I'm currently a little confused as to how to properly wait for the promise to finish before returning the result from a query
Here is my current code:
const getLeaderboardValues = async () => {
const SQLConnection = await getSQLConnection();
return new Promise((resolve, reject) => {
SQLConnection.query(getValuesSQLQuery, (err, result) => {
if (err) { reject(err) }
return resolve(result);
});
SQLConnection.end()
})
}
const runtime = () => {
getLeaderboardValues().then((result) => {
console.log(result);
})
}
The code above does log the correct results while debbugging, i believe this is because i'm giving the code more time to render with the breakpoints, however when running normally i get undefined
I believe the SQLConnection.end() line is executing before the query is returned, given it is outside the query statement.
The below may solve your issue, however I do not recommend opening a connection and closing it on every request in production systems.
const getLeaderboardValues = async () => {
const SQLConnection = await getSQLConnection();
return new Promise((resolve, reject) => {
SQLConnection.query(getValuesSQLQuery, (err, result) => {
if (err) {
reject(err)
return SQLConnection.end()
}
SQLConnection.end()
return resolve(result);
});
})
}
const runtime = () => {
getLeaderboardValues().then((result) => {
console.log(result);
})
}
Related
I know the database is connecting with the code as, when i console.log(res) with the code below it is returning the correct data,
const orm={
selectAll(){
connection.query('SELECT * FROM burgers', (err,res) => {
if (err) throw err;
console.log(res)
return res;
});
},
Yet when i console.log(burgers) from this function in the code below it is returning an undefined
app.get(`/`, function (req, res) {
const burgers = orm.selectAll();
console.log(burgers)
res.render(`index`, burgers);
});
I understand this may be a very simple answer but i personally just cannot work it out any help is welcomed.
selectAll is using a callback style method inside it. You can not get the response syncronously. You need to either pass the callback to selectAll or change it to use promise like this
function selectAll() {
return new Promise((reoslve, reject) => {
connection.query("SELECT * FROM burgers", (err, res) => {
if (err) {
reject(err);
}
reoslve(res);
});
});
}
You can then use it like this
app.get(`/`, function async (req, res) {
const burgers = await selectAll();
console.log(burgers)
res.render(`index`, burgers);
});
Your selectAll method will not return any value.
query get an lambda callback function as second parameter AND query is asyncron
One way is to return an Promise from selectAll
const orm = {
selectAll(callback) {
return new Promise((resolve, reject) => {
connection.query('SELECT * FROM burgers', (err, res) => {
if (err) {
reject(err);
} else {
resolve(res)
}
})
})
},
Than you can get your result:
app.get(`/`, function (req, res) {
orm.selectAll().then( burgers => {
console.log(burgers)
res.render(`index`, burgers);
});
});
Can't I use promise for nodeJS mysql query?
// My DB settings
const db = require('../util/database');
db.query(sqlQuery, [param1, param2])
.then(result => {
console.log(result);
})
.catch(err => {
throw err;
});
It is returning: TypeError: db.query(...).then is not a function
You mentioned in the comments that you want logic after the query block to be awaited, without placing that logic inside of the callback. By wrapping the method with a Promise, you can do that as such:
try {
const result = await new Promise((resolve, reject) => {
db.query(sqlQuery, (error, results, fields) => {
if (error) return reject(error);
return resolve(results);
});
});
//do stuff with result
} catch (err) {
//query threw an error
}
Something like this should work
function runQuery(sqlQuery){
return new Promise(function (resolve, reject) {
db.query(sqlQuery, function(error, results, fields) {
if (error) reject(error);
else resolve(results);
});
});
}
// test
runQuery(sqlQuery)
.then(function(results) {
console.log(results)
})
.catch(function(error) {
throw error;
});
mysql package does not support promise. We can use then only a function call returns a promise.You can use mysql2 which has inbuilt support for Promise. It will also make your code more readable. From mysql2 docs:
async function main() {
// get the client
const mysql = require('mysql2/promise');
// create the connection
const connection = await mysql.createConnection({host:'localhost',
user: 'root', database: 'test'});
// query database
const [rows, fields] = await connection.execute(query);
// rows hold the result
}
I would aslo recommend you to learn about callbacks, promise and async-await
I trying to execute 2 MySQL queries sequentially in Node.JS. MySQL queries work properly by itself.
I would like to do it with async/await function to be sure record is inserted before it's updated.
Here is the code:
router.post('/assign_new_item_id', async (req, res) => {
.....
try {
let qr1= "INSERT INTO foo1 ........;"
await pool.query( qr1, (err) => {
if (err) throw err;
});
let qr2= "UPDATE foo1 .....;"
await pool.query( qr2, (err) => {
if (err) throw err;
});
}catch(err){
console.log(err)
}
It seems that execution "hangs" within first await await block. What is the best way the ensure that both queries are executed consequently.
