Why won't the titlesec package modify \chapter headings? (PDFLaTeX)

Question

It seems to me that titlesec is failing to set \chapter heading styles in documents.

The documentation says that it should have support to set \chapter heading styles.

Here is a minimum reproducible example:

\documentclass[12pt,letterpaper]{article}
\usepackage[margin=0.5in]{geometry}
\usepackage[compact]{titlesec}

\titleformat{\chapter}[hang]
{\centering\bfseries\huge}
{}
{0pt}
{\titlerule[0.8pt]}[]
\titlespacing{\chapter}
{0pt}
{0pt}
{0pt}[0pt]

\titleformat{\section}[hang]
{\centering\Large}
{}
{0pt}
{\titlerule[1.6pt]}[]
\titlespacing{\section}
{0pt}
{0pt}
{0pt}[0pt]

\begin{document}
\chapter{CHAPTER TEXT}\label{chapter-text}
\section{SECTION TEXT}\label{section-text}
\end{document}

This fails both in my local installation of PDFLatex and in Overleaf.

Perhaps I am missing something. Does anyone have a clue as to what might resolve this issue for me? Thank you!

Answer 1

The issue was that the article document class does not define \chapter headings, and therefore it is not possible to give them styles.

The solution (Suggested in a comment by @Ulrike Fischer) is to switch to another document class, such as report or book. This works!

I chose to switch to the report document class. Note that this caused a migration issue: my headers (set by \pagestyle) refused to work on the same page as the \chapter heading. I found the solution to this issue in another forum post here: https://tex.stackexchange.com/a/19741/315958

Answer 2

You could instead wrap your definition for \chapter inside an \@ifundefined (note to enclose this with \makeatletter ... \makeatother).

\makeatletter
\@ifundefined{chapter}{}{
    \titleformat{\chapter}[hang]
    {\centering\bfseries\huge}
    {}
    {0pt}
    {\titlerule[0.8pt]}[]
    \titlespacing{\chapter}
    {0pt}
    {0pt}
    {0pt}[0pt]
}
\makeatother

This way, you can keep the \chapter style independently of the documentclass.

---

Clarification: This solution was never meant to provide the \chapter command for article class, but to keep the \chapter style redefinitions regardless of the document class used. As @UlrikeFischer mentioned, article does not provide \chapter, and this is for a reason, which also justifies the existence of book and report. I personally find it very wrong to provide chapters in articles.

However, as OP asked, one can use titlesec and titletoc to define a \chapter command. This could be done as follows (I tried to be close to the original, might not be perfect):

\documentclass{article}

\usepackage{titlesec}
\usepackage{titletoc}

\newcommand{\chaptername}{Chapter}
\makeatletter
\newcommand{\@chapapp}{\chaptername}
\makeatother

\titleclass{\chapter}{top}[\part]
\newcounter{chapter}

\titleformat{\chapter}[display]
    {\bfseries\huge}
    {\chaptertitlename\space\thechapter}
    {20pt}
    {\Huge\bfseries}

\titlespacing*{\chapter}
    {0pt}
    {50pt}
    {40pt}

\titlecontents{chapter}
    [1.5em]
    {\vspace*{10pt}\bfseries}
    {\contentslabel{1.3em}}
    {\hspace*{-1.3em}}
    {\hfill\contentspage}

\AddToHook{cmd/appendix/before}{
    \setcounter{chapter}{0}
    \gdef{\thechapter}{\Alph{chapter}}
}

\begin{document}

\chapter{A}

\end{document}

Note however that this is wrong on many levels. For example, the book and report classes use \chapter* to automatically issue the headings of \tableofcontents, \begin{thebibliography}{...}, \printindex, etc., while the article class uses \section* for this. Moreover, I have not accounted for markings which are used in the header of each page (similar to other information like page number). The book and report classes define \chaptermark for this, but obviously article does not have that.