Create a Promise that resolves when variable is not undefined

Question

I'm trying to Create a Promise that resolves when variable is not undefined.

Code example

https://codesandbox.io/s/quizzical-feynman-ktvox?file=/src/index.js

let fetchedPromise = new Promise((resolve, reject) => {
  const check = ()=>{
    setTimeout(() =>{
    console.log("checking")
    if (dataFetched) resolve(dataFetched);
    else check()
    }, 100);
  }
  check()
});

const waitForDataFromAnotherComponent = async () => {
  let result = await fetchedPromise;
  console.log("Result: ", result);
};

const assignData = () => {
  setTimeout(() => {
    dataFetched = 1000;
    console.log(dataFetched);
  }, 5000)
};

waitForDataFromAnotherComponent();
assignData();

This works but I find it inefficient as it's callstack prone and seems wrong.

Other non-working solutions I've tried:

//-------------SOLUTION 1
let fetchedPromise = new Promise((resolve, reject) => {
  const check = ()=>{
    if (dataFetched) resolve(dataFetched);
    else check()
  }
  check()
});

//--------------------SOLUTION 2
let fetchedPromise = new Promise((resolve, reject) => {
   if (dataFetched) resolve(dataFetched);
});

Scenario

I need a function like solution 3 that doesn't rely on setTimeout

Answer 1

Solved by using Javascript Proxy

Basically I assign a Proxy Object to dataFetched that listens to changes. I re-create the function of listening due to the fact that it must include resolve()

let dataFetched
let x = {
  aListener: function (val) {},
  set a(val) {
    dataFetched = val;
    this.aListener(val);
  },
  get a() {
    return dataFetched;
  },
  registerListener: function (listener) {
    this.aListener = listener;
  }
};

let fetchedPromise = new Promise((resolve, reject) => {
  x.registerListener(function (val) {
    console.log("yeyyyyyyyyyyyyyyy");
    if (dataFetched) resolve(dataFetched);
  });
});

const waitForDataFromAnotherComponent = async () => {
  let result = await fetchedPromise;
  console.log("Result: ", result);
};
const assignData = async () => {
  await new Promise((resolve, reject) =>
    setTimeout(() => {
      x.a = 1000;
      console.log(dataFetched);
      resolve(dataFetched);
    }, 1000)
  );
};

waitForDataFromAnotherComponent();
assignData();

EDIT

Actually it's possible to externalize the resolve() function of the promise but with some downsides as stated here

example

let dataFetched
var promiseResolve, promiseReject;

let x = {
  aListener: function (val) {
    if (dataFetched) promiseResolve(dataFetched);
},
  set a(val) {
    dataFetched = val;
    this.aListener(val);
  },
  get a() {
    return dataFetched;
  },
  registerListener: function (listener) {
    this.aListener = listener;
  }
};

let fetchedPromise = new Promise((resolve, reject) => {
 promiseResolve = resolve;
  promiseReject = reject;
});