Thanks in advance for any help.
To await you need a Promise, Not Callback. In your case you are not returning a promise to await.
router.post('/assign_new_item_id', async (req, res) => {
// .....
try {
let qr1 = "INSERT INTO foo1 ........;"
await new Promise((res, rej) => {
pool.query(qr1, (err, row) => {
if (err) return rej(err);
res(row);
});
});
let qr2 = "UPDATE foo1 .....;"
await new Promise((res, rej) => {
pool.query(qr2, (err, row) => {
if (err) return rej(err);
res(row);
});
});
} catch (err) {
console.log(err)
}
});
Here I am promisifing the pool.query method and returning a promise.
I am fetching some exchange data from DB, then extracting the name of distinct exchanges and passing again into MYSQL query to fetch data from a different table.
The problem I am facing is that async await does not return the value rather just return Promise { }.
Below is the code that I am trying, wondering where I am going wrong.
//Function that fetches the exchanges from DB
const getExchange = () => {
return new Promise((resolve, reject) => {
db.connection.query(`
SELECT *
FROM,
(
SELECT
exchange,
COUNT(pair) as noOfMarkets
FROM ticker_data
) as t
`, (err, resp) => {
if (!err) {
resolve(resp)
} else {
reject(err)
}
})
})
}
// push unique exchanges to an array.
const getExchangesData = async () => {
const allExchanges = await getExchanges();
let exchanges = []
allExchanges.forEach(item => {
let exchange = {
exchange: item.exchange
}
exchanges.push(exchange)
})
return await exchanges
}
// mapping through an array of exchanges and passing to DB query to get data from the DB.
const getSingleExchange = async () => {
const exchanges = await getExchangesData()
await Promise.all(exchanges.map(async (item) => {
db.connection.query(`
SELECT
exchange_rank,
name
volume24hUSD
(
SELECT
volume24hUSD as tradingVolYesterday
FROM exchanges
WHERE name = '${item.exchange}'
AND createdAt >= now() -interval 1 day
AND createdAt < now() -interval 1 day + interval 120 second
LIMIT 1
) volumeDay1
FROM exchanges
WHERE name = '${item.exchange}'
`, (err, resp) => {
if (!err) {
console.log(resp) // getting all the values
let volData = {
name: resp[0].name,
exchange_rank: resp[0].exchange_rank,
icon: resp[0].icon
}
return volData
}
})
}))
}
const data = getSingleExchange()
console.log(data) // returning Promise { <pending> }
Edit
After making changes suggested in an answer, I still have an issue:
//Function that fetches the exchanges from DB
const getExchange = () => {
return new Promise((resolve, reject) => {
db.connection.query(`
SELECT *
FROM,
(
SELECT
exchange,
COUNT(pair) as noOfMarkets
FROM ticker_data
) as t
`, (err, resp) => {
if (!err) {
resolve(resp)
} else {
reject(err)
}
})
})
}
// push unique exchanges to an array.
const getExchangesData = async () => {
const allExchanges = await getExchanges();
let exchanges = []
allExchanges.forEach(item => {
let exchange = {
exchange: item.exchange
}
exchanges.push(exchange)
})
return await exchanges
}
// mapping through an array of exchanges and passing to DB query to get data from the DB.
const getSingleExchange = async () => {
const exchanges = await getExchangesData()
await Promise.all(exchanges.map((item) => {
return new Promise((resolve, reject) => {
db.connection.query(`...`, (err, resp) => {
if (!err) {
resolve(resp)
} else {
reject(err)
}
}).then(resp => {
console.log(resp)
let volData = {
name: resp[0].name,
exchange_rank: resp[0].exchange_rank,
icon: resp[0].icon
}
return volData
})
})
}))
}
getSingleExchange().then(data => {
console.log(data)
});
I now get this error:
(node:30583) UnhandledPromiseRejectionWarning: TypeError: db.connection.query(...).then is not a function
at Promise (/getExchanges.js:217:16)
at new Promise ()
at Promise.all.exchanges.map (/getExchanges.js:145:16)
at Array.map ()
at getSingleExchange (/getExchanges.js:144:33)
The main issue is in this part:
await Promise.all(exchanges.map(async (item) => {
That map callback is not returning anything, and it has no await, so using async makes no sense.
Instead remove async:
await Promise.all(exchanges.map((item) => {
... and return a promise in the callback function, much like you had done in the first function:
return new Promise((resolve, reject) => {
db.connection.query(`...`), (err, resp) => {
if (!err) {
resolve(resp)
} else {
reject(err)
}
})
}).then(resp => {
console.log(resp)
let volData = {
name: resp[0].name,
exchange_rank: resp[0].exchange_rank,
icon: resp[0].icon
}
return volData
});
You would benefit from writing one generic function that promisifies query, so that you don't have to do that new Promise-thing for every single query you need.
