How to use promise in JS and Node JS

Question

I have connected my sql server database to my nodejs application like this :

DAO.js

const sql = require('mssql')

class DAO {
    constructor() {
        this.sqlConfig = {user: 'connexionMartin', password: 'InfoMartin', server: '192.168.102.232\\SQLEXPRESS', database: 'PROFACE'}
    }

    async connect() {
        try {
            console.log("Connecting database.....");
            let pool = await sql.connect(this.sqlConfig);
            if (pool)
                console.log("Database connected");
        } catch (err) {
            console.log(err);
        }
    }

    async getDataLastHour() {
        try {
            let result = await sql.query('SELECT * FROM PROFACE.dbo.SuiviProduction WHERE Time_Stamp >= DATEADD(DAY,DATEDIFF(DAY,0,GETDATE()),0) AND DATEPART(HOUR,Time_Stamp) = DATEPART(HOUR,GETDATE())-1');
            console.dir(result);
        } catch (err) {
            console.log(err);
        }
    }
}

app.js

const Server = require('./server/Server');
const DAO = require('./server/DAO');
const express = require('express');

const server = new Server();
const dao = new DAO();

server.start();
dao.connect();

Now I want to request my database using dao.getDataLastHour() in app.js, but the function is executed before application is connected to database. I have tried to fix this problem by using promise, like this :

const promise = dao.connect();
promise.then(dao.getDataLastHour());

But it doesn't seem to work.

Perhaps I don't use Promise correctly.

If you are using await, then you dont have to use then. In this case if sql.connect is returing a promise, you can either return that promise and use then to execute the callback or call connect with await and just return. — Arun Selin, Nov 21 '19 at 12:53
@Arun Selin I am using await but dao.getDataLastHour() is executed before database is connected ... — MissKnacki, Nov 21 '19 at 12:56
In this case, can you make sure if the sql.connect function return a promise or not. One other thing that can be done is sql.connect(config, function(err) { <>}); — Arun Selin, Nov 21 '19 at 13:02

score 0 · Answer 1 · answered Nov 21 '19 at 13:00

To use a then in you function,it need to return any result and turn it in a promise, or not use await, might it will work!

    async connect() {

            console.log("Connecting database.....");
            sql.connect(this.sqlConfig).then(pool => {
              if (pool)
                console.log("Database connected");
            }).catch (err{ 
            console.log(err);
        });
    }

sorry for the bad identification!

Node_Ninja · Answer 2 · 2019-11-21T13:22:36.187

Your method : dao.connect() does not return a promise. So the first thing would be to change that method to return a promise which you can then listen to then decide whether to run a query or not. :

...
connect() {
     return new Promise((resolve, reject) => {
        try {
            console.log("Connecting database.....");
            let pool = sql.connect(this.sqlConfig);
            if (pool)
                console.log("Database connected");
                resolve("Success");
        } catch (err) {
            console.log(err);
            reject(err);
        }
     });
}
...

And then call your connect method like this:

dao.connect().then(
    success => { ... }, // you can continue querying 
    error => { ... } // something went wrong
);

Also try to read a bit about how to use promises here : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

Differences between promise and async: What is the difference between JavaScript promises and async await?

I try this but it doesn't work anyway SyntaxError: await is only valid in async function — MissKnacki, Nov 21 '19 at 13:20
@MissKnacki I modified my answer. Removed await from the code. — Node_Ninja, Nov 21 '19 at 13:24
I have an error : ConnectionError: Connection is closed., but no message that database is connected like I had before — MissKnacki, Nov 21 '19 at 13:33

Arun Selin · Answer 3 · 2019-11-21T13:25:15.503

0

You can try something like this:

connect() {
 return new Promise((resolve, reject) => {

        let pool = sql.connect(this.sqlConfig, function(err) {
            if(!err) {
                resolve();
            }
            else {
                reject();
            }
        });
 });
}
 dao.connect().then(<<call function here>>)

edited Nov 21 '19 at 13:25

answered Nov 21 '19 at 13:07

Arun Selin

603
4
11

I try this but it doesn't work anyway ```SyntaxError: await is only valid in async function``` – MissKnacki Nov 21 '19 at 13:19
@MissKnacki that was a typo. Can you call it without await. – Arun Selin Nov 21 '19 at 13:25
Yes but I have an error : ```ConnectionError: Connection is closed.```, but no message that database is connected like I had before. – MissKnacki Nov 21 '19 at 13:30
@MissKnacki sql.connect accepts a callback function. So you can move the promise resolve into that callback – Node_Ninja Nov 21 '19 at 14:15
@Node_Ninja what do you mean ? – MissKnacki Nov 21 '19 at 14:59

How to use promise in JS and Node JS

3 Answers3