The newcommand extension provides the \def, \newcommand, \renewcommand, \let, \newenvironment, and \renewenvironment macros for creating new macros and environments in TeX. For example,

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

defines a macro \RR that produces a bold “R”, while \bold{math} typesets its argument using a bold font. See Defining TeX macros for more information.

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

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

Alternatively, use \require{newcommand} in a TeX expression to load it dynamically from within the math on the page, if the require package is loaded.