Natbib Footnote Citation

Question

Building on the helpful suggestions offered by @moewe both here and here, I wanted seek assistance for a more complex footnote citation command that would allow the user to include page numbers as well. Long story short, I'm helping a colleague with an old project that uses natbib.

Minimal working example as follows,

\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{A01,
    author = {Author Name},
    publisher = {Publisher Co.},
    title = {Title of the Book},
    year = {2022}}
\end{filecontents}
\listfiles
\documentclass{article} 
\usepackage{natbib}
\begin{document}
\NewDocumentCommand{\footcite}{om}{\footnote{\IfNoValueTF{#1}{\cite{#2}}{\cite[#1]{#2}}}}
Test function of footnote citation without page number.\footcite{A01}
Test function of footnote citation with page number.\footcite[2]{A01}
\bibliographystyle{plainnat} 
\bibliography{\jobname}
\end{document}

Unfortunately the command \NewDocumentCommand{\footcite}{om}{\footnote{\IfNoValueTF{#1}{\cite{#2}}{\cite[#1]{#2}}}} results in undefined control sequence errors.

Any guidance or suggestions would be greatly appreciated!

Answer 1

After some tinkering I finally figured it out! Please see the now functional MWE with desired output. Also note that I added some stylistic options. I hope this helps future users in similar need!

\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{A01,
    author = {Author Name},
    publisher = {Publisher Co.},
    title = {Title of the Book},
    year = {2022}}
\end{filecontents}
\listfiles
\documentclass{article} 
\usepackage{natbib}
\setcitestyle{authoryear,open={(},close={)}} %Citation-related command, replaces default brackets to parentheses 
\setcitestyle{aysep={}} %Citation-related command, removes default comma between author & year
\usepackage{xparse}
\NewDocumentCommand\footcite{o m}
{%
\footnote{ %
\IfNoValueTF {#1}%
{\citet{#2}}%
{\citet[#1]{#2}}%
}
}
\begin{document}
Test function of footnote citation without page number.\footcite{A01}
Test function of footnote citation with page number.\footcite[2]{A01}
\bibliographystyle{plainnat} 
\bibliography{\jobname}
\end{document}