How to define a function which returns Promise to a Express Route function?

Question

I have a business leve database module called "db_location" which uses the node-fetch module to get some data from a remote server via REST API.

**db_location.js** DB LOGIC

const p_conf = require('../parse_config');

const db_location = {
    getLocations: function() {

        fetch(`${p_conf.SERVER_URL}/parse` + '/classes/GCUR_LOCATION', { method: 'GET', headers: {
            'X-Parse-Application-Id': 'APPLICATION_ID',
            'X-Parse-REST-API-Key': 'restAPIKey'
        }})
        .then( res1 => {
            //console.log("res1.json(): " + res1.json());
            return res1;
        })
        .catch((error) => {
            console.log(error);
            return Promise.reject(new Error(error));
        })
    }

};

module.exports = db_location

I would need to call this function within a Route function so as to separate database processing from controller.

**locations.js** ROUTE

var path = require('path');
var express = require('express');
var fetch = require('node-fetch');
var router = express.Router();

const db_location = require('../db/db_location');

/* GET route root page. */
router.get('/', function(req, res, next) {

  db_location.getLocations()
  .then(res1 => res1.json())
  .then(json => res.send(json["results"]))
  .catch((err) => {
    console.log(err);
    return next(err);
  })
});

When I ran http://localhost:3000/locations, I received the following error.

Cannot read property 'then' of undefined

TypeError: Cannot read property 'then' of undefined

It seems the Promise was empty or something wrong down the Promise chain going from one response object to another? What is a best practise for solving this kind of scenario?

EDIT 1

If I changed the getLocations to return res1.json() (which I think is a non-empty Promise according to the node-fetch documentation):

fetch(`${p_conf.SERVER_URL}/parse` + '/classes/GCUR_LOCATION', { method: 'GET', headers: {
        'X-Parse-Application-Id': 'APPLICATION_ID',
        'X-Parse-REST-API-Key': 'restAPIKey'
    }})
    .then(  res1 => {
       return res1.json();     // Not empty as it can be logged to `Promise Object`
    })
    .catch((error) => {
        console.log(error);
        return Promise.reject(new Error(error));
    })

And the route code was changed to :

db_location.getLocations()
  .then(json => res.send(json["results"]))
  .catch((err) => {
    console.log(err);
    return next(err);
  })

The exactly same error was thrown.

score 2 · Accepted Answer · answered Jul 19 '18 at 01:26

2

You need getLocations to return a Promise. At the moment, it's running a fetch, but that fetch isn't connected to anything else, and getLocations is returning undefined (and of course you can't call .then on uundefined)

Instead, change to:

const db_location = {
  getLocations: function() {
    return fetch( ...

Also, since you're not doing anything special in the getLocations catch block, you might consider omitting it entirely and let the caller handle it.

answered Jul 19 '18 at 01:26

CertainPerformance

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Many thanks for your answer! I have a further question. Please refer to EDIT 1 as above. – alextc Jul 19 '18 at 01:39
Your `EDIT 1` doesn't look to be `return`ing the initial `fetch`, which is exactly what you need to to to chain the promises together, as said in the answer...? – CertainPerformance Jul 19 '18 at 01:45
Does it have to return the initial fetch? Is there a way to return the response.json() from the fetch to the route function? response.json() from the fetch call is a Promise accordin to node-fetch documentation. – alextc Jul 19 '18 at 01:47
Yes, you need to both return the `res1.json()` from the `.then` chained right after the `fetch` *in addition to returning the `fetch` itself*, that way the consumer of `getLocations` can see and chain onto the end of the returned Promise chain. `return fetch` – CertainPerformance Jul 19 '18 at 01:49
Did you mean return both res1.json() and fetch from the getLocations() function? Can you please show an example? – alextc Jul 19 '18 at 01:58
eg `getLocations: () => fetch( ... ).then(res1 => res1.json())` using arrow functions' implicit return – CertainPerformance Jul 19 '18 at 02:00

score 0 · Answer 2 · answered Jul 19 '18 at 01:25

0

Your function doesn't return anything.

If you want to use a promise, you need return it.

answered Jul 19 '18 at 01:25

SLaks

868,454
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Many thanks! I have made some changes to the code and it returned a Promise object but the same error was thrown. Please refer to EDIT as above. – alextc Jul 19 '18 at 01:41

How to define a function which returns Promise to a Express Route function?

2 Answers2