JavaScript - General Monkeypatch

example.js

import { patch } from './monkeyPatch';

(() => {
  patch(EventTarget, 'dispatchEvent', function() {
    const args = Array.from(arguments);

    const customEvent = new CustomEvent('GLOBAL_LOGGER', {
      details: args[0],
    });

    this.original.apply(this.sender, [customEvent, args.slice(0, 1)]);
    this.original.apply(this.sender, arguments);
  });

  patch(EventTarget, 'dispatchEvent', function() {
    const args = Array.from(arguments);

    console.log('called dispatchEvent w/ args', args);
  });

  document.addEventListener('GLOBAL_LOGGER', (event) => {
    console.log('GLOBAL_LOGGER_EVENT', event);
  });

  document.addEventListener('hello', (event) => {
    console.log('hello event listener', event);
  });

  document.dispatchEvent(new CustomEvent('hello', {
    detail: 'world',
  }));
})();

monkeyPatch.js

/**
 * Patch a function on a class with a new function
 * 
 * @param targetClass - Class to patch
 * @param targetFunctionName - Function to patch
 * @param patchFunction - Function to patch with
 */
export const patch = function (targetClass, targetFunctionName, patchFunction) {
  targetClass.prototype._originalFunctions ??= new Map();
  targetClass.prototype._patches ??= new Map();

  if (!targetClass.prototype._originalFunctions.has(targetFunctionName)) {
    targetClass.prototype._originalFunctions.set(targetFunctionName, targetClass.prototype[targetFunctionName]);
  }

  if (!targetClass.prototype._patches.has(targetFunctionName)) {
    targetClass.prototype._patches.set(targetFunctionName, new Set());
  }

  targetClass.prototype._patches.get(targetFunctionName).add(patchFunction);

  const proxy = new Proxy(targetClass.prototype[targetFunctionName], {
    apply: function (target, thisArg, argumentsList) {
      const context = { sender: thisArg, original: targetClass.prototype._originalFunctions.get(targetFunctionName) };

      targetClass.prototype._patches.get(targetFunctionName).forEach((patchFunction) => {
        patchFunction.apply(context, argumentsList);
      });
    }
  });

  targetClass.prototype[targetFunctionName] = proxy;
};