writestream and express for json object? - javascript

I might be out of depth but I really need something to work. I think a write/read stream will solve both my issues but I dont quite understand the syntax or whats required for it to work.
I read the stream handbook and thought i understood some of the basics but when I try to apply it to my situation, it seems to break down.
Currently I have this as the crux of my information.
function readDataTop (x) {
console.log("Read "+x[6]+" and Sent Cached Top Half");
jf.readFile( "loadedreports/top"+x[6], 'utf8', function (err, data) {
resT = data
});
};
Im using Jsonfile plugin for node which basically shortens the fs.write and makes it easier to write instead of constantly writing catch and try blocks for the fs.write and read.
Anyways, I want to implement a stream here but I am unsure of what would happen to my express end and how the object will be received.
I assume since its a stream express wont do anything to the object until it receives it? Or would I have to write a callback to also make sure when my function is called, the stream is complete before express sends the object off to fullfill the ajax request?
app.get('/:report/top', function(req, res) {
readDataTop(global[req.params.report]);
res.header("Content-Type", "application/json; charset=utf-8");
res.header("Cache-Control", "max-age=3600");
res.json(resT);
resT = 0;
});
I am hoping if I change the read part to a stream it will allievate two problems. The issue of sometimes receiving impartial json files when the browser makes the ajax call due to the read speed of larger json objects. (This might be the callback issue i need to solve but a stream should make it more consistent).
Then secondly when I load this node app, it needs to run 30+ write files while it gets the data from my DB. The goal was to disconnect the browser from the db side so node acts as the db by reading and writing. This due to an old SQL server that is being bombarded by a lot of requests already (stale data isnt an issue).
Any help on the syntax here?
Is there a tutorial I can see in code of someone piping an response into a write stream? (the mssql node I use puts the SQL response into an object and I need in JSON format).
function getDataTop (x) {
var connection = new sql.Connection(config, function(err) {
var request = new sql.Request(connection);
request.query(x[0], function(err, topres) {
jf.writeFile( "loadedreports/top"+x[6], topres, function(err) {
if(err) {
console.log(err);
} else {
console.log(x[6]+" top half was saved!");
}
});
});
});
};

Your problem is that you're not waiting for the file to load before sending the response. Use a callback:
function readDataTop(x, cb) {
console.log('Read ' + x[6] + ' and Sent Cached Top Half');
jf.readFile('loadedreports/top' + x[6], 'utf8', cb);
};
// ...
app.get('/:report/top', function(req, res) {
// you should really avoid using globals like this ...
readDataTop(global[req.params.report], function(err, obj) {
// setting the content-type is automatically done by `res.json()`
// cache the data here in-memory if you need to and check for its existence
// before `readDataTop`
res.header('Cache-Control', 'max-age=3600');
res.json(obj);
});
});

Related

May I have a res.json and res.sendFile together in the same api endpoint in express?

I want upload the content of an excel file into the server in order to get its data and do some stuff...
I came up with the following code, however it seems like it is not working properly as the following error displays in the console Error: Can't set headers after they are sent.
The file is getting uploaded into the folder and the json message is being displayed... However I do not know if I am going to face any issue in the future...
Actually I just need the excel data no need for the excel being uploaded... Maybe you could give me a workaround, guys...
const router = express.Router();
const storage = multer.diskStorage({
destination(req, file, cb) {
cb(null, 'uploads/');
},
filename(req, file, cb) {
cb(
null,
`${file.fieldname}-${Date.now()}${path
.extname(file.originalname)
.toLowerCase()}`
);
},
});
const excelFilter = (req, file, cb) => {
if (
file.mimetype.includes('excel') ||
file.mimetype.includes('spreadsheetml')
) {
cb(null, true);
} else {
cb('Please upload only excel file.', false);
}
};
const upload = multer({
storage,
fileFilter: excelFilter,
});
router.post('/', upload.single('file'), (req, res) => {
var workbook = XLSX.readFile(req.file.path);
var sheet_name_list = workbook.SheetNames;
var xlData = XLSX.utils.sheet_to_json(workbook.Sheets[sheet_name_list[0]]);
res.json(xlData).sendFile(`/${req.file.path}`, { root: path.resolve() });
});
May I have a res.json and res.sendFile together in the same api endpoint in express?
No, you cannot. Each of those methods, sends a complete http response (including calling res.end() which terminates the http request) and you can only send one http response to each incoming request. The particular error you're getting has to do with the res.sendFile() trying to configure the response that it's getting ready to send and finding that the http response object has already been used for sending a response and can't be used again.
Ordinarily, if you wanted to sent two different pieces of data, you would just combine them into a single Javascript object and just call res.json() on the object that contains both pieces of data.
But, sending a binary file is not something you can easily put in a JSON package. You could construct a multipart response where one part was the JSON and one part was the file. You could JSON encode binary data (though that's inefficient). I presume there are probably some modules that would help you do that, but for most clients, that isn't what they are really expecting or equipped to handle.
The only way to a proper solution is for us to understand what client/server workflow you're trying to implement here and why you're trying to send back the same file that was just uploaded. There would normally not be a reason to do that since the client already has that data (they just uploaded it).

