I am planning to create a Node.js desktop application. I am concerned if I write the application in javascript, the source code of it might not be secured just like scripts served on web browsers. Is it possible to secure both server-side script and client-side script including css when building a Node.js desktop application?
Considering that you want to protect your JS source code - short answer is NOT all of the JavaScript File's source code will be safe.
Tools Kit like NW.js helps to protect JS file to some extent. Depends on how the JavaScript Files are used in your application.
Read about packaging Application using NW.js here
And Protecting JS Files is documented in below link
Since you said you are in planning stage- you can also select Electron, which allows to create Application Package