipleReturn([out]BSTR* arg1, [out, retval] BSTR* arg2);

implemented as

STDMETHODIMP SomeCoolObject::MultipleReturn(BSTR* arg1, BSTR* arg2)
{
    *arg1 = SysAllocString(L"test1");
    *arg2 = SysAllocString(L"test2");
    return S_OK;
}

In Python I can call it as

import comtypes.client as cc

obj = cc.CreateObject('SomeCoolObject')
a = obj.MultipleReturn()
print(a) # gives (u'test1', u'test2'), Python, I see you don't bite :)

same in JavaScript

var obj = new ActiveXObject("SomeCoolObject")

// this gives error, kind of obvious
// 'Wrong number of arguments or invalid property assignment'
// var val = obj.MultipleReturn();

var a = "holaaa!";
var val = obj.MultipleReturn(a);
alert(val); // gives "test2"
alert(a);   // gives "holaaa!", may be could have given "test1"

this proves, JavaScript won't play this ball. Why? If not, then how do you return multiple values from COM to JavaScript. For this particular job, I returned a JSON though.

Yes. I thought about using SAFEARRAY but question is about collecting two [out] variables in JavaScript. Not possible? — Logan
– Logan, Commented Jun 10, 2015 at 14:01

Igor Tandetnik · Accepted Answer · 2015-06-10 14:49:26Z

3

JavaScript/COM binding doesn't support [out] parameters - only [out, retval] (of which, of course, there can only be one). JavaScript generally doesn't have a notion of pass-by-reference.

There are several ways you can get closer to your goal.

Return a SAFEARRAY of two strings. In JavaScript, consume it via VBArray object.
Implement a simple COM object with two BSTR properties, create and return an instance of that object via [out, retval] IDispatch**.
Take IDispatch* as an [in] parameter, set a new property on it via IDispatchEx::GetDispID(fdexNameEnsure). JavaScript would consume it like this:

.

var outParam = {};
var result = obj.MultipleReturn(outParam);
var secondResult = outParam.value;

(where value is the name of the property your method creates on the object).

answered Jun 10, 2015 at 14:49

Igor Tandetnik

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5 Comments

acelent Over a year ago

It's not JavaScript per se, it's COM automation (late-binding, dispatching, whatever it's called these days) that only supports [out, retval].

Igor Tandetnik Over a year ago

@PauloMadeira Nothing in Automation spec precludes [out] parameters. They work fine with VB, VBA and VBScript. So no, it's specifically JavaScript, and its COM binding, that have this limitation.

acelent Over a year ago

You're right. But with late binding, you have to inform the language of such arguments (e.g. ByRef). I'm almost sure the op's Python code, as-is, could only return those values with early binding. So, it's not even a fair comparison with the VB* languages.

Igor Tandetnik Over a year ago

@PauloMadeira I'm not familiar with Python/COM binding. My guess would be, it's reading the type library, via IDispatch::GetTypeInfo. The information about parameter types and direction of data flow is available there. I don't see any evidence of early binding in the sample shown.

acelent Over a year ago

You're right again. I can't bother to check comtypes' code right now, but that's plausible.

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