javascript - 如何知道异步 forEach 何时完成

Question

我想在两个 forEach 完成时调用回调。我想知道它们何时完成异步处理并调用回调。 console.log("Done") 似乎在两个 forEach

之前完成

const getDates =  () => {
    const ref  = db.ref("reminders");
    const dateTime = new Date();
    const currentDate = dateFormat(dateTime, "yyyy-mm-dd");

    ref
        .orderByChild('date')
        .endAt(currentDate)
        .once('value', (reminderDates) => {
             reminderDates.forEach((singleDate) => {
                 // iterate over reminder dates
                  singleDate.forEach( (notificationValues) => {
                      // iterate over notification codes
                    if (!notificationValues.key.includes('date')) {
                        processNotifications(notificationValues, () => {
                            console.log(`Sent notification reminder at ${notificationValues.key}`);
                        });
                    }
                });
            });
    }).catch( (error) => {
        console.log(error);
    });
    console.log("Done")
};

输出

Done

AB000001_AB0977 { subtitle: 'Time to start thinking about making a payment',
  title: 'School Semester 1, 2019 School Fees',
  userId: 'kXnfHPyxfpeLQ1aCjvl8Pu09sssslou1' } d-ktpdo45SQ:APA91bF5rJtaHvtNUE42GDssssXoOAP_r7omRmsIs44WKnABsMC8lintdoDBzUYrZ5lutEKECwuaOOIQtdZkKW5Apt4A0ssssyZwdl_epdI2dYHkhk0h-Yns6jzlMbIltSHasA40YL725sssL9TmyCd
Sent notification reminder at AB000001_AB0977

Answer 1

来自文档:

once

once(eventType: EventType, successCallback?: function, failureCallbackOrContext?: Object | null, context?: Object | null): Promise<DataSnapshot>

once返回 Promise这意味着它是异步的，因此 console.log("Done")将在您的 forEach() 之前打印。您无法知道异步操作何时完成。

所以解决的最好办法就是加上console.log("Done")里面forEach() :

        .once('value', (reminderDates) => {
         reminderDates.forEach((singleDate) => {
             // iterate over reminder dates
              singleDate.forEach( (notificationValues) => {
                  // iterate over notification codes
                if (!notificationValues.key.includes('date')) {
                    processNotifications(notificationValues, () => {
                        console.log(`Sent notification reminder at ${notificationValues.key}`);
                       console.log("Done");
                    });
                }
            });
        });