Getting Data from Firestore for web

I currently try to use firebase for web and get data from Firestore. This is my code for bring the all my data from Firestore.

function FireStoreGetAllData() {

let db = firebase.firestore();
let dataMap = new Map();

db.collection("User").get().then((querySnapshot) => {
        querySnapshot.forEach((doc) => {
            console.log(doc.id, " => ", doc.data());
          
            let dataset = new Map();
            let docId = doc.id;
            let docdata = doc.data();

            let firstName = docdata.first;
            let lastName = docdata.last;
            let bornData = docdata.born;

            dataset.set('first', firstName);
            dataset.set('last', lastName);
            dataset.set('born', bornData)

            dataMap.set(docId, dataset);
        });

    });
console.log('dataSet',dataMap.entries());
return dataMap;}

Problem is the "dataMap" always return null every time. I debug this issue and find out that "db.colloction" is execute the code after "return dataMap".

What am I missing?