如何在Node.js中基于Promise的业务级功能中处理Error上返回的对象？

Question

I need to create a business level function called "getLocationById" which retrieves some data from a remote server via REST API. This function is then called by a router to display the data on a web page.

If the fetch call is successful, the json result is returned as Promise. However, what should be returned to the router if fetch caught an error, eg remote server was not responding or responding with a 500 error?

Furthermore, how does the route respond to the error?

const fetch = require('node-fetch');    
const p_conf = require('../parse_config');  // Configuration

const db = {
    getLocationById: function(locId) {
        fetch(`${p_conf.SERVER_URL}/parse` + '/classes/location', { method: 'GET', headers: {
            'X-Parse-Application-Id': p_conf.APP_ID,
            'X-Parse-REST-API-Key': p_conf.REST_API_KEY
        }})
        .then(res1 => return res1.json())  // RETURN A PROMISE ON SUCCESS
        .catch((error) => {
            console.log(error);
            **WHAT TO RETURN TO THE ROUTER ON ERROR HERE?**
        });
    }
};

EDIT:

const db_location = {
    getLocations: function() {
        //res.send("respond with 'locations' router.");
        fetch(`${p_conf.SERVER_URL}/parse` + '/classes/GCUR_LOCATION', { method: 'GET', headers: {
            'X-Parse-Application-Id': p_conf.APP_ID,
            'X-Parse-REST-API-Key': p_conf.REST_API_KEY
        }})
        .then(res1 => res1)
        .catch((error) => {
            console.log(error);
            return Promise.reject(new Error(error));
        })
    }
};

In router:

router.get('/', function(req, res, next) {
  db_location.getLocations()
  .then(r => res.send(r.json()))      // WHERE AN ERROR WAS THROWN
  .catch((err) => {
    console.log(err);
    return next(err);
  })
});

The following error was thrown:

TypeError: Cannot read property 'then' of undefined

on .then(r => res.send(r.json()))

FURTHER EDITS:

I then made the following changes.

business layer

getLocations: function() {
    // According to node-fetch documentation, fetch returns a Promise object.
    return fetch(`${p_conf.SERVER_URL}/parse` + '/classes/GCUR_LOCATION', { method: 'GET', headers: {
        'X-Parse-Application-Id': p_conf.APP_ID,
        'X-Parse-REST-API-Key': p_conf.REST_API_KEY
      } });

}

Router side:

router.get('/', function(req, res, next) {
   db_location.getLocations()
  .then(r => {
    console.log("r.json(): " + r.json());
    res.send(r.json())})
  .catch((err) => {
    console.log(err);
    return next(err);
  })  
});

Then a new error was thrown:

(node:10184) UnhandledPromiseRejectionWarning: TypeError: body used already for: http://localhost:1337/parse/classes/GCU
R_LOCATION
    at Response.consumeBody (C:\Work\tmp\node_modules\node-fetch\lib\index.js:326:30)
    at Response.json (C:\Work\tmp\node_modules\node-fetch\lib\index.js:250:22)
    at db_location.getLocations.then.r (C:\Work\tmp\ExpressApps\express-parse-server\routes\locations.js:30:13)
    at <anonymous>
    at process._tickDomainCallback (internal/process/next_tick.js:228:7)
(node:10184) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing ins
ide of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejectio
n id: 5)
(node:10184) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejection
s that are not handled will terminate the Node.js process with a non-zero exit code.

I believed the fetch function returned a Promise object that can be received by the calling function from the route?

Answer 1

On failure or error, the system needs to do the following:

1. Register the error
2. Determine the error type
3. Operational errors, such as failing to connect to a server
4. Programmer error, which are bugs and needs to be fixed by a developer

Operational errors can be ignored by

1. Unit Testing
2. Taking care of asynchronous and synchronous calls
3. Ensuring all arguments are not passed to the function
4. Checking for empty arguments, if false, show an error asking the user to provide required fields

In case of server no connecting, the system should retry after an interval of 60 seconds

Programming errors:

• Terminate the function with 'throw '
• Record the error
• Show the user with an appropriate message, without presenting any technical jargony

Answer 2

If your error needs to be handled by the router you either need to not catch() in getLocationById or catch() there, process what you need, and then return Promise.reject(error) so the router can still take care of it downstream.

Something like:

 function getLocation() { return Promise.reject('some error') // fake fetch error .then(r => r ) .catch(error => { console.log("handling error in getlocation") // maybe logging or other functions return Promise.reject(error) }) } // in routes: getLocation() .then(r => console.log("sending to user")) .catch(error => console.log("sending error to user: ", error)) 

If you don't have process the error in getLocation() just don't catch it and the error will go down the chain:

 function getLocation() { return Promise.reject('some error') .then(r => r ) } // in routes: getLocation() .then(r => console.log("sending to user")) .catch(error => console.log("sending error to user: ", error)) 

Answer 3

Your new edits are close! First let's clear up a misconception you have about fetch . Non-OK responses do not result in the fetch promise being rejected. In order to determine if a call has a successful response, check response.ok .

Next we need to investigate the json method. Looking at the documentation we see that it also returns a promise and not JSON.

Here's a version of your router that's a closer to what you're looking for:

router.get('/', function(req, res, next) {
   db_location.getLocations()
   .then(r => {
        if (r.ok) { return r.json(); }
        throw 'Something went wrong!';
    })
   .then(data => res.json(data))
   .catch((err) => {
        console.log(err);
        next(err);
    })  
});

I think it's great that you're learning promises. Once you feel comfortable with promises check out async/await . It'll make your code easier to read but having an understanding of promises is important.

Answer 4

Try switching to the following code and let me know if it works:

router.get('/', function(req, res, next) {
   return db_location.getLocations().then(r => {
       console.log("r.json(): " + r.json());
       return res.json(r);
     })
     .catch(err => {
       console.log(err);
       return next(err);
   })  
});