javascript - 如何将 Firestore 中的数据放入全局变量？

Question

如何让值在 onSnapshot 函数之外返回？

function checkIfExisting(){

  const collection = firebase.firestore().collection("audit");
  const getCaseID = collection.where("caseID", "==", "00001234");

  getCaseID.onSnapshot(function(querySnapshot) {
    let wordLists = [];
    querySnapshot.forEach(function(doc) {
    //get all the scanned words under "word" field in the Firestore Database
        wordLists.push(doc.data().word);
    });

    console.log("words: ", wordLists);// return a value

  });

  console.log("words1: ", wordLists);// undefined

  }

我知道 console.log("words1: ", wordLists) 在函数之外，这就是我无法获取其值的原因。您能否给我一个关于如何在函数外调用它的想法(如果可能)。

Answer 1

要在外部访问它，您可以使用Promise:

function checkIfExisting(){
  return new Promise((resolve, reject) => {
   const collection = firebase.firestore().collection("audit");
   const getCaseID = collection.where("caseID", "==", "00001234");

   getCaseID.get().then(function(querySnapshot) {
     let wordLists = [];
     querySnapshot.forEach(function(doc) {
     //get all the scanned words under "word" field in the Firestore Database
        wordLists.push(doc.data().word);
          resolve(wordLists);
     });
     console.log("words: ", wordLists);// return a value
  });

然后要访问外部执行以下操作:

checkIfExisting().then((result) => {
   console.log(result);
});

result 将包含 wordLists

查看以下内容以获取更多信息:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise