I am quite new to express and I created a database in Postgres to extract the data about blog posts to place the information in an ejs file.
I get the error:
Cannot read property 'send' of undefined
I've tried to call db.getPosts() with res and req, but it's not possible to set a header again, returns an error.
The problematic chunk of code in my query.js file:
const getPosts = (_req, res) => {
pool.query('SELECT * FROM blog_posts', (error, results) => {
console.log(error);
// console.log(results.rows);
if (error) {
throw error
}
return res.send(results.rows );
})
}
send(results.rows) or render('blog', {posts: results.rows}) called on res give the exact same error.
Function in server.js that is supposed to use this data is as follows:
app.get("/blog", function (req, res) {
const posts = db.getPosts();
res.render("blog", { posts: posts });
});
What do I do wrong? I lack some knowledge, that is for sure, so please, if you can help, explain this briefly to me if possible.
Also, is send() function a correct function to get the data to operate on in server.js? Many tutorials suggest json() but then I don't really get the proper data format, it is just displayed in the browser.
Thank you very much.
Make getPosts receive a callback:
const getPosts = (callback) => {
pool.query('SELECT * FROM blog_posts', (error, results) => {
console.log(error);
// console.log(results.rows);
if (error) {
throw error
}
callback(results.rows);
})
}
Usage would be something like:
app.get("/blog", function (req, res) {
db.getPosts(function(rows) {
res.render("blog", {posts: rows})
});
});
in your getPosts method do not use send. just return results.rows. upate your code like below.
const getPosts = () => {
pool.query('SELECT * FROM blog_posts', (error, results) => {
console.log(error);
// console.log(results.rows);
if (error) {
throw error
}
return results.rows;
})
}
also you need to use async await while calling getposts as it is a async function. update the code like below.
app.get("/blog", async function (req, res) {
const posts = await db.getPosts();
res.render("blog", { posts: posts });
});
Related
I want to add data to my MongoDB collection. I'm getting this data via a local Flask API. I'm GETting the data on my React Frontend and it's displaying fine. I'm not sure why I can't do the same thing on my express nodejs backend. I want to get that same data and use it to build the entity that I'm going to store.
This is how I'm attempting to get the data
app.get('/', async (req, res) => {
let initialData = {};
axios.get('http://localhost:3000/details').then((res) => {
initialData = res.data;
});
const recruit = new RecruitModel({ email:initialData.email,
mobile_number:initialData.mobile_number,
name:initialData.name});
try {
await recruit.save()
res.send("inserted data")
} catch (error) {
console.log(error)
}
})
I'm pretty sure something wrong there and nowhere else. Because if I pass static information instead it's correctly stored, no issues.
You are saving to the database's Recruit Collection before the promise is resolved. Since data to save in the Recruit Collection is dependent upon the result from the API which will initially return the promise, therefore, use promise resolving functions to wait for its result.
Solution#1 (using .then function):
app.get('/', async (req, res) => {
let initialData = {};
try {
axios.get('http://localhost:3000/details').then((response) => {
initialData = response.data;
const recruit = new RecruitModel({
email: initialData.email,
mobile_number: initialData.mobile_number,
name: initialData.name,
});
recruit.save().then((response) => res.send('inserted data'));
});
} catch (error) {
console.log(error);
}
});
Solution#2 (using async await keywords):
app.get('/', async (req, res) => {
try {
const response = await axios.get('http://localhost:3000/details');
const recruit = new RecruitModel({
email: response.data.email,
mobile_number: response.data.mobile_number,
name: response.data.name,
});
await recruit.save();
res.send('inserted data');
} catch (error) {
console.log(error);
}
});
Either solution will work in your case.
I am new to this. There are a couple of solutions posted for a similar problem but none of them helped.