Finally, you cannot expect to get an asynchronous result synchronously: async functions do not return the asynchronous result synchronously, but return a promise for it. So your last lines (main code) should still await. So either do this:
(async () => {
const data = await getSingleExchange()
console.log(data)
})(); // immediately executing (async) function expression
Or:
getSingleExchange().then(data => {
console.log(data)
});
NB: doing return await exchanges in the second function makes no sense (exchanges is not a promise), so you can just do return exchanges.
I'm exploring the possibilities of promises and callbacks in node.js
I'm trying to find a way for this code to work. Currently the issue I'm facing is that when I'm calling a function and want to use the return value, it is not ready yet. I know what I have to do, but don't know how. Basically, I have to make that insertAddress() returns a promise (so I can use the .then() on it), or takes a callback as a param. To do this, I also think databaseWork() should return a promise. But I don't know where to add it.
The issue is located in the 'console.log(out)', that runs before out variable is set (because insertAddress is still running).
Here is my code
app.js
-----
const databaseWork = require('./db/mysql.js').databaseWork;
app.use('/test', (req, resp) => {
var address = {
country : "Country",
city : "Randomcity",
street : "Random",
number : 6,
postalcode : "A789",
province : "a province"
}
var out = insertAddress(address); //<== takes time to finish, is not ready when the next console.log finishes
console.log(out);
});
function insertAddress(address){
var rows
databaseWork(
//Following anonymous function contains the actual workload. That has to be done inside a transaction
async (connection) => {
rows = await insertAddressQuery(address,connection);
console.log(rows); //this one waits for insertAddressQuery to be complete
})
return rows; //this will run before insertAddressQuery is complete
}
function insertAddressQuery(address,connection) {
return new Promise( (resolve, reject) => {
//async job
connection.query('INSERT INTO address (country,city,Street,number,postalcode,province) VALUES(?,?,?,?,?,?)', [address.country,'4','5',6,'7','8'] , (err, rows) => {
if (err) {reject(err);}
resolve(rows);
});
});
};
/db/mysql.js
------------
var mysql = require('mysql');
var dbpool = mysql.createPool({
host: process.env.HOST_DB,
user: process.env.USER_DB,
password: process.env.PWD_DB,
database: process.env.DB
});
function databaseWork(workload){
dbpool.getConnection( async (err, connection) => {
await beginTransaction(connection);
await workload(connection);
await commitTransaction(connection)
connection.release();
});
}
function beginTransaction(connection){
return new Promise( (resolve, reject) => {
//async job
connection.beginTransaction( (err) => {
if (err) {reject(err);}
resolve();
});
});
};
function commitTransaction(connection) {
return new Promise( (resolve, reject) => {
//async job
connection.commit( (err) => {
if (err) {reject(err);}
resolve();
});
});
};
exports.databaseWork = databaseWork;
You would do that in your databaseWork:
function databaseWork(workload) {
return new Promise((resolve, reject) => {
dbpool.getConnection(async (err, connection) => {
try {
await beginTransaction(connection);
var result = await workload(connection);
await commitTransaction(connection)
resolve(result);
} catch( err ) {
reject(err)
} finally {
connection.release();
}
});
})
}
The Promise returned by databaseWork will be resolved by the result of workload. And now you can change insertAddress to this:
async function insertAddress(address){
return databaseWork(connection => {
return insertAddressQuery(address,connection);
})
}
You then need to change the route to this:
app.use('/test', async (req, resp) => {
var address = {
country: "Country",
city: "Randomcity",
street: "Random",
number: 6,
postalcode: "A789",
province: "a province"
}
var out = await insertAddress(address); // use await here to wait for insertAddress to be finished
console.log(out);
});
*UPDATE code with an getConnection function that returns a Promise:
function getConnection() {
return new Promise((resolve, reject) => {
dbpool.getConnection((err, connection) => {
if (err) {
reject(err)
} else {
resolve(connection);
}
})
});
}
async function databaseWork(workload) {
var connection = await getConnection();
var result;
try {
await beginTransaction(connection)
result = await workload(connection)
await commitTransaction(connection)
} catch (err) {
// a rollback might be neccesaary at that place
throw err
} finally {
connection.release();
}
return result;
}
One way you can do this is by using async await.
var example = async (req, res) => {
var response = await myAsyncTask();
// this will get logged once the async task finished running.
console.log(response)
}
// Use async await to get response
var myAsyncTask = async () => {
try {
var response = await asyncTaskINeedDataFrom()
return response;
}
catch(err) {
return console.log(err);
}
}
Here's the npm module: https://www.npmjs.com/package/async