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.

Since the nnewcommand extension is included in the combined components that contain the TeX input jax, it may already be in the package list. In that case, if you want to disable it, you can remove it:

window.MathJax = {
  tex: {packages: {'[-]': ['newcommand']}}