I have posted below a very simple toy example that will help you to debug.
index.js
const express = require('express')
port=3001
var controller = require('./controller');
app.use('/api', controller);
controller.js
var model = require('./model')
var router = express.Router();
router.get('/bl', function(req, res) {
model.getData( function (err, objects) {
if(err) return res.send(err);
return res.status(200).json(objects);
});
});
module.exports = router;
model.js
const bl = {"hello":"world"}
const getData= (request, response) => {
return(bl);
}
module.exports = {
getData
}
Issue:
invoking : http://localhost:3001/api/bl => no response
, console : no error
Note:
in my model.js, I am querying in the Postgres database, and I can see the results in console.log.
but I don't see any such result when I try to see data using console.log in controller.js. Similar behavior I observed in the above toy example
I can see an error and something worrying me.
The error is in following two lines
// In controller.js
model.getData( function (err, objects) {
// In models.js
const getData= (request, response) => {
You define a function accepting two arguments (request, response) (both of them are objects) and than you call it passing only one argument of type function.
The thing worrying me is the getData function itself.
const bl = {"hello":"world"}
const getData= (request, response) => {
return(bl);
}
Apart from parameters, it is a sync function, but in your question you pointed that in models.js you query in the Postgres database, and you can see the results in console.log; now you give us no details about how you query Postgress, but I suppose you are doing it with something asynchronous (probably pg). The other relevant detail you didn't revealed is if you query Postgress inside or outside the body of getData function. I hope I'm wrong, but since in your example you are returning something define outside the body of the function, I have the doubt.
Let's consider only the good option, you query Postgress inside the body of the function, probably your complete getData function looks like follows.
const getData = (request, response) => {
client.query('SELECT $1::text as message', ['Hello world!'], (err, res) => {
// Here you get your message in console
console.log(err ? err.stack : res.rows[0].message);
client.end();
})
return something;
// I don't know what, but fore sure something wrong
}
To solve both the problems you just need to fix getData (by chance or by cut&paste you are calling it in the right way in controller.js).
Let's start fixing its signature makeing it accept only one argument: the callback function:
const getData = (done) => {
then let's asynchronously "return" the data we got from Postgress through the done callback function.
const getData = (done) => {
client.query('SELECT $1::text as message', ['Hello world!'], (err, res) => {
// We can leave this for debugging purposes
console.log(err ? err.stack : res.rows[0].message);
// Here we are not handling errors from client.end()...
// but we can neglect about this right now
client.end();
// Let's calle the callback function passing it the result
done(err, res);
})
// There's nothing to return
}
It should be enough.
Hope this helps.
You need a third argument, you will mostly see callback or just cb and execute it in the function and pass some data to it. The first parameter false will be the error argument later and bl will be the objects argument that is passed
const getData= (request, response, callback) => {
callback(false, bl);
}
and then pass req and res arguments to it:
model.getData(req, res, function (err, objects) {
if(err) return res.send(err);
return res.status(200).json(objects);
});
Or the modern way to do it with async / await you could return an promise
const getData= (request, response) => {
return new Promise((resolve, reject) => {
if(someErrorAppear){
reject("some error occured");
}
resolve(bl);
})
}
Now you can go with async / await
router.get('/bl', async function(req, res) {
try {
let objects = await model.getData(req, res);
res.status(200).json(objects);
}catch(err){
res.send(err);
}
});
});
There is a simple solution to this.
Actually, the error is in model.js. Try below code:
const bl = {"hello":"world"}
const getData= (callback) => {
callback (bl)
}
module.exports = {
getData
}
I have a generic Node+Express server where I serve GET requests. Some of these GET requests need multiple DB queries which are callbacks.
Here is an example of my code:
GET router:
router.get('/getbalance', function(req, res, next) {
wallet.createNewAddress()
.then(result => {
res.send(result);
})
.catch(err => {
console.log(err);
});
This is the function with callbacks:
async createNewAddress()
{
pool.query(`SELECT ...`)
.then (dbres1 => {
pool.query(`SELECT ...`)
.then(dbres2 => {
(async() => {
var pubkeys = await this.getPublicKeysFromIndexes(wallet.id, index_wallet_1, index_wallet_2, index_wallet_3);
var script = this.generateScript(pubkey1, pubkey2, pubkey3);
})();
})
.catch(e => {
console.log(e.stack);
})
}
})
.catch(e => {
console.log(e.stack);
});
}
I have removed long statements for brevity.
