如何避免 javascript 中的异步功能？

Question

function CanPurchase() {
return new Promise(function(resolve, reject){
  var name = document.getElementById('name').value;
      var civilNumber = document.getElementById('civilNumber').value;

      var canPurchase_query = db.collection("observer").doc("purchase_record").collection("record_set").where("name",
        "==", name).where("civilNumber", "==", civilNumber);
      var result = "";

     canPurchase_query
        .get()
        .then(function (querySnapshot) {
          querySnapshot.forEach(function (doc) {
            // doc.data() is never undefined for query doc snapshots
            result += doc.data().time;
          });
          if (result) {
            console.log("canPurchase", false);
            alert(false);
            resolve(false);
          } else {
            alert(true);
            console.log("canPurchase", true);
            resolve(true);
          }
        })
        .catch(function (error) {
          alert(false);
          console.log("Error getting documents: ", error);
          resolve(false);
        });


});

    }

function StoreBuyerListInDB() {
      var serialNumber = document.getElementById("serialNumber3").value;
      var name = document.getElementById("name").value;
      var count = document.getElementById("count3").value;
      var civilNumber = document.getElementById("civilNumber").value;
      var canPurchase = CanPurchase().then(function(resolvedData){
        return resolvedData;
      });
      sleep(3222);
      //DB에 시리얼넘버, 이름, 개수, 주민번호 넣기
      var docRef = db.collection("users").doc(firebaseEmailAuth.currentUser.uid);
      var docRef2 = db.collection("users").doc(firebaseEmailAuth.currentUser.uid).collection("buyers");
      var docRef3 = db.collection("observer").doc("purchase_record").collection("record_set");
      //로그인된 사용자가 seller인지 확인
      docRef.get().then(function (doc) {
        if (doc.exists) {
          if (doc.data().index != "seller") {
            alert('seller가 아니라면 등록할 수 없습니다! 돌아가세요!');
            return;
          }
          console.log("Document data:", doc.data());
        } else {
          // doc.data() will be undefined in this case
          console.log("No such document!");
          return;
        }
      }).catch(function (error) {
        console.log("Error getting document:", error);
      });

      //어느 칸이라도 비어있으면 안됨
      if (!serialNumber || !name || !count || !civilNumber) {
        alert('제대로 입력해주세요!');
        return;
      }
      //갯수가 0개이상 3개이해
      else if (count < 0 || count >= 3) {
        alert('1인당 3개 이하의 마스크만 구매 가능합니다! 다시 입력해주세요!');
        return;
      }
      //주민번호 유효성
      else if (!ValidateCivilNumber(civilNumber)) {
        alert('주민번호가 유효하지 않습니다.');
        return;
      }
      //전체 구매자 db에 이름, 주민번호가 같은사람이 있을경우 등록 불가
      else if (!canPurchase) {
        alert('이번주에 이미 구매하셨습니다.');
        return;
      }
      else{
        //해당seller의 db에 저장
        docRef2.add({
            name: name,
            serialNumber: serialNumber,
            count: count,
            civilNumber: civilNumber,
            time: firebase.firestore.FieldValue.serverTimestamp()
          })
          .then(function (docRef) {
            console.log("Document written with ID: ", docRef.id);
            alert("seller 저장 완료!");
            setTimeout(GetDataFromDB(), 3000);
          })
          .catch(function (error) {
            console.error("Error adding document: ", error);
            alert("seller 저장 실패!");
          });
        //감사자인 observer db에 저장
        docRef3.add({
            name: name,
            serialNumber: serialNumber,
            count: count,
            civilNumber: civilNumber,
            time: firebase.firestore.FieldValue.serverTimestamp()
          })
          .then(function (docRef) {
            console.log("Document written with ID: ", docRef.id);
            alert("observer 저장 완료!");
            setTimeout(GetDataFromDB(), 3000);
          })
          .catch(function (error) {
            console.error("Error adding document: ", error);
            alert("observer 저장 실패!");
          });
        }
    }

I wanna know user can purchase items or not by using CanPurchase() function and if process. But in real case, it doesn't work.

 else if (!canPurchase) {
        alert('이번주에 이미 구매하셨습니다.');
        return;
      }

here it just pass this logic. so whatever canPurchase variable is, it just go on. So I searched some ways to avoid this problem. I used promise. but also it doesn't work too. How can i solve it? Thank you so much !!

Answer 1

There is no way to make code that calls an asynchronous API behave synchronously. If you want to interact with cloud-based (and most other modern) APIs, you will have to learn to work with asynchronous calls.

In your case, the code for CanPurchase can be a bit simpler, as you don't need to declare your own promise as far as I can see. It should be something like this:

function CanPurchase() {
  var name = document.getElementById('name').value;
  var civilNumber = document.getElementById('civilNumber').value;

  var canPurchase_query = db.collection("observer").doc("purchase_record")
    .collection("record_set")
    .where("name", "==", name)
    .where("civilNumber", "==", civilNumber);
  var result = "";

  return canPurchase_query
    .get()
    .then(function (querySnapshot) {
      querySnapshot.forEach(function (doc) {
        result += doc.data().time;
      });
      return !result
    })
    .catch(function (error) {
      return false;
    });
});

So this no longer has a custom Promise , but instead returns the value from within then and catch and then the query. This is a common pattern when using promises, and is called bubbling up the results.

You can now call this function with:

canPurchase().then(function(result) {
  if(!result) {
    alert('이번주에 이미 구매하셨습니다.');
  }
}

---

If you're OK with using some more modern JavaScript feature, you can use async / await to make this code look a bit more familiar:

async function CanPurchase() {
  ...
}

let result = await canPurchase();
if(!result) {
  alert('이번주에 이미 구매하셨습니다.');
}

Just keep in mind that this is syntactic sugar around the existing flow with promises, and does not change the fact that these are asynchronous calls.