The require extension defines the non-standard \require macro that allows you to load extensions from within a math expression in a web page. For example:

\(\require{enclose} \enclose{circle}{x}\)

would load the enclose extension, making the following \enclose command available for use.

An extension only needs to be loaded once, and then it is available for all subsequent typeset expressions.

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

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