using a Proxy to monkey-patch a method

monkey_patch_with_proxy.js

class Foo {
  method() {
    return 'foo';
  }
}

class Wrapper {
  constructor(wrapped) {
    this.latch = 2; // arbitrary condition; "proxy it this many times"
    return new Proxy(wrapped, this);
  }

  get(target, key, proxy) {
    const actual = Reflect.get(target, key, proxy);
    if ((key === 'method') && (this.latch !== 0)) {
      return (...args) => {
        return `${ Reflect.apply(actual, target, args) } @ ${ this.latch-- }`;
      };
    }
    return actual;
  }
}

const wrapped = new Wrapper(new Foo());

for (let i = 0; i < 5; ++i) {
  console.log(i, wrapped.method());
}