Why can’t I use module export to export variable

Question

Explanation:

Good guys in my app.js file have many lines of codic ... Some functions ... But I think it doesn’t matter much, so what I want to do is export a variable that I need to use in another already tried file everything else will not, I tried even in the simplest ways.

What I want to do:

//app.js
token = 'Teste' //Variável que quero exportar
module.exports = {
 token:token
}

And in the file Account.js

//account.js
x = require("./app.js");
console.log(x.token);

Problem: Guys if I test this comic works, however in my project not only has this, I have other functions and imports of lib ... But it doesn’t work ... It always does Undefined.

Another solution I found:

//app.js
token = 'Teste'
exports.token = token

//account.js
x = require("./app.js")
console.log(x.token)

Anyway, it worked, but why didn’t it work out the way I wanted it to? I do not want to use this way just because it works, even because the way I wanted to do works in a file that contains only this snippet, but in my file there is not only this, it has some more functions ... And I don’t know if it affects the way I want to export the variable.

The scenario where I want to apply the first way I want to do:

async function verify(req,res,username,password){              
 db.serialize(async function (){
  const query = 'SELECT * from Users WHERE User = (?) AND Password = (?)'                                                     
  db.all(query,[username, password],async function(err,rows){    
   try{
    if(rows.length == 1){                                          
     console.time("time1")                                         
     await console.log(rows);                                      
     console.log("Correct user");             
     res.render(__dirname + "/View/Home/index.handlebars");        
     module.exports = {
      rows:rows //Desta forma era pra funcionar 
     }                            
     console.timeEnd("time1")                                      
    }                                                        
    else{
    console.log("Incorrect user!")}}
   catch{
    console.error("\x1b[31m","Problema com a função de autenticação, erro: \n", err);
   }
  })
})}

But in my file Account.js:

const express = require('express');
const app = express();                                        
const router = express.Router();                              
const rows = require("./app.js")                              
const io = require('socket.io')                               
const handlebars = require("express-handlebars");             
const multer = require("multer");                             
const path = require("path");

app.engine("handlebars", handlebars({defaultLayout:false}));
app.set("view engine","handlebars");                         
app.get('/',(req,res) => {                                    
 res.render(__dirname + "/View/Account/account.handlebars")
 console.log(rows.rows);
 //Quando eu entrasse nessa rota eu queria que aparecesse o valor da variável da forma que eu queria exportar como havia explicado
});

module.exports = app

Type of error: Undefined

There are more things in my app.js file but I only put the function where I want to use the export module.

Answer 1

The context is somewhat confused, but I believe the error is in the asynchronous function.

The function verify that you created is an asynchronous function, but the reserved word await is at the beginning of console.log(...). So when you export the variables, they may not have had time to be generated.

the await should come before a Promise, as for example before db.all(....) that will make a query in the database.

Answer 2

Apparently, what you want is to keep some variables for a specific user between requests, if this is the case, you should use sessions.

take a look at the express-Session npm package, it might satisfy you:

https://www.npmjs.com/package/express-session

Exports is a variable equivalent to global in the scope of the function you run...it is not recommended to use global variables for storing variables that depend on user to user, they will overwrite each other

or

if the question is not is, but simply returns the ROW value from within the Verify function I advise you to refactor it to use Promise, I decided to refactor and left some comments:

//removi o parametro req pós não é utilizados
function verify(res,username,password){
    return new Promise((resolve,reject)=>{
        //não precisa de async, pós não há await dentro
        db.serialize(function (){
            const query = 'SELECT * from Users WHERE User = (?) AND Password = (?)'
            //não precisa de async, pós não há await dentro
            db.all(query,[username, password],function(err,rows){
                try{
                    if(rows.length == 1) {
                        console.time("time1");
                        //console.log não é uma função assincrona, não precisa de await
                        console.log(rows);
                        console.log("Correct user");
                        res.render(__dirname + "/View/Home/index.handlebars"/*, objeto com variaveis para o handlerbars*/);
                        //não retorne a variavel pelo exports, exports é um globlal
                        //em um ambiente com varias requisições, passar as respostas de varias requisições por uma global não é nada recomendado, pós entre o tempo de definila e finalizar e retorna, ela já pode ter sido sobreescrita por outra requisição      
                        //isso ira retorna pelo await...;
                        resolve(rows);
                        console.timeEnd("time1")
                    } else {
                        console.log("Incorrect user!");
                        //adicionei um res.send, pós se a requisição nunca chamar res.send, o browser ira aguarda até gerar um erro "timeout" pós, nenhuma respostas foi enviada
                        res.send("Incorrect user!");
                    }
                } catch (e){
                    res.send("Problema com a função de autenticação");
                    console.error("\x1b[31m","Problema com a função de autenticação, erro: \n", err);
                }
            }
        });
    });
}

*Obs.: because I didn’t know the DB API used, I didn’t add the cases where the query goes wrong (which may also be a reason Row never be set)...

Answer 3

if your Node JS does not support ES6 (version earlier than 10), the error is this:

module.exports = {
 token:token
}
//que pode ser
module.exports = {
 token
}
//ou
module.exports = {
 'token':token
}

when doing token:token, it creates an object with a variable that has both the name and the value equal to 'Test', if you do not want to put a new name to the variable, just put it, if you want to give a new name inside the object, to prevent the value of another variable from being used as a key, use a string to prevent the value of another variable from being used as a name within the object

'novoNome':token,

This is one of the features that before ES6 made Java script easy to write and hard to break..., which can sometimes leave you confused.

OBS.: if the name of the variable inside the object were not a defined variable it would work...