Windows Mobile Client (APPX): JavaScript support (optional chaining)

we built a Windows app using some JavaScript library (pdfjs) with their source code using optional chaining operator (?.).
The app builds and installs fine but won’t start complaining about syntax errors in this JavaScript library.
I know it’s not directly related to the build process but do you have any insights as to which JS engine is used to run Windows mobile clients and/or which limitations exist on Windows in terms of JS usage.
Thanks a lot,

The Cordova Windows implementation is based on Edge - in other words, JavaScript UWP, using Visual Studio 2017.

Microsoft dropped support for this in VS 2019, so it’s likely the version of JavaScript it uses is older.

There is a fuller discussion here:

Thanks for the insight!