Sync way of obtaining promise result

gistfile1.js

Promise.prototype.then = (()=> {
    const _then = Promise.prototype.then;
    return function(...args) {
        _then.call(this, (res) => {
            promiseResultMap.set(this, res);
        })
        this.then = _then;
        return _then.apply(this, args);
    }
  })();

  const promiseResultMap = new WeakMap()

function getPromiseValue(p) {
    return promiseResultMap.get(p);
}

const sleep = (duration)=> new Promise((resolve) => setTimeout(()=>resolve(0), duration));

(()=> {
    const p = sleep(500);
    console.log("before", getPromiseValue(p));
    p.then(()=> {
        console.log("after", getPromiseValue(p));
    })
})()