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ActiveX event handlers in an HTA using JScript

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忘了有多久
忘了有多久 2020-12-06 15:48

In C# I can write event handlers as follows:

var wdApp = new Microsoft.Office.Interop.Word.Application();
wdApp.DocumentBeforeSave += (Document doc, ref bool
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  • 不知归路
    2020-12-06 16:09

    I could not make the object::event syntax work. So, I delegated the event handling to an auxiliary script, which follows.

    caller.js file:

    var WordApp = WScript.CreateObject("Word.Application", "wd_");
WordApp.Visible = true;

WScript.Sleep(20000);

function wd_Quit() { //'object_event' works with js inside wscript.exe
    WScript.Echo("word quit");
}

    Then, call this script from within a HTA file

    main.hta file:

    <html>
<script LANGUAGE="JScript">
    var shell = new ActiveXObject("WScript.Shell");
    shell.Run("caller.js");
</script>
</html>

    Both files must be on the same folder. Run main.hta and the code in caller.js will be called, without polluting the global namespace of the HTA.

    This example is for testing purposes. There is a 20 seconds timeout for the script. After this time, it will quit and events won't be processed.

    If it works, tell me so we can create a better way of communicating from the HTA (mshta.exe) to the JS (wscript.exe). There is a chance of using "GetObject" from the HTA to control the word application (I did it once, but with VBScript), while the events are processed by the JS file.

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  • 南笙
    2020-12-06 16:11

    The error appears to be because

    1. In Javascript/JScript function declarations are evaluated first, before the variable is initialized. It's as if the code was written thus:

       var wdApp;
 function wdApp::Quit() { ... }
 wdApp = new ActiveXObject('Word.Application');

    2. The Microsoft JScript parser interprets the specially-named declaration (with ::) as an instruction to attach the function as an event handler.

    3. But at the point of the declaration, the handler cannot be attached to the object referred to by wdApp, because wdApp at that point is still undefined. Hence, the error.

    The solution is to force the function declaration to be evaluated after wdApp is initialized. This can be done in one of three ways1:

    1. Since the function declaration is hoisted only to within the function scope, wrap the function declaration in an IIFE:

       var wdApp = new ActiveXObject('Word.Application');
 (function() {
     function wdApp::Quit() {
         //do stuff here
     }
 })();

    2. Create the declaration using some sort of string -> code mechanism -- eval, setTimeout with a string, window.execScript, or new Function:

       var wdApp = new ActiveXObject('Word.Application');
 eval('function wdApp::Quit() { ... }`);

    3. Initialize the variable before the current SCRIPT block. Most of the examples in the Scripting Events article do this by setting the id property on some element before the SCRIPT block:

    <object progid="ordersystem.clsorder" id="myorder" events="true"/>
<script language="jscript">
    function myorder::onNew() {
      WScript.Echo("new order received from myorder")
    }
//...

    but this could also be done using multiple SCRIPT blocks:

        <SCRIPT>
        var wdApp = new ActiveXObject('Word.Application');
    </SCRIPT>
    <SCRIPT>
        function wdApp::Quit() {
            //do stuff here
        }
    </SCRIPT>

    As far as polluting the global namespace, only the third variant requires wdApp to be in the global namespace; the other two variants can be wrapped in an IIFE.

    1. Technically, there is a fourth way under IE and HTAs, but it involves non-standard HTML instead of non-standard Javascript; But it can only be used if the object is declared using the HTML OBJECT tag, not with new ActiveXObject( ... ). 

    <script for="wdApp" event="Quit">
    //do stuff here
</script>
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