As you can see, I have multiple levels of nested promises.
What is the proper way to handle a request like this? Should I return each promise or should I run everything synchronously using async()?
What I need to do is to return the script at the very middle of the statements. This last call that returns the script is a normal synchronous function.
Appreciate any advice.
Thank you.
I believe using async/await will give you much more readable code, while essentially following the same logic. Of course you will have to be aware that you'll need to add try/catch handler(s) to the code.
If you use async/await you'll end up with something like this:
async function createNewAddress()
{
try {
let dbres1 = await pool.query(`SELECT ...`);
let dbres2 = await pool.query(`SELECT ...`);
var pubkeys = await this.getPublicKeysFromIndexes(wallet.id, index_wallet_1, index_wallet_2, index_wallet_3);
return this.generateScript(pubkey1, pubkey2, pubkey3);;
} catch (err) {
// ok something bad happened.. we could skip this handler and let the error bubble up to the top level handler if we're happy with that approach.
console.error(err);
// Rethrow or create new error here.. we don't want to swallow this.
throw err;
}
}
You can then call as before:
router.get('/getbalance', function(req, res, next) {
wallet.createNewAddress()
.then(result => {
res.send(result);
})
.catch(err => {
console.log(err);
});
Or use an async handler:
router.get('/getbalance', async function(req, res, next) {
try {
let result = await wallet.createNewAddress();
res.send(result);
} catch (err) {
// Also consider sending something back to the client, e.g. 500 error
console.log(err);
};
})
Right now i have this code
router.get('/export', function(req, res, next) {
var postData, eventData, messageData, userData
Posts.list().then(data=> {
var jsonOutput=JSON.stringify(data)
postData=jsonOutput //this doesnt work
})
.catch(erro => res.status(500).send('error'))
Events.list().then(data=> {
var jsonOutput=JSON.stringify(data)
eventData=jsonOutput //this doesnt work
})
.catch(erro => res.status(500).send('error'))
Messages.list().then(data=> {
var jsonOutput=JSON.stringify(data)
messageData=jsonOutput //this doesnt work
})
.catch(erro => res.status(500).send('error'))
Users.list().then(data=> {
var jsonOutput=JSON.stringify(data)
userData=jsonOutput //this doesnt work
})
.catch(erro => res.status(500).send('error'))
//Then when all data from colections is retrieve i want to use the 4 variables that i created in the beggining
});
So basicly im trying to retrieve the data from my mongo database and then assign the results to that 4 variables that i create, but im not getting success.
For what i´ve been seeing i have to use async but im having some trouble doing it.
I don't like too much mrlanlee solution. This is a typical situation where using async / await can really make sense. Anyway, the Hugo's solution (the second one, with async await), even if it just works, will make the four queries in sequence, one after another to. If you want a clean, working and parallel solution, check this:
router.get('/export', async function(req, res, next) {
let data
try {
data = await Promise.all([
Posts.list(),
Events.list(),
Messages.list(),
Users.list()
]);
// at this point, data is an array. data[0] = Posts.list result, data[1] = Events.list result etc..
res.status(200).json(data)
} catch (e) {
res.status(500).send('error');
}
});
The other answer from Sashi is on the right track but you will probably run into errors. Since your catch statement on each promise returns 500, if multiple errors are caught during the query, Express will not send an error or 500 each time, instead it will throw an error trying to.
See below.
router.get('/export', function(req, res, next) {
var postData, eventData, messageData, userData
try {
postData = Posts.list().then(data=> {
return JSON.stringify(data);
});
eventData = Events.list().then(data=> {
return JSON.stringify(data)
});
messageData = Messages.list().then(data=> {
return JSON.stringify(data);
})
userData = Users.list().then(data=> {
return JSON.stringify(data)
});
} catch (err) {
// this should catch your errors on all 4 promises above
return res.status(500).send('error')
}
// this part is optional, i wasn't sure if you were planning
// on returning all the data back in an object
const response = {
postData,
eventData,
messageData,
userData,
};
return res.status(200).send({ response })
});
For explanation of why you weren't able to mutate the variables, see Sashi's answer as he explains it.
