Having difficulties with node.js res.send() loop - node.js

Question

I'm attempting to write a very basic scraper that loops through a few pages and outputs all the data from each url to a single json file. The url structure goes as follows:
http://url/1
http://url/2
http://url/n
Each of the urls has a table, which contains information pertaining to the ID of the url. This is the data I am attempting to retrieve and store inside a json file.
I am still extremely new to this and having a difficult time moving forward. So far, my code looks as follows:
app.get('/scrape', function(req, res){
var json;
for (var i = 1163; i < 1166; i++){
url = 'https://urlgoeshere.com' + i;
request(url, function(error, response, html){
if(!error){
var $ = cheerio.load(html);
var mN, mL, iD;
var json = { mN : "", mL : "", iD: ""};
$('html body div#wrap h2').filter(function(){
var data = $(this);
mN = data.text();
json.mN = mN;
})
$('table.vertical-table:nth-child(7)').filter(function(){
var data = $(this);
mL = data.text();
json.mL = mL;
})
$('table.vertical-table:nth-child(8)').filter(function(){
var data = $(this);
iD = data.text();
json.iD = iD;
})
}
fs.writeFile('output' + i + '.json', JSON.stringify(json, null, 4), function(err){
console.log('File successfully written! - Check your project directory for the output' + i + '.json file');
})
});
}
res.send(json);
})
app.listen('8081')
console.log('Magic happens on port 8081');
exports = module.exports = app;
When I run the code as displayed above, the output within the output.json file only contains data for the last url. I presume that's because I attempt to save all the data within the same variable?
If I include res.send() inside the loop, so the data writes after each page, I receive the error that multiple headers cannot be sent.
Can someone provide some pointers as to what I'm doing wrong? Thanks in advance.
Ideal output I would like to see:
Page ID: 1
Page Name: First Page
Color: Blue
Page ID: 2
Page Name: Second Page
Color: Red
Page ID: n
Page Name: Nth Page
Color: Green

Answer 1

I can see a number of problems:
Your loop doesn't wait for the asynchronous operations in the loop, thus you do some things like res.send() before the asynchronous operations in the loop have completed.
In appropriate use of cheerio's .filter().
Your json variable is constantly being overwritten so it only has the last data in it.
Your loop variable i would lose its value by the time you tried to use it in the fs.writeFile() statement.
Here's one way to deal with those issues:
const rp = require('request-promise');
const fsp = require('fs').promises;
app.get('/scrape', async function(req, res) {
let data = [];
for (let i = 1163; i < 1166; i++) {
const url = 'https://urlgoeshere.com/' + i;
try {
const html = await rp(url)
const $ = cheerio.load(html);
const mN = $('html body div#wrap h2').first().text();
const mL = $('table.vertical-table:nth-child(7)').first().text();
const iD = $('table.vertical-table:nth-child(8)').first().text();
// create object for this iteration of the loop
const obj = {iD, mN, mL};
// add this object to our overall array of all the data
data.push(obj);
// write a file specifically for this invocation of the loop
await fsp.writeFile('output' + i + '.json', JSON.stringify(obj, null, 4));
console.log('File successfully written! - Check your project directory for the output' + i + '.json file');
} catch(e) {
// stop further processing on an error
console.log("Error scraping ", url, e);
res.sendStatus(500);
return;
}
}
// send all the data we accumulated (in an array) as the final result
res.send(data);
});
Things different in this code:
Switch over all variable declarations to let or const
Declare route handler as async so we can use await inside.
Use the request-promise module instead of request. It has the same features, but returns a promise instead of using a plain callback.
Use the promise-based fs module (in latest versions of node.js).
Use await in order to serialize our two asynchronous (now promise-returning) operations so the for loop will pause for them and we can have proper sequencing.
Catch errors and stop further processing and return an error status.
Accumulate an object of data for each iteration of the for loop into an array.
Change .filter() to .first().
Make the response to the request handler be a JSON array of data.
FYI, you can tweak the organization of the data in obj however you want, but the point here is that you end up with an array of objects, one for each iteration of the for loop.
EDIT Jan, 2020 - request() module in maintenance mode
FYI, the request module and its derivatives like request-promise are now in maintenance mode and will not be actively developed to add new features. You can read more about the reasoning here. There is a list of alternatives in this table with some discussion of each one. I have been using got() myself and it's built from the beginning to use promises and is simple to use.