The ams extension implements AMS math environments and macros, and macros for accessing the characters in the AMS symbol fonts. This extension is already loaded in all the components that include the TeX input jax, other than input/tex-base. See the list of control sequences for details about what commands are implemented in this extension.

To load the ams extension explicitly (when using input/tex-base for example), add '[tex]/ams' to the load array of the loader block of your MathJax configuration, and add 'ams' to the packages array of the tex block.

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

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