Retrieve value passed to a called back function in a mathjax-node Node.js example

Question

I am trying to use the example

// a simple TeX-input example
var mjAPI = require("mathjax-node/lib/mj-single.js");
mjAPI.config({
  MathJax: {
    // traditional MathJax configuration
  }
});
mjAPI.start();

var yourMath = 'E = mc^2';

mjAPI.typeset({
  math: yourMath,
  format: "TeX", // "inline-TeX", "MathML"
  mml:true, //  svg:true,
}, function (data) {
  if (!data.errors) {console.log(data.mml)}
});

from this page.

From what I see the method typeset passes the output I need to a function input in its second argument.

I don't want to print the output data.mml. I need to assign it to an external variable for further processing.

Question: How can I assign data.mml to a variable that is outside the scope of the anonymous function passed to typeset?

The anonymous function is not able to modify variables from the global scope (outside typeset) because that is not the immediate scope from the the anonymous function is called.