If you want to modify code that is loaded as an extension, you can use a StartupHook to synchronize your code with the loading of the extnsion. For example, we can modify the definition of the \iiiint macro, which is defined in the AMSmath extension, as follows: <script type="text/x-mathjax-config"> MathJax.Hub.Register.StartupHook("TeX AMSmath Ready",function () { var TEX = MathJax.InputJax.TeX; TEX.Macro("iiiint","\\unicode{x2A0C}"); }); </script> MathJax provides many signals about what it is doing that you can hook onto in this way. More about MathJax signals and hooks