Calling methods from other files with Node.js

Question

I'm failing to require methods from my ./db/index.js into my server.js file to select data from the database and display it.

The /db/index.js is like this:

'use strict';

const pgp = require('pg-promise')();
const pg = pgp(process.env.DATABASE_URL);

let select = () => {
    pg.any('SELECT username, status FROM status')
        .then(function(data){
            for (var item of data) {
                return item.username + "'s status is " + item.status;
            }
        })
        .catch(function(err) {
            return 'Error: ' + err.message || err;
        });
};

module.exports = () => {
    select
};

and I want to call it in from a different file:

'use strict';

const port = process.env.PORT || 3000;
const bodyParser = require('body-parser');
const express = require('express');
const app = express();
const db = require('./db/');

app.use(bodyParser.urlencoded({extended: true}));

app.post('/logdash', function(req, res, next) {
    res.status(200).send(db.select());
});

app.listen(port, function() {
    console.log('Server is running on port', port);
});

I'm using Heroku, and like this, watching the logs, no error is shown in both terminal and Slack (it's a slash command). I can't find help on how to properly separate the functions. How can I call this select method and any other one from a different file?

Answer 1

The code in your module is asynchronous. You can't return a value directly. Instead, you should return the promise and then use the promise from the caller to obtain the final asynchronous value.

For further discussion of this general concept see this answer:

How do I return the response from an asynchronous call?

Change your code to this (see embedded comments):

'use strict';

const pgp = require('pg-promise')();
const pg = pgp(process.env.DATABASE_URL);

let select = () => {
    // return the promise here
    return pg.any('SELECT username, status FROM status')
        .then(function(data){
            return data.map(function(item) {
                return item.username + "'s status is " + item.status;
            });
        })
        .catch(function(err) {
            // to keep this an error, we have to rethrow the error, otherwise
            // the rejection is considered "handled" and is not an error
            throw 'Error: ' + err.message || err;
        });
};

// export the function
module.exports.select = select;

And call it like this:

'use strict';

const port = process.env.PORT || 3000;
const bodyParser = require('body-parser');
const express = require('express');
const app = express();
const db = require('./db/');

app.use(bodyParser.urlencoded({extended: true}));

app.post('/logdash', function(req, res, next) {
    db.select().then(function(data) {
        res.status(200).json(data);
    }).catch(function(err) {
        // add some sort of error response here
        res.status(500).json(err);
    });
});

app.listen(port, function() {
    console.log('Server is running on port', port);
});

Summary of changes here:

1. In select(), return the promise
2. In the .catch() in select(), rethrow the error to keep it a rejected promise. If you add a handler for a .catch() and don't either rethrow or return a rejected promise, then the error is handled and the promise becomes resolved.
3. You need to fix your for loop. It should not be doing a return inside the for loop with no conditional checks. That code is probably just wrong (though I'm not sure what you intended to do).
4. When you call db.select(), use a .then() handler to get teh final resolved value.
5. Add an error handler for the db.select() promise.
6. Change the exports so that db.select() is your function.
7. Revised the way you were referencing data in the for loop so it will actually fetch the desired property.

Answer 2

There are many problems in your code, some of them listed in the previous answer by @jfriend00.

I will only add that you also do not return any data from the method when it is successful.

Considering how many errors you got there, rather than re-iterating them, I will give you a corrected code example instead.

The database module:

'use strict';

const pgp = require('pg-promise')();
const db = pgp(process.env.DATABASE_URL);

let select = (req, res, next) =>
    db.map('SELECT username, status FROM status', null, row=> {
        return row.username + "'s status is " + row.status;
    })
        .then(data=> {
            res.status(200).send(data);
        })
        .catch(err=> {
            res.status(500).send(err.message || err);
        });

module.exports = {
    select
};

And the HTTP service file:

'use strict';

const port = process.env.PORT || 3000;
const bodyParser = require('body-parser');
const express = require('express');
const app = express();
const db = require('./db/');

app.use(bodyParser.urlencoded({extended: true}));

app.post('/logdash', db.select);

app.listen(port, function () {
    console.log('Server is running on port', port);
});

The code is based on pg-promise v.4.3.x (upgrade, if you have an older one).

I wouldn't say it is a good approach to organizing your code, but at least it is a working example. You can check out pg-promise-demo for a complete application example that may give you a better idea of how to organize your database code.

---

API references: map

---

Answer 3

A few things. I would make sure that your select function returns a Promise. I would also handle the promise in your route. This way you can properly send the appropriate status codes and responses.

db/index.js

'use strict';

const pgp = require('pg-promise')();
const pg = pgp(process.env.DATABASE_URL);

let select = () => {
  return pg.any('SELECT username, status FROM status')
}

module.exports = () => {
    select
};

server.js

'use strict';

const port = process.env.PORT || 3000;
const bodyParser = require('body-parser');
const express = require('express');
const app = express();
const db = require('./db/');

app.use(bodyParser.urlencoded({extended: true}));

app.post('/logdash', function(req, res, next) {
    db.select()
        .then((data) => {
          res.status(200).json(data)
        })
        .catch((error) => {
          res.status(500).json(error)
        })
});

app.listen(port, function() {
    console.log('Server is running on port', port);
});

I did not test this, but it should do the trick.