JSFiddle - React, Tailwind, and code Playground

JavaScript

function foo() {
 console.log('foo');
}

function appendToFunction(fn, callback) {
  window[fn] = (function(fn){
    return function() {
      fn();
      callback();
    }
 }(window[fn]));
}

appendToFunction('foo', function(){console.log('appended!')});

foo();