Defining TeX macros

You can use the \def, \newcommand, \renewcommand, \newenvironment, \renewenvironment, and \let commands to create your own macros and environments. Unlike actual TeX, however, in order for MathJax to process such definitions, they must be enclosed in math delimiters (since MathJax only processes macros in math-mode). For example

\(
   \def\RR{{\bf R}}
   \def\bold#1{{\bf #1}}
\)

would define \RR to produce a bold-faced “R”, and \bold{...} to put its argument into bold face. Both definitions would be available throughout the rest of the page.

Configuring Macros at Startup

You can include macro definitions in the macros section of the tex block of your configuration, but they must be represented as javascript objects. For example, the two macros above can be pre-defined in the configuration by

window.MathJax = {
  tex: {
    macros: {
      RR: "{\\bf R}",
      bold: ["{\\bf #1}", 1]
    }
  }
};

Here you give the macro as a name: value pair, where the name is the name of the control sequence (without the backslash) that you are defining, and value is either the replacement string for the macro (when there are no arguments) or an array consisting of the replacement string followed by the number of arguments for the macro and, optionally, default values for optional arguments.

Note that the replacement string is given as a javascript string literal, and the backslash has special meaning in javascript strings. So to get an actual backslash in the string you must double it, as in the examples above, or use the javascript String.raw method for specifying the string literal.

Similarly, you can create new environments with the environments section of the tex block of your configuration, and active characters (single characters that act as a macro without the need for a backslash) using the active section of the tex block of your configuration.

See configmacros Options for more details on the macros, environments, and active configuration blocks.

Running TeX Code During Startup

Alternatively, you can use the ready() function in the startup block of your configuration to process a TeX expression that defines the macros, as in the following example:

window.MathJax = {
  startup: {
    ready() {
      MathJax.startup.defaultReady();
      const {STATE} = MathJax._.core.MathItem;
      MathJax.tex2mml(String.raw`
        \newcommand{\RR}{\mathbf{R}}
        \newcommand{\bold}[1]{\mathbf{#1}}
        \let\star=\ast
      `);
    }
  }
};

This allows you to use actual TeX commands, but without having to have an actual TeX block within the page to make the definitions.

If you need to have definitions that are different for each page, then you could include them in a script that specifies the definitions, and then include that in the ready() call. For example,

<script type="text/x-tex-macros">
  \newcommand{\RR}{\mathbf{R}}
  \newcommand{\bold}[1]{\mathbf{#1}}
  \let\star=\ast
</script>
<script>
  window.MathJax = {
    startup: {
      ready() {
        MathJax.startup.defaultReady();
        const {STATE} = MathJax._.core.MathItem;
        const defs = document.querySelector('script[type="text/x-tex-macros"]');
        MathJax.tex2mml(defs?.textContent || '');
      }
    }
  };
</script>

will take the definitions from the first script, if any, and process them during its startup phase. You can have the first script be part of the page, and the second be part of the scripts loaded into every page, and that will allow you to have page-specific definitions. It would also be possible to use a regular <script> tag to set a javascript variable (rather than using type="text/x-tex-macros") and use that value in the convert() call that rather than looking up the contents of a script.

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