了解 node.js 中的承诺拒绝

Question

我正在尝试理解 node.js 中的承诺。这是一个示例代码

con.queryReturnPromise("SELECT * FROM bookings WHERE driverId = " + accId + " AND bookingStatus = " + config.get('BOOKING_STATUS_ACTIVE') + " LIMIT 1")
  .catch((err) => {

    callback({
      "message": "Success",
      "error": true,
    });
    console.log("mysql query error");
    return Promise.reject();
  })
  .spread((rows, fields) => {
    if (rows.length != 1) {
      return Promise.reject();
    }
    mBooking = rows[0];
    var query = "INSERT INTO locations SET timeStamp = " + timeStamp + " latitude = " + lat + ", longitude = " + lng + ", bookingId = " + mBooking.id;
    return con.queryReturnPromise(query)
  })
  .catch((err) => {
    if (err)
      console.log("Adding Location error" + err);
    return Promise.reject();
  })
  .spread(() => {
    return funcs.getBooking(con, mBooking.id)
  })
  .then((booking) => {
    if (mBooking.userId.toString() in userSockets) {
      userSockets[mBooking.userId].emit(config.get('EVENT_USER_UPDATE_BOOKING'), {
        "message": "Success",
        "error": false,
        "booking": booking
      });
      userId = mBooking.userId.toString();
    }
    callback({
      "message": "Success",
      "error": false,
      "booking": booking
    });
  })
  .catch((err) => {
    if (err)
      console.log(err);
  });

代码很简单。但是我有一个困惑。如果正在调用return Promise.reject()，该函数将指向何处，将调用哪个代码。例如，如果第一个 catch 子句被调用并且它调用 return Promise.reject() 下面的部分代码将在之后运行

for 循环中的承诺

data = JSON.parse(data);
            var promisesArray = [];
            for (var i = 0; i < data.length; i++) 
            {
                var location = data[i];
                var lng       = location.lng;
                var lat       = location.lat;
                var bearing   = location.bearing;
                var deltaTime = location.deltaTime;
                var timeStamp = location.timeStamp;

                var query = "INSERT INTO locations SET timeStamp = " + timeStamp + " latitude = " + lat + ", longitude = " + lng + ", bookingId = " + mBooking.id;


                var promise = con.queryReturnPromise(query)                    
                        .then( () => {                            
                        });

                promisesArray[] = promise;
            }

            Promise.all(promisesArray)
            .then(function(results)
            {
                    callback({
                        "error": false,
                    });            
            });

Answer 1

每次执行return Promise.reject() 时，都会调用该链中的下一个.catch() 处理程序。它将跳过任何.then() 处理程序，直到下一个.catch() 处理程序。

从.then() 处理程序或.catch() 处理程序返回拒绝承诺会使当前承诺链被拒绝。因此，随着链中遇到更多处理程序，链中的下一个拒绝处理程序将被调用。

当您点击.catch() 处理程序时，.catch() 之后会发生什么取决于.catch() 所做的事情。如果它抛出或返回一个被拒绝的 Promise，则该 Promise 将保持被拒绝状态，并且链中的下一个 .catch() 将执行。如果它不返回任何内容或任何常规值（最终拒绝的承诺除外），则承诺将被解决（不再拒绝），然后链中的下一个 .then() 处理程序运行。

这是一个例子：

a().then(function() {
    // makes promise chain assume rejected state
    return Promise.reject();
}).then(function() {
    // this will not get called because promise is in rejected state
}).catch(function() {
    // this will get called
    // returning a reject promise will keep the promise in the reject state
    return Promise.reject();
}).then(function() {
    // this will not get called because promise is in rejected state
}).catch(function() {
    // this will get called
    return 2;
}).then(function(val) {
    // this will get called because prior `.catch()` "handled" the rejection
    // already
    console.log(val);    // logs 2
}).catch(function() {
     // this is not called because promise is in resolved state
});