Pausing Node.js Readable Stream

I'm building a bar code scanning app using the node-serialport. Where I'm stuck is making a AJAX call to trigger a scan and then have Express server respond with the data from the readable stream.
Initialize Device:
// Open device port
var SerialPort = require('serialport');
var port = '/dev/cu.usbmodem1411';
var portObj = new SerialPort(port, (err) => {
if(err) {
console.log('Connection error ' + err);
}
});
//Construct device object
var Scanner = {
// Trigger Scan
scan : () => {
portObj.write(<scan cmd>), (err) => {
if(err) {
console.log('Error on scan' + err);
}
});
}
}
I've tried two approaches and neither produce the 'scan-read-respond' behavior I'm looking for.
First, I tried putting a event listener immediately following a scan, then using a callback in the listener to respond to the AJAX request. With this approach I get a 'Can't set headers after they are sent' error'. From what I understand Node is throwing this error because res.send is being called multiple times.
First Approach -- Response as callback in listener:
app.get('/dashboard', (req, res) => {
Scanner.scan(); //fire scanner
portObj.on('data', (data) => {
res.send(data); //'Can't set headers after they are sent' error'
});
}
In the second approach, I store the scan data into a local variable ('scanned_data') and move the response outside the listener block. The problem with this approach is that res.send executes before the scanned data is captured in the local variable and so comes up as 'undefined'. Also intriguing is the scanned_data that is captured in the listener block seems to multiple with each scan.
Second Approach -- Response outside listener:
app.get('/dashboard', (req, res) => {
var scanned_data; //declare variable outside listener block
Scanner.scan(); //trigger scan
portObj.on('data', (data) => {
scanned_data = data;
console.log(scanned_data); //displays scanned data but data multiplies with each scan. (e.g. 3 triggers logs 'barcode_data barcode_data barcode_data')
});
console.log(scanned_data); //undefined
res.send(scanned_data);
}
I'm a front end developer but have gotten to learn a lot about Node trying to figure this out. Alas, I think I've come to a dead end at this point. I tinkered with the .pipe() command, and have a hunch that's where the solution lies, but wasn't able to zero in on a solution that works.
Any thoughts or suggestions?
You should not make assumptions about what chunk of data you get in a 'data' event. Expect one byte or many bytes. You need to know the underlying protocol being used to know when you have received a full "message" so you can stop listening for data. At that point you should then send a response to the HTTP request.

