documentclass{article}
usepackage{listings}
usepackage{xcolor}

lstset{
    basicstyle=ttfamily,
    keywordstyle=color{blue},
    keywords={function},
    escapeinside={|}{|}
}
deffbar{function bar ();}

begin{document}
begin{lstlisting}[]
function foo ();
|fbar|
end{lstlisting}
end{document}

documentclass{article}
usepackage{listings}
usepackage{xcolor}

lstset{
    basicstyle=ttfamily,
    keywordstyle=color{blue},
    keywords={function},
    escapeinside={|}{|}
}
deffbar{scantokens{function bar ();}}

begin{document}
begin{lstlisting}[]
function foo ();
|expandafter|fbar
end{lstlisting}
end{document}

documentclass{article}
usepackage{listings}
usepackage{xcolor}

lstset{
    basicstyle=ttfamily,
    keywordstyle=color{blue},
    keywords={function},
    escapeinside={|}{|}
}
deffbar{scantokens{|function bar ();}}

begin{document}
begin{lstlisting}[]
function foo ();
|fbar
end{lstlisting}
end{document}