extend arrow to match width of the above line

Question

How can you extend arrow to match the width of text in the above line?

I want to get a result like this.

X -> Y -> Z
 ------->

And the best way I could think of was:

X
\begin{array}{c}
    \stackrel{f}{\longrightarrow} Y \stackrel{g}{\longrightarrow} \\
    \xrightarrow{\hfill h \hfill}
\end{array}
Z

But the arrow is short. I want to extend the width of the

\xrightarrow{\hfill h \hfill}

to match the width of

\stackrel{f}{\longrightarrow} Y \stackrel{g}{\longrightarrow}

Please give me some advice.

Answer 1

Like this?

\documentclass{article}

\usepackage{mathtools, amssymb}
\usepackage{eqparbox} 
\newcommand{\eqmathbox}[2][M]{\eqmakebox[#1]{$#2$}}

\begin{document}

\begin{align*}
  X
\begin{array}{c}
    \eqmathbox{\stackrel{f}{\longrightarrow} Y \stackrel{g}{\longrightarrow}} \\[-1ex]
    \xrightarrow[\hskip-0.7ex\eqmathbox{\scriptstyle h}\hskip-0.8ex]{}
\end{array}
Z
\end{align*}

\end{document}