Passing function on express js Route not working

I'm just really new on Node and Express. Trying to pass a function instead of text on my route but it seems not working. I just looked up at documentation there, They mentioned only text with req.send() method. I'm trying to pass here function's but it's not working. and also the alert() not working like this req.send(alert('Hello world')) it say's alert isn't defined or something similar.
**Update: ** I'm trying to execute this library with express and node https://github.com/przemyslawpluta/node-youtube-dl
I'm trying to do here pass functions like this
function blaBla() {
var youtubedl = require('youtube-dl');
var url = 'http://www.youtube.com/watch?v=WKsjaOqDXgg';
// Optional arguments passed to youtube-dl.
var options = ['--username=user', '--password=hunter2'];
youtubedl.getInfo(url, options, function(err, info) {
if (err) throw err;
console.log('id:', info.id);
console.log('title:', info.title);
console.log('url:', info.url);
console.log('thumbnail:', info.thumbnail);
console.log('description:', info.description);
console.log('filename:', info._filename);
console.log('format id:', info.format_id);
});
}
app.get('/', (req, res) => {
res.send(blaBla());
})
**Instead of **
app.get('/', function (req, res) {
res.send('Hello World!')
})
I hope you guy's understood my question.
res.send() expects a string argument. So, you have to pass a string.
If you want the browser to execute some Javascript, then what you send depends upon what kind of request is coming in from the browser.
If it's a browser page load request, then the browser expects an HTML response and you need to send an HTML page string back. If you want to execute Javascript as part of that HTML page, then you can embed a <script> tag inside the page and then include Javascript text inside that <script> tag and the browser will execute that Javascript when the page is parsed and scripts are run.
If the route is in response to a script tag request, then you can return Javascript text as a string and you need to make sure the MIME type appropriately indicates that it is a script.
If the route is in response to an Ajax call, then it all depends upon what the caller of the Ajax call expects. If they expect a script and are going to execute the text as Javascript, then you can also just send Javascript text as a string. If they expect HTML and are going to process it as HTML, then you probably need to embed the <script> tag inside that HTML in order to get the Javascript executed.
In your example of:
response.send(blaBla());
That will work just fine if blaBla() synchronously returns a string that is formatted properly per the above comments about what the caller is expecting. If you want further help with that, then you need to show or describe for us how the request is initiated in the browser and show us the code for the blaBla() function because the issue is probably in the blaBla() function.
There are lots of issues with things you have in your question:
You show req.send(alert('Hello world')) in the text of your question. The .send() method belongs to the res object, not the req object (the second argument, not the first). So, that would be res.send(), not req.send().
In that same piece of code, there is no alert() function in node.js, but you are trying to execute it immediately and send the result with .send(). That won't work for a bunch of reasons.
Your first code block using blaBla() will work just fine as long as blaBla() returns a string of the right format that matches what the caller expects. If that doesn't work, then there's a problem with what blaBla() is doing so we need to see that code.
Your second code block works because you are send a string which is something the caller is equipped to handle.
Update now that you've shown the code for blaBla().
Your code for blaBla() does not return anything and it's asynchronous so it can't return the result. Thus, you cannot use the structure response.send(blaBla());. There is no way to make that work.
Instead, you will need to do something different like:
blaBla(response);
And, then modify blaBla() to call response.send(someTextValue) when the response string is known.
function blaBla(res) {
var youtubedl = require('youtube-dl');
var url = 'http://www.youtube.com/watch?v=WKsjaOqDXgg';
// Optional arguments passed to youtube-dl.
var options = ['--username=user', '--password=hunter2'];
youtubedl.getInfo(url, options, function(err, info) {
if (err) {
res.status(500).send("Internal Error");
} else {
console.log('id:', info.id);
console.log('title:', info.title);
console.log('url:', info.url);
console.log('thumbnail:', info.thumbnail);
console.log('description:', info.description);
console.log('filename:', info._filename);
console.log('format id:', info.format_id);
// construct your response here as a string
res.json(info);
}
});
}
Note also that the error handling does not use throw because that is really not useful inside an async callback.
No one just could help me with that and after finding things are alone I got to know how to do this. In express there is something called middleware we have to use that thing to get this kind of matter done. Those who are really expert or have working experience with express they know this thing.
to using functions with express you need to use middleware.
like below I'm showing
const express = require('express')
const youtubedl = require('youtube-dl');
const url = 'https://www.youtube.com/watch?v=quQQDGvEP10';
const app = express()
const port = 3000
function blaBla(req, res, next) {
youtubedl.getInfo(url, function(err, info) {
console.log('id:', info.id);
console.log('title:', info.title);
console.log('url:', info.url);
// console.log('thumbnail:', info.thumbnail);
// console.log('description:', info.description);
console.log('filename:', info._filename);
console.log('format id:', info.format_id);
});
next();
}
app.use(blaBla);
app.get('/', (request, response) => {
response.send('Hey Bebs, what is going on here?');
})
app.listen(port, (err) => {
if (err) {
return console.log('something bad happened', err)
}
console.log(`server is listening on ${port}`)
})
And remember that you must need to use app.use(blaBla); on top of getting your route. Otherwise this might not work.

