How to make JavaScript Proxy's handler methods to be async functions?

Question

Refer the Mozilla document about Proxy()

A simple Proxy example:

const handler = {
  get: function(target, prop, receiver) {
    return Reflect.get(target, prop, receiver);
  },
  set: function(target, prop, receiver) {
    return Reflect.set(target, prop, receiver);
  }
};

const proxy = new Proxy(target, handler);

I have some async functions in get and set, so I would like to let get and set to be async functions.

I expect something like:

const handler = {
  get: async function(target, prop, receiver) {
    await foo();
    return Reflect.get(target, prop, receiver);
  },
  set: async function(target, prop, receiver) {
    await bar();
    return Reflect.set(target, prop, receiver);
  }
};

const proxy = new Proxy(target, handler);

await (proxy.prop1 = xxx);
yyy = await proxy.prop2;

How can I achieve my goal? Thanks!

Answer 1

This is not possible. An assignment expression proxy.property = value always¹ evaluates to value, not some return value of the proxy's set trap. You cannot make it return an awaitable promise.

_{1: unless the assignment, including the proxy traps, throws an exception of course.}