How to call module export function from another file in javascript

Question

i am new in javascript/node js. So a little thing about function in js makes me confused. Here i have problem.

I have a function in a file (appModel.js) called getUserById to return a row from database by id as a parameter.

User.getUserById = function getUserById(id,result){
    var koneksi = new newDb();

    koneksi.query('Select ID, email, name, status, telp_no, company_id, created_at, updated_at from user where ID = ?',id)
        .then(rows=>{
            console.log("user: ",rows);
            result(null,rows);
        })
        .catch(err=>{
            console.log('error: ',err);
            koneksi.close().then(()=>result(err,null));
        });
};

module.exports = User;

and i access appModel file through different file from (appRoute.js) using

var User = require('../models/appModel');

I've tried to access getUserById function in appRoute.js by

var _a;
var _check = User.getUserById(req.params.id, function(err,user){
    if(err){
        _a = err;
    }else{
        _a = user;
    }
    return _a;
});
console.log('res', _check);

when i print _check itself the result is res undefined

so, i expect the output from this function is an error or user itself. How can i get that?

Answer 1

Your problem isn't actually modules related.

Its actually a lack of understanding of asynchronous processes and closures in general.

If you add a console.log in there as follows, you'll find that your 'res' runs before 'done'.

var _a;
var _check = User.getUserById(req.params.id, function(err,user){
    console.log('done', err, user); // <-- Add this in here.
    if(err){
        _a = err;
    }else{
        _a = user;
    }
    return _a;
});
console.log('res', _check);

The second part about closures, the return value in here is ignored:

_check = User.getUserById(req.params.id, function(err,user){
   ... 
   return _a; // This is ignored. It will never be assigned to _check.
}

The return value actually comes from:

function getUserById(id,result){
    var koneksi = new newDb();

    ...

    return "test"; // Here. Add this last line and try again.
};

Answer 2

You're not returning anything from getUserById , so until an explicit return is added to a function in JavaScript, the result from calling it will always be undefined .

Your result function is simply a callback. While it is indeed returning a value, that value isn't further being used in getUserById or being passed on to anything else, mainly because you're not returning anything from your then / catch blocks.

Suggested reading on Promises: https://developers.google.com/web/fundamentals/primers/promises