The variables defined outside the async code is out of scope of the async functions. Hence you cannot store the returned value from the async functions in those variables.
This should work.
router.get('/export', function(req, res, next) {
var postData, eventData, messageData, userData
postData = Posts.list().then(data=> {
var jsonOutput=JSON.stringify(data);
return jsonOutput;
}).catch(erro => res.status(500).send('error'));
eventData = Events.list().then(data=> {
var jsonOutput=JSON.stringify(data);
return jsonOutput;
}).catch(erro => res.status(500).send('error'));
messageData = Messages.list().then(data=> {
var jsonOutput=JSON.stringify(data);
return jsonOutput;
}).catch(erro => res.status(500).send('error'));
userData = Users.list().then(data=> {
var jsonOutput=JSON.stringify(data);
return jsonOutput;
}).catch(erro => res.status(500).send('error'));
});
Using Async/Await is a much neater solution.
router.get('/export', async function(req, res, next) {
var postData, eventData, messageData, userData;
try{
postData = await Posts.list();
eventData = await Events.list();
messageData = await Messages.list()
userData = await Users.list();
catch (e){
res.status(500).send('error');
}
});
I have the following Express endpoint:
const all = require('promise-all');
router.post('/verify', upload.single('photo'), async (req, res) => {
...
await all({'p1': p1, 'p2': p2}).then((response) => {
...
console.log("Response:",
ruleCtrl.manageRule(detection, res);
});
});
ruleCtrl.manageRuleis as follows:
export async function manageRule(identifierDetected, res) {
let rule = db.getRule(identifierDetected);
await all([rule]).then((ruleExtracted) => {
...
res.json(ruleExtracted);
}).catch((err) => {
res.status(418).send("DOCUMENT_NOT_RECOGNIZED");
});
}
and db.getRule:
export async function getRule(idRule) {
return new Promise((resolve, reject) => {
Rule.findOne({ruleID: idRule}, (err, rule) => {
if (err) {
reject("MongoDB Rule error: " + err);
} else {
resolve(rule);
}
});
})
}
My response is into manageRule and this function depends of the values extracted into the await all. So, right now, Express is returning a response before get the information from mongoose database (db).
Which is the way to handle this issue?
Thanks everyone!
I would refactor your code a bit to make it easier to read, and also return the result from ruleCtrl.manageRule(detection, res);.
The request might simply be timing out since your original code is missing a return there or an await (to make sure it finishes executing)
Express endpoint:
const all = require('promise-all');
router.post('/verify', upload.single('photo'), async (req, res) => {
...
// Catch any exceptions from the promises. This is the same as using .catch
try {
// Lets assign the returned responses to variable
let [p1Result, p2Result] = await all({'p1': p1, 'p2': p2});
...
console.log("Responses:", p1Result, p2Result);
// return the response from manageRule method
return ruleCtrl.manageRule(detection, res);
} catch(err) {
// Handle err here
}
});
One of the great benefits with async await is moving away from chained promises, so simply return the result from the await to a variable instead of using .then()
ruleCtrl.manageRule
export async function manageRule(identifierDetected, res) {
// Use try catch here to catch error from db.getRule. Assign to variable and return
// res.json
try {
let ruleExtracted = await db.getRule(identifierDetected);
...
return res.json(ruleExtracted);
} catch(err) {
return res.status(418).send("DOCUMENT_NOT_RECOGNIZED");
}
}
You dont have to return res.json or res.status here, I just like to keep track of when I want to end function execution.
You could refactor the ruleCtrl.manageRule method even further by not sending in res as a parameter but by returning the result from db.getRule instead. Let router.post('/verify) handle req and res, so to make it even easier to read.