insertStuffDeco.js

const insertStuff = (target, name, descriptor) => {
  const original = descriptor.value;  

  if (typeof original === 'function') { 
    const paramNames = getParamNames(original)
    descriptor.value = function () {
      const args = paramNames.reduce((arr, pn, i) => {
        arr[i] = this.newStuff[pn];
        return arr;}, [] )

      const result = original.apply(this, [...args]);
      // console.log(`${name}(${JSON.stringify(args)}) = ${result}`)
    }
  }
}

class MyClass {
  @insertStuff
  getStuff(isWombat, sugar) {
    console.log({isWombat, sugar})
  }
}