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Question

我正在使用Firestore，Cloud Functions和Dialogflow來創建Google Home應用程序。 我的雲函數應該根據集合中的值返回計算，但我只能返回一個Promise。

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
var accountbalance = await transactions.get()
    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })
    .catch(err => {
        console.log('Error getting transactions', err);
    });

console.log(accountbalance)
balanceresponse = {
    "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
    "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
    "data": {},
    "contextOut": [],
    "source": "system"
}

response.send(balanceresponse);    

在調用response.send之前，如何等待promise返回實際值？ 我還嘗試將response.send放在then中，但該函數完成並在操作完成之前發送頭文件。

Answer 1

如果您正在使用promises，則需要將值作為Promise.then鏈的一部分返回。 這通常是通過將一堆.then()調用/塊鏈接在一起來完成的。 請記住，只有在Promise結算時才會調用then()部分。

在您的情況下，看起來您可以生成balanceresponse並在計算workingbalance后在同一代碼塊內調用response.send() 。 對response.send()的調用不需要在函數的末尾。 所以你可能看起來像這樣的塊：

.then(snapshot => {
    var workingbalance = 0
    snapshot.forEach(doc => {
        workingbalance = workingbalance + doc.data().amount;
    });

    balanceresponse = {
      "speech": request.body.result.parameters.accountowner + " has a balance of $" + workingbalance,
      "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + workingbalance,
      "data": {},
      "contextOut": [],
      "source": "system"
    }

    response.send(balanceresponse); 
})

如果由於某種原因，您想要將計算分開並計算不同塊中的balanceresponse ，則可以這樣做。 我不一定推薦這個，但它有一些優點（例如，你可以在每個部分之后放置多個catch塊，我在這里展示）：

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
await transactions.get()

    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })

    .catch(err => {
        console.log('Error getting transactions', err);
    });

    .then( accountbalance => {
        balanceresponse = {
            "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
            "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
            "data": {},
            "contextOut": [],
            "source": "system"
        }
        response.send(balanceresponse);    
    })

    .catch(err => {
        console.log('Error sending response', err);
    });