问如何解析异步变量？

EN

// var passkey = {"shared_key":""} // Declaring passkey as global variable but didn't work.
const onSubmit = (formData) => {
        const email_id = getUserDetails().email;
        const from_db = db.collection(email_id); 
        const to_db = db.collection(formData.to);
        var passkey = {"shared_key": ""};

        // Check if there exists a shared key between `from` and `to`
        // Checking for shared key in either of the db is enough.
        to_db.doc(email_id).get().then((res) => {
            // assume that res.exists is true.
            if (res.exists) {
                passkey.shared_key = res.data().shared_key; // passkey is set here
            } else {
                // Generate and use a shared key
                ...some code to generate a shared key ...
                passkey.shared_key = generated_key;
            }
        });
        console.log(passkey); 
// above prints {"shared_key": ""} instead of printing the value of shared key taken from the database.
// or the one that is generated!
};