I was looking for a way to log down messages the installer log using JScript and stumbled upon this answer:
How to debug an MSI Custom Action that is implemented in Javascript?
It works great. However there is something intriguing in the code that I haven't been able to figure out. In this method:
// 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);
}
How does the line record.StringData(0) = "CustomAction:: " + msg; work, from a syntax/semantic perspective? It looks to me this is trying to assign a value to the return value of a function call, which should be illegal or a no-op at best? Yet it works, and the message is printed out in the log.
What am I missing?
It is a JScript way to access lists and not an actual function. Therefore it does not throw an Invalid left-hand side in assignment.
StringData is a list of values. What you are actually setting is the value of the 0 index. It is like setting the value to an array using arr[0] = 'xyz'. In your example you could also omit it:
record.StringData = "CustomAction:: " + msg;
propVal = Record.StringData
Record.StringData = propVal
Required field number of the value within the record, 1-based.
The returned value of a nonexistent field is an empty string. To set a record string field to null, use either an empty variant or an empty string. Attempting to store a value in a nonexistent field causes an error.
Source: Patrick