GeometriX - it works like this:
Firstly "" inside a quoted string is actually read as a single " in the output string. For example the expression text
"Here's a quote: "" <-- there it is"
outputs
Here's a quote: " <-- there it is
By using two "" it inserts a double quote instead of being the " that ends the string.
So a string containing just a double quote is " "" ", or without spaces, four double quotes: """"
Add to that the app folder, then add the filename, which also has a quote at the end, e.g. "something.xxx"" ", or without the space, "something.xxx""".
So you have an expression
"""" & NodeWebkit.AppFolder & "something.xxx"""
that actually outputs a string like
"C:\some\path\something.xxx"
or, just a normal path wrapped in double-quotes, which is what the OS needs to handle paths with spaces.