A way to detect mathjax: \ifmathjax

It is not possible to share symbol definition files between full LaTeX and programs using MathJax as a renderer (e.g. previews in VSCode LaTeX plugins). For example, MathJax chokes up at the following definition of the symbol for weak* convergence:

\makeatletter
\def\weaktostar@sym{\setbox0=\hbox{$\rightharpoonup$}\rlap{\hbox 
        to\wd0{\hss\raise1ex\hbox{$\scriptscriptstyle{*\,}$}\hss}}\box0}
\def\weaktostar{\mathrel{\weaktostar@sym}}
\makeatother

This is the case even if \makeatother, \makeatletter, and use of @ are removed; in the first case all renderings are full of \makeatother gibberish, in the second case only \weaktostar renders full of gibberish.

For rendering in MathJax it would be sufficient to

\def\weaktostar{\stackrel{\hbox{$\scriptscriptstyle{*\,}$}}{\rightharpoonup}}

However, this is a suboptimal rendering for actual LaTeX output. So it would be good if there was a \ifmathjax{…MathJax content…}{…LaTeX content…} macro, and/or, considering also the use of KaTeX by other applications (which could implement the same mechanism), a more general \iffulllatex macro .

No source file, test, or entry point is named in the issue. Start by reviewing the MathJax handling of custom LaTeX macros and the supplied \weaktostar examples; done means a documented conditional mechanism can select MathJax-compatible content while preserving the preferred full-LaTeX rendering.