The noundefined extension causes undefined control sequences to be shown as their macro names rather than generating error messages. So $X_{\xyz}$ would display as an “X” with a subscript consisting of the text \xyz in red.


In version 2, the styling for the undefined macro could be configured. In version 3, this is not yet implemented.

This extension is already loaded in all the components that include the TeX input jax, other than input/tex-base. To load the ams extension explicitly (when using input/tex-base for example), add '[tex]/noundefined' to the load array of the loader block of your MathJax configuration, and add 'noundefined' to the packages array of the tex block.

window.MathJax = {
  loader: {load: ['[tex]/noundefined']},
  tex: {packages: {'[+]': ['noundefined']}}