proxy_2.js

const test = {
  spy: function(fn) {
    const calls = [];
    return new Proxy(stuff, {
      apply(target, thisArg, args) {
        const result = target.apply(thisArg, args);
        calls.push([args, result]);
      },

      get(target, p){
        if (p === 'calls') {
          return calls;
        }

        return target[p];
      }
    });
  }
};

function stuff(arg1, arg2) {
  return `${arg1} ${arg2} cool`;
}

function doSomeStuff(fn) {
  return stuff(1, 2);
}

const spyStuff = test.spy(stuff);

spyStuff("hello", "cool");

doSomeStuff(spyStuff);

const calls = spyStuff.calls;
assert(calls[0][0][0] === "hello");
assert(calls[1][1] === "hello cool cool");