Node (Express) - Trying to save a PDF from an API call

I've tried all sorts to get this to work. I'm trying to request a PDF from an API on node, then send this back to the client who called it to begin with.
For the minute I just want to successfully save and view the PDF on the node server.
The issue is the PDF file is always empty when I open it (Even though it has a size of 30kb).
The basic flow is like this (removed a few bits, but the below code works and returns me the PDF fine)
// We pass through session ID's, request dates etc through in body
app.post("/getPayslipURL", function(client_request, res) {
// create request, which will simply pass on the data to the database (In order to get the NI number we need for the pay API)
const NI_NUMBER_REQUEST = db_api.createRequestTemplate({
body: JSON.stringify(client_request.body)
});
// Create a chain of HTTPS Requests, Starting with our call to the DB
requestPromise(NI_NUMBER_REQUEST)
.then((db_response) => {
const PAY_API_OPTIONS = /*Code to generate options based on furhter DB info (Includes dates etc)*/
return requestPromise(PAY_API_OPTIONS); // Call pay API
})
.then((pay_pdf_data) => {
console.log(typeof pay_pdf_data); // It's a string
// At this point I can log pay_pdf_data, But if I try to save it to file it's always empty
// No matter how I encode it etc
fs.writeFile("./test.pdf", pay_pdf_data, 'binary', function(err) {
if(err) {
return console.log(err);
}
console.log("The file was saved!");
});
})
.catch(err => `Error caught: ${console.log}`) // Catch any errors on our request chain
});
}
I've tried saving with / without the binary flag as suggested in other posts in both the file save aswell as within the requests itself. Also various types of decoding methods have been tried, I always get an empty PDF saved.
My return data looks like this (is much bigger, when saved as test.pdf I get a 30kb file as before mentioned)
%PDF-1.4
%����
1 0 obj
0 obj
<
I've found a post which says about piping the data all the way through, I have a feeling my pdf_data is corrupted when getting converted to a string
Any ideas how would I go about doing this with the current setup?
e/ RequestPromise is a library, could also use the standards request library if it's easier
https://github.com/request/request-promise -
https://github.com/request/request
Thanks!
Your code doesn't work because the underlying request library (used by request-promise) requires the option encoding set to null for binary data - see https://github.com/request/request#requestoptions-callback.
Here's how you download binary data using that module -
app.post("/getPayslipURL", function(client_request, res) {
const NI_NUMBER_REQUEST = db_api.createRequestTemplate({
body: JSON.stringify(client_request.body),
encoding: null
});
requestPromise(NI_NUMBER_REQUEST)
.then((db_response) => {
const PAY_API_OPTIONS = /*Code to generate options based on furhter DB info (Includes dates etc)*/
return requestPromise(PAY_API_OPTIONS); // Call pay API
})
.then((pay_pdf_data) => {
fs.writeFile("./test.pdf", pay_pdf_data, 'binary', (err) => {
if(err) {
return console.log(err);
}
console.log("The file was saved!");
});
})
.catch(err => `Error caught: ${console.log}`) // Catch any errors on our request chain
});
}

Hide an API key (in an environment variable perhaps?) when using Angular

I'm running a small Angular application with a Node/Express backend.
In one of my Angular factories (i.e. on the client side) I make a $http request to Github to return user info. However, a Github-generated key (which is meant to be kept secret) is required to do this.
I know I can't use process.env.XYZ on the client side. I'm wondering how I could keep this api key a secret? Do I have to make the request on the back end instead? If so, how do I transfer the returned Github data to the front end?
Sorry if this seems simplistic but I am a relative novice, so any clear responses with code examples would be much appreciated. Thank you
Unfortunately you have to proxy the request on your backend to keep the key secret. (I am assuming that you need some user data that is unavailable via an unauthenticated request like https://api.github.com/users/rsp?callback=foo because otherwise you wouldn't need to use API keys in the first place - but you didn't say specifically what you need to do so it is just my guess).
What you can do is something like this: In your backend you can add a new route for your frontend just for getting the info. It can do whatever you need - using or not any secret API keys, verify the request, process the response before returning to your client etc.
Example:
var app = require('express')();
app.get('/github-user/:user', function (req, res) {
getUser(req.params.user, function (err, data) {
if (err) res.json({error: "Some error"});
else res.json(data);
});
});
function getUser(user, callback) {
// a stub function that should do something more
if (!user) callback("Error");
else callback(null, {user:user, name:"The user "+user});
}
app.listen(3000, function () {
console.log('Listening on port 3000');
});
In this example you can get the user info at:
http://localhost:3000/github-user/abc
The function getUser should make an actual request to GitHub and before you call it you can change if that is really your frontend that is making the request e.g. by cheching the "Referer" header or other things, validate the input etc.
Now, if you only need a public info then you may be able to use a public JSON-P API like this - an example using jQuery to make things simple:
var user = prompt("User name:");
var req = $.getJSON('https://api.github.com/users/'+user);
req.then(function (data) {
console.log(data);
});
See DEMO

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