let reject;
new Promise((_, rejectFn) => {
  reject = rejectFn;
});

window.onunhandledrejection = (evt) => {
    // Creates a new unhandled promise based on `promise`,
    // which will cause this listener to be called (again and again).
    evt.promise.then();
};

reject();

Asking because the former is roughly what internally happens with an async function's return value. In those case, do we want the event handler to run in the context of the initial rejection, or in the context of the call to the async function ?

const ctx = new AsyncContext()

window.onunhandledrejection = (evt) => {
    ctx.get(); // ??
};

const wrap = async (p) => p;

let reject;
ctx.run(1, () => wrap(new Promise((_, rejectFn) => {
  reject = rejectFn;
}));

ctx.run(2, reject);