How to debug an MSI Custom Action that is implemented in Javascript?

Question

I'm having difficulty figuring out why my Javascript Custom action is failing.

I thought I saw a topic in the WIX.chm file on debugging; now I cannot find it.

Q1
is there doc on how to debug Javascript or VBScript custom actions?

Q2
Is there a way to emit something into the MSI log from a custom action?

---

Addendum:
Some people think script is the wrong tool for writing CAs.
I don't agree. I think Javascript is a very good tool for the job.

Answer 1

For the doc, look for Session.Message.

To emit messages into the MSI log from a Javascript Custom Action, follow this boilerplate code:

//
// CustomActions.js 
// 
// Template for WIX Custom Actions written in Javascript.
// 
// 
// Mon, 23 Nov 2009  10:54
// 
// ===================================================================


// http://msdn.microsoft.com/en-us/library/sfw6660x(VS.85).aspx
var Buttons = {
        OkOnly           : 0,
        OkCancel         : 1,
        AbortRetryIgnore : 2,
        YesNoCancel      : 3
};

var Icons = {
        Critical         : 16,
        Question         : 32,
        Exclamation      : 48,
        Information      : 64
};

var MsgKind = {
        Error            : 0x01000000,
        Warning          : 0x02000000,
        User             : 0x03000000,
        Log              : 0x04000000
};

// http://msdn.microsoft.com/en-us/library/aa371254(VS.85).aspx
var MsiActionStatus = {
        None             : 0,
        Ok               : 1, // success
        Cancel           : 2,
        Abort            : 3,
        Retry            : 4, // aka suspend?
        Ignore           : 5  // skip remaining actions; this is not an error.
};


function MyCustomActionInJavascript() {
    try {
        LogMessage("Hello from MyCustomActionInJavascript");
        // ...do work here...
        LogMessage("Goodbye from MyCustomActionInJavascript");
    }
    catch (exc1) {
        Session.Property("CA_EXCEPTION") = exc1.message ;
        LogException(exc1);
        return MsiActionStatus.Abort;
    }
    return MsiActionStatus.Ok;
}

// Pop a message box.  also spool a message into the MSI log, if it is enabled. 
function LogException(exc) {
    var record = Session.Installer.CreateRecord(0);
    record.StringData(0) = "CustomAction: Exception: 0x" + decimalToHexString(exc.number) + " : " + exc.message;
    Session.Message(MsgKind.Error + Icons.Critical + Buttons.btnOkOnly, record);
}


// spool an informational message into the MSI log, if it is enabled. 
function LogMessage(msg) {
    var record = Session.Installer.CreateRecord(0);
    record.StringData(0) = "CustomAction:: " + msg;
    Session.Message(MsgKind.Log, record);
}


// popup a msgbox
function AlertUser(msg) {
    var record = Session.Installer.CreateRecord(0);
    record.StringData(0) = msg;
    Session.Message(MsgKind.User + Icons.Information + Buttons.btnOkOnly, record);
}

// Format a number as hex.  Quantities over 7ffffff will be displayed properly.
function decimalToHexString(number) {
    if (number < 0)
        number = 0xFFFFFFFF + number + 1;
    return number.toString(16).toUpperCase();
}