如何从承诺中返回价值 [英] How to return value from a Promise

问题描述

我一直在与Promises挣扎，想知道事情在Promises中如何工作。
在我的项目中，我正在使用Bookshelfjs ORM从Postgres中获取数据。

I have been struggling with Promises and would like to know how things work with Promises. In my project, I am using Bookshelfjs ORM for fetching the data from Postgres.

这是我现在正在处理的代码。我在此请求中获得了一组设备ID，每个设备都以两种模式之一运行。

Here is the code that I am working on right now. I get an array of device ids in this request and each device is running in one of the two modes.

router.post('/devices', function (req, res, next) {
var currentData = [];
var deviceIds = req.body.devices;
loadash.forEach(deviceIds, function (device) {
    var deviceid = device.deviceid;
    Device.forge()
        .where({deviceid: deviceid})
        .fetch({columns: ['id', 'mode']})
        .then(function (fetchedDevice) {
            if(fetchedDevice.get('mode') === 1) {
                Model_1.forge()
                    .where({device_id: fetchedDevice.get('id')})
                    .orderBy('epoch_time', 'DESC')
                    .fetch()
                    .then(function (modelOne) {

                        //first push
                        currentData.push(modelOne.toJSON()); 

                        //array with first push data                
                        console.log(currentData)                                    
                    })
                    .catch(function (err) {
                        console.log(err);
                    });
            }
            else if(fetchedDevice.get('mode') === 2) {
                Model_2.forge()
                    .where({device_id: fetchedDevice.get('id')})
                    .orderBy('epoch_time', 'DESC')
                    .fetch()
                    .then(function (modelTwo) {

                        //second push
                        currentData.push(modelTwo.toJSON());

                        //array not empty here(shows data from both push)                
                        console.log(currentData);                                   
                    })
                    .catch(function (err) {
                        console.log(err);
                    });
            }
        })
        .catch(function (err) {
            console.log(err);
        });
   });
//This shows an empty array
console.log('Final: ' +currentData);                                                           
});

现在，我知道这是由于Javascript的异步特性而发生的。我的问题是

Now, I know this is happening because of async nature of Javascript. My question is

1. 在所有 push（）已经执行了吗？我尝试使用 Promise.all（）方法执行此操作，但没有成功。

1. how can I display final array after all push() have been executed? I tried doing this using Promise.all() method but did not have any success.

是是否有可能在每个promise中返回 modelOne 或 modelTwo 然后推送到数组？我该如何实现？

is it possible to return modelOne or modelTwo out of every promise and then push to an array? How can I achieve this?

推荐答案

使用 .map（）和 Promise.all（）， return 从函数传递给 .then（）

Use .map() and Promise.all(), return a value from function passed to .then()

var currentData = loadash.map(deviceIds, function (device) {
    var deviceid = device.deviceid;
    return Device.forge()
        .where({deviceid: deviceid})
        .fetch({columns: ['id', 'mode']})
        .then(function (fetchedDevice) {
            if(fetchedDevice.get('mode') === 1) {
                // return value from `.then()`
                return Model_1.forge()
                    .where({device_id: fetchedDevice.get('id')})
                    .orderBy('epoch_time', 'DESC')
                    .fetch()
                    .then(function (modelOne) {
                        // return value from `.then()`
                        return modelOne.toJSON(); 

                    })
                    .catch(function (err) {
                        console.log(err);
                    });
            }
            else if(fetchedDevice.get('mode') === 2) {
                // return value from `.then()`
                return Model_2.forge()
                    .where({device_id: fetchedDevice.get('id')})
                    .orderBy('epoch_time', 'DESC')
                    .fetch()
                    .then(function (modelTwo) {
                        // return value from `.then()`
                        return modelTwo.toJSON();

                    })
            }
        })

   });

   var res = Promise.all(currentData);
   res
   .then(function(results) {console.log(results)})
   .catch(function (err) {
     console.log(err);
   });

这篇关于如何从承诺中返回价值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！