How to highlight code inside verbatim environment?

Question

I am writing a book containing tutorials and I need to show code:

Write this: 

\begin{verbatim}
function doit() {
  console.log("Hello");
}
\end{verbatim}

And then add these lines:

\begin{verbatim}
function doit() {
  var s = "Hello";
  console.log(s);
}
\end{verbatim}

I would like to highlight var s = "Hello"; so that it is clear it is the new code added. I would like to set a different color for this text so the new line appears black and the rest appears grey.

How do I achieve this?

This is with a very standard LaTeX document, so I did not include the whole file. fg

Answer 1

Here is an option with fancyvrb where you can change the formatting of specific lines; the code should be self-explanatory:

\documentclass{article}

\usepackage{fancyvrb,xcolor}

\begin{document}

Write this: 

\begin{Verbatim}
function doit() {
  console.log("Hello");
}
\end{Verbatim}

And then add these lines:

\renewcommand{\FancyVerbFormatLine}[1]{%
  \ifnum\value{FancyVerbLine}=2
    \textcolor{red}{#1}% Format specific line
  \else
    #1% Else, do nothing different
  \fi
}
\begin{Verbatim}
function doit() {
  var s = "Hello";
  console.log(s);
}
\end{Verbatim}

\renewcommand{\FancyVerbFormatLine}{}% Clear how lines are formatted differently
\begin{Verbatim}
function doit() {
  var s = "Hello";
  console.log(s);
}
\end{Verbatim}

\end{document}

It should work for elementary setups and small code formatting.

Answer 2

Took this: How can I highlight some lines from source code?

Made this:

\documentclass{article}
\usepackage{soul,listings,xcolor}

    \lstnewenvironment{teX}[1][]
      {\lstset{language=[LaTeX]TeX}\lstset{escapeinside={(*@}{@*)},
       numbers=left,numberstyle=\normalsize,stepnumber=1,numbersep=5pt,
       breaklines=true,
       %firstnumber=last,
           %frame=tblr,
           framesep=5pt,
           basicstyle=\normalsize\ttfamily,
           showstringspaces=false,
           keywordstyle=\itshape\color{blue},
          %identifierstyle=\ttfamily,
           stringstyle=\color{maroon},
        commentstyle=\color{black},
        rulecolor=\color{black},
        xleftmargin=0pt,
        xrightmargin=0pt,
        aboveskip=\medskipamount,
        belowskip=\medskipamount,
               backgroundcolor=\color{white}, #1
    }}
    {}
    \begin{document}


Write this: 

\begin{teX}
function doit() {
console.log("Hello");
}
\end{teX}

And then add these lines:

\begin{teX}
function doit() {
(*@ \hl{var s = "Hello";} @*)
console.log(s);
}
\end{teX}

    \end{document}

Got this: