Getting the result value out of Sqlite3 callback function - javascript

I'm using Sqlite3 in an Express back-end with a React front-end. So I'm trying to check if there is a user with a certain email in the database. I've got the function below, which is not finished yet, but the issue I'm having is that I cannot return the result.
Is there a way to return the result variable from the callback function or do I only have access to it inside the callback function?
When finished the function should be returning the email of the user if they exists or false if they don't exist in the database.
getUserByEmail = () => {
database.get(getUserByEmailQuery, [], (err, result) => {
if (err) {
console.log(err.message);
}
console.log(result.emailAddress);
return result;
});
};

I just solved it an hour or so ago. Not the smartest solution, but what I did was just use a different package for the database.
So I used better-sqlite3 which makes it possible to just store the result from the database query into a variable.
Here is the updated code:
exports.getUserByEmail = (email) => {
const result = betterSqlite3DB.prepare(getUserByEmailQuery).get(email);
return result;
};

Related

issue with Mongoose assigning JSON to variable

Thank you in advance for anyone that reads this. I really appreciate any and all help.
so this is my first app personal app. I have setup a DB in Atlas on mongodb.com and I can write to it with out an issue. But when my app tries and pull from my db I can get it to print to console. But I am able to assign the data to any variable to use anywhere else in my my app.
here is my code that works to print to the console. But not sure what setup or package I am missing so i can store it as a local variable. I am using a callback function to return the api call and the console print out works. just dont know what to do next
function getTerms() {
let allTerms = []
termAdd.find({}, '_id', (err, term) => {
term.map((term) => {
allTerms.push(term)
// if I understand Push() correctly this should store my output to allTerms.
});
//this works to print out to console in JSON.
console.log(allTerms, 'getTerms')
return allTerms;
});
};
if (res.statusCode === 200) {
//callback function to return the console from the function
getTerms()
// when i do a let foo = getTerms() it will return undefined
//so I am really dont understand how to assign the return from
//the function to a variable.
//How do I assign this console output to variable to use for output
console.log('200 statusCode')
};
//Random Number Generator
let ranNum = Math.floor((Math.random() * 10) + 1);
console.log('random number = ',ranNum);
res.render('fs', {
flashCard : 'Test Card',
items: ranNum
});
});
I have the full code on GitHub if that would help I can link to this project.
Should I be using a delay in anyway. I was given a suggestion. But I did not understand what I was reading.
Before anything, please abstain of pushing your .env file to Github or sharing anything in it publicly.
You separated the functionality of getting terms into its own function, which is good:
// Get function for all items in mongodb
function allItems(all) {
let allTerms = []
termAdd.find({}, 'term', (err, term) => {
term.map((term) => {
allTerms.push(term)
});
//this works to print out to console.
console.log(allTerms, 'function allTerms')
//Question is how to I get this JSON to save to a VAR or be passed to another function
});
};
First of all I would rename it to getAllTerms, since it's what it does. And it seems the all parameter is not necessary.
**
Anyway, usually, what you would do is simply return the allTerms variable as such:
// Get function for all items in mongodb
function getAllTerms() {
let allTerms = []
termAdd.find({}, 'term', (err, term) => {
term.map((term) => {
allTerms.push(term)
});
//this works to print out to console.
console.log(allTerms, 'function allTerms')
//Question is how to I get this JSON to save to a VAR or be passed to another function
});
return allTerms
}
However, this wouldn't work because since you are making a call to a database, it might take some time for the database to get the terms; in this case, allTerms might return an empty array, [].
What you have to do is wait for the database to return the terms, push them into the allTerms array, and finally return it.
// Get function for all items in mongodb
async function getAllTerms() {
let allTerms = []
const fetchedTerms = await termAdd.find({}, 'term')
fetchedTerms.forEach(fetchedTerm => allTerms.push(fetchedTerm))
return allTerms
}
If you don't know what async and await are, no worries, here is a good article explaining the why and when to use them.
If you still have any questions, let me know.
Using Async/Await the code above can be optimized as the code below:
`
async function getTerms() {
//using async/await
try{
const terms = await termAdd.find({}, '_id')
const allTerms = terms.map((term)=>term)
// return allTerms here
return allTerms
}catch(err){
throw err
}
}
`
for more insight on Async/Await here is a good read

Can I reduce this Passport code?

This is more of a conceptual question as logically, I can reduce the code below. Per the comments, user information is extracted and put in props. The same user data was then pulled from the database to be returned using done.
I think the original author of the code wanted to make sure it was saved to the database. But I think that is overkill.
Error checking will let us know if anything went wrong and we don't need to pull the data immediately after saving it. We can just return the same data that was stored.
This is passport authentication code.
// the user was not found
// create the user, get the user, and return the user object
function createUser (done, profile) {
let props = obtainProps(profile);
DBM.createUser(props).then(() => {
DBM.getUser(props.id_google).then((res) => {
return done(null, res[0]);
}).catch( error => {
return done(error, null);
});
});
}
to this code:
// the user was not found
// create the user then return props
function createUser (done, profile) {
let props = obtainProps(profile);
DBM.createUser(props).then(() => {
return done(null, props);
}).catch( error => {
return done(error, null);
});
}
I think that you can omit the check too, in addition, as commented, it would make sense to use the Promise and return that, instead of passing a callback and execute it in the promise's then. Promises were made to replace callbacks, your code is going in the other direction. Your createUser function should look like
function createUser (profile) {
let props = obtainProps(profile);
return DBM.createUser(props);
}
and if you need the result somewhere else, chain the function with then
// somewhere else in your code
createUser().then(e => /* e is result of DBM.createUser */)

how to get all keys and values in redis in javascript?

I am creating a node API using javascript. I have used redis as my key value store.
I created a redis-client in my app and am able to get values for perticular key.
I want to retrieve all keys along with their values.
So Far I have done this :
app.get('/jobs', function (req, res) {
var jobs = [];
client.keys('*', function (err, keys) {
if (err) return console.log(err);
if(keys){
for(var i=0;i<keys.length;i++){
client.get(keys[i], function (error, value) {
if (err) return console.log(err);
var job = {};
job['jobId']=keys[i];
job['data']=value;
jobs.push(job);
});
}
console.log(jobs);
res.json({data:jobs});
}
});
});
but I always get blank array in response.
is there any way to do this in javascript?
Thanks
First of all, the issue in your question is that, inside the for loop, client.get is invoked with an asynchronous callback where the synchronous for loop will not wait for the asynchronous callback and hence the next line res.json({data:jobs}); is getting called immediately after the for loop before the asynchronous callbacks. At the time of the line res.json({data:jobs}); is getting invoked, the array jobs is still empty [] and getting returned with the response.
To mitigate this, you should use any promise modules like async, bluebird, ES6 Promise etc.
Modified code using async module,
app.get('/jobs', function (req, res) {
var jobs = [];
client.keys('*', function (err, keys) {
if (err) return console.log(err);
if(keys){
async.map(keys, function(key, cb) {
client.get(key, function (error, value) {
if (error) return cb(error);
var job = {};
job['jobId']=key;
job['data']=value;
cb(null, job);
});
}, function (error, results) {
if (error) return console.log(error);
console.log(results);
res.json({data:results});
});
}
});
});
But from the Redis documentation, it is observed that usage of
Keys are intended for debugging and special operations, such as
changing your keyspace layout and not advisable to production
environments.
Hence, I would suggest using another module called redisscan as below which uses SCAN instead of KEYS as suggested in the Redis documentation.
Something like,
var redisScan = require('redisscan');
var redis = require('redis').createClient();
redisScan({
redis: redis,
each_callback: function (type, key, subkey, value, cb) {
console.log(type, key, subkey, value);
cb();
},
done_callback: function (err) {
console.log("-=-=-=-=-=--=-=-=-");
redis.quit();
}
});
Combination of 2 requests:
import * as ioredis from 'ioredis';
const redis = new ioredis({
port: redisPort,
host: redisServer,
password: '',
db: 0
});
const keys = await redis.collection.keys('*');
const values = await redis.collection.mget(keys);
Order will be the same for both arrays.
This will get all keys but with no values:
const redis = require('redis');
const client = redis.createClient();
client.keys('*', (err, keys) => {
// ...
});
Now you need to get the values for those keys in a usual way. For example:
Promise.all(keys.map(key => client.getAsync(key))).then(values => {
// ...
});
or with async module or in any way you like.
You should never do this. First off, it is not recommended to use KEYS * in production. Second, this does not scale (cluster).
You can organise your cached entries into SETs and query for the items within the SET, then retrieve the references keys. This also makes invalidation easier.
Have a look at some data storage best practices.
https://redis.io/topics/data-types-intro
how to get all keys and values in redis in javascript?
You may find something useful in this link
https://github.com/NodeRedis/node_redis/tree/master/examples

Database call in a function without callback with Node.js and Postgresql

I'm trying out the framework node.js on one of my projects.
I'm really seeing some good advantages on what they called "event-driven, non-blocking I/O model" however if my project there are some moments where I don't necessarily want to have some asynchronous calls and to be able to several operation before launching some asynchronous call.
Especially when I want to do some factorization and create some functions.
Typically I have the following case:
I know that in several part of my program I have to check if a media is existing in my database for a given string or id.
So as a guy who tried to stay organize I want to create a function that I will call each time I need to check this.
However, I did not find the way to do that with node.js and pg (the npm PostgreSQL library (https://github.com/brianc/node-postgres/) . Indeed, there is always a callback in the function and the return is null because of the callback. Here is an example below
/*
Function which is supposed to check if a media existing
*/
function is_media_existing (url_or_id){
log.debug("is_media_existing : begin of the function", {"Parameter" : url_or_id});
pg.connect(connectionstring, function (err, client, done) {
if (err) {
log.warning("is_media_existing : Problem with Database connection", {
"Parameter": url_or_id,
"Error": err
});
}
if (isNaN(url_or_id)) {
// Case is parameter is not a number (string)
var query = client.query('SELECT COUNT(*) as count FROM media WHERE url = $1::string ', url_or_id);
query.on('error', function (error) {
log.warning("is_media_existing : Problem with Database query (connection to db passed but not query " +
"", {"Parameter": url_or_id, "Error": error});
});
return query;
} else {
// Case is parameter is a int
log.debug("is_media_existing : Type of Parameter is a string");
// Case is parameter is not a number (string)
var query = client.query('SELECT COUNT(*) as count FROM media WHERE id = $1::id ', url_or_id);
query.on('error', function (error) {
log.warning("is_media_existing : Problem with Database query (connection to db passed but not query " +
"", {"Parameter": url_or_id, "Error": error});
});
return query;
}
});
}
// Executing the function
var test = is_media_existing("http://random_url_existing_in_db");
// test is always null as the return is in a callback and the callback is asynchronous
i have the feeling my question is touching the core concepts of node.js, and perhaps my approach is wrong and I apologize in advance.
I know it's not good to wait for a response before doing something.
But what's the alternative? How can I factorize my code into functions when I need some functionalities in several part of my code?
So if there would be anyone who could explain how to do that with a best practice of programming it would be great.
Thanks
Anselme
As Cody says, you probably dont want to do synchronous function.
The way you should handle the situation in your example is to pass in your own callback like this
function is_media_existing (url_or_id, callback){
and then instead of return query; use your callback like this-
callback(query);
or probably better to follow the node convention for callback functions to have two parameters (err, result) so your callback would look like this
callback(null, query);
Here is a rework of your sample
function is_media_existing (url_or_id, callback){ /* callback(err, result) */
log.debug("is_media_existing : begin of the function", {"Parameter" : url_or_id});
pg.connect(connectionstring, function (err, client, done) {
if (err) {
done(err);
log.warning("is_media_existing : Problem with Database connection", {
"Parameter": url_or_id,
"Error": err
});
return callback(err, null);
/* note that this return is simply used to exit the connect's callback and the return value is typically
* not used it is the call to callback() that returns the error value */
}
var qrystr;
if (isNaN(url_or_id)) {
log.debug("is_media_existing : Type of Parameter is a string");
qrystr = 'SELECT COUNT(*) as count FROM media WHERE url = $1::string;';
} else {
qrystr = 'SELECT COUNT(*) as count FROM media WHERE id = $1::id;';
}
client.query(qrystr, [url_or_id], function(err, result){
done();
if(err){
/* .. */
}
callback(err, result);
});
});
}
// Executing the function
var test = is_media_existing("http://random_url_existing_in_db", function(err, result){
if(err){
}else {
}
});
If you end up with a hard nest of callbacks, promises are really worth looking into.
I don't think you really do want a synchronous call. The problem with synchronous calls in node is that it stops the entire process from doing anything while a synchronous function is running as it will stop the event loop. As an example lets say your sync function takes 2 seconds to complete. Your server will then do nothing for 2 full seconds. That 2 seconds includes everything (accepting new connections, everything else, etc). The reason blocking functions don't exist is because they are (very) bad. Here is an example how your function will react in an async manor.
is_media_existing("http://random_url_existing_in_db", function(exists){
if (exists){
//do stuff
} else {
//do this other stuff
}
});
Then within is_media_existing you will need to call that callback function when your query completes.
//psuedo
function is_media_existing(url, callback){
query('select COUNT(*) as count FROM media WHERE id = $1::id '. [url], function(err, result){
if (err)
callback(false)
else
callback(result.count > 0)
})
}
With the new ES6 plus async stuff and babel its simpler. You can npm i -g babel npm i babel-runtime then compile and run the following with babel test.js --optional runtime --stage 2 | node. Please read the following example carefully to see how to adapt it to your use case:
let testData = [
{ id: 0, childIds: [1,2]},
{ id: 1, childIds:[] }
];
function dbGet(ids) {
return new Promise( r=> {
// this an example; you could do any db
// query here and call r with the results
r(ids.map((id) => { return testData[id];}));
});
}
async function dbExists(ids) {
let found = await dbGet(ids);
return (found && found.length>0);
}
async function test() {
var exists = await dbExists([0]);
console.log(exists);
}
test().then(f=>{}).catch( e=> {console.log('e',e)});

nodejs function doesnt return a value [duplicate]

This question already has answers here:
How to return value from an asynchronous callback function? [duplicate]
(3 answers)
Closed 8 years ago.
I have this nodejs function, which is for inserting some data into db, and when it finishes inserting it should return 'success' or 'failure'.
This function is in the file insert.js
function insert (req,res) {
var d = req.body;
new dbModel({
name: d.dname,
desc: d.desc
}).save(false,function(err){
if (err){
return 'failure';
}else{
return 'success';
}
});
}
module.exports.insert = insert;
It inserts data into the db, but it doesnt return any value.
This is the route which invokes the above function and this is in the routes.js file.
router.post('/insert', function(req, res) {
var result = insert.insert(req,res);
res.render('insert',{title: 'insert',message: result});
});
Am I doing anything wrong or does this have something to do with async nature of nodejs.
Please help. Thanks.
EDIT
I tried as #Salehen Rahman said in the answers below, but the page isn't rendering and the browser keeps on waiting for reply from the server and i tried to log the value of result inside the callback function and again no output, it seems the code inside the callback function is not running. I checked the db and data has been inserted successfully.
That dbModel#save method is asynchronous, and so, the return keyword will only return to the inner the anonymous function. You want to use callbacks, instead. Also remove false from the save method. It can have only callback function as a parameter.
So, your insert function will look like this:
function insert (req, res, callback) {
var d = req.body;
new dbModel({
name: d.dname,
desc: d.desc
}).save(function(err){
if (err){
// Instead of return, you call the callback
callback(null, 'failure');
}else{
// Instead of return, you call the callback
callback(null, 'success');
}
});
}
module.exports.insert = insert;
And your route function will look like this:
router.post('/insert', function(req, res) {
insert.insert(req, res, function (err, result) {
res.render('insert',{title: 'insert', message: result});
});
});
Ideally, though, whenever an inner callback returns an error, you should also call your callback with the error, without any result; absent an error, and at the presence of a successful function call, you set the first parameter to null, and the second parameter to your result.
Typically, the first parameter of the callback call represents an error, where as the second parameter represents your result.
As a reference, you may want to read up on Node.js' callback pattern.

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