How does counting instances of a callback function work in node.js?
I was working on the 9th exercise of learnyounode (below the official solution).
As I understand it, the httpGet function is called three times, running through process.argv[2], [3] and [4]. But how could count ever === 3? Don't the individual functions just get to one? How does one call of httpGet know of the other ones?
var http = require('http')
var bl = require('bl')
var results = []
var count = 0
function printResults () {
for (var i = 0; i < 3; i++)
console.log(results[i])
}
function httpGet (index) {
http.get(process.argv[2 + index], function (response) {
response.pipe(bl(function (err, data) {
if (err)
return console.error(err)
results[index] = data.toString()
count++
if (count == 3)
printResults()
}))
})
}
for (var i = 0; i < 3; i++)
httpGet(i)
But how could count ever === 3?
count is defined outside of httpGet and thus its value is independent of the those function calls. count++ is the same as count = count + 1, i.e. every call to httpGet increases the value of count by 1. The third time the function is called, count's value will be 3.
We can easily replicate this:
var count = 0;
function httpGet() {
count++;
console.log('count: ', count);
if (count === 3) {
console.log('count is 3');
}
}
httpGet();
httpGet();
httpGet();
First, prefer not using var.
var is defined in the global scope so it’s value updated between calls
Read more about var here
Related
I have two problems i cant figure out. When i call GetParams first to get used defined values from a text file, the line of code after it is called first, or is reported to the console before i get data back from the function. Any data gathered in that function is null and void. The variables clearly are being assigned data but after the function call it dissapears.
let udGasPrice = 0;
let udGasLimit = 0;
let udSlippage = 0;
I want to get data from a text file and assign it to variables that need to be global. able to be assigned in a function but used outside it. So above is what i was doing to declare them outside the function. because if i declare them inside, i lose scope. It doesnt seem right to declare with 0 and then reassign, but how else can i declare them gloabaly to be manipulated by another function?
next the code is called for the function to do the work
GetParams();
console.log('udGasPrice = " + udGasPrice );
The code after GetParams is reporting 0 but inside the function the values are right
The data is read and clearly assigned inside the function. its not pretty or clever but it works.
function GetParams()
{
const fs = require('fs')
fs.readFile('./Config.txt', 'utf8' , (err, data) => {
if (err) {
console.error(err)
return;
}
// read file contents into variable to be manipulated
var fcnts = data;
let icnt = 0;
for (var x = 0; x < fcnts.length; x++) {
var c = fcnts.charAt(x);
//find the comma
if (c == ',') {
// found the comma, count it so we know where we are.
icnt++;
if (icnt == 1 ) {
// the first param
udGasPrice = fcnts.slice(0, x);
console.log(`udGasPrice = ` + udGasPrice);
} else if (icnt == 2 ) {
// second param
udGaslimit = fcnts.slice(udGasPrice.length+1, x);
console.log(`udGaslimit = ` + udGaslimit);
} else {
udSlippage = fcnts.slice(udGaslimit.length + udGasPrice.length +2, x);
console.log(`udSlippage = ` + udSlippage );
}
}
}
})
}
Like i said i know the algorithm is poor, but it works.(Im very noob) but why are the variables not retaining value, and why is the code after GetParams() executed first? Thank you for your time.
The code is executed before the GetParams method finishes, because what it does is an asynchronous work. You can see that by the use of a callback function when the file is being read.
As a best practice, you should either provide a callback to GetParams and call it with the results from the file or use a more modern approach by adopting promises and (optionally) async/await syntax.
fs.readFile asynchronously reads the entire contents of a file. So your console.log('udGasPrice = " + udGasPrice ); won't wait for GetParams function.
Possible resolutions are:
Use callback or promise
let udGasPrice = 0;
let udGasLimit = 0;
let udSlippage = 0;
GetParams(() => {
console.log("udGasPrice = " + udGasPrice);
});
function GetParams(callback) {
const fs = require('fs')
fs.readFile('./Config.txt', 'utf8', (err, data) => {
if (err) {
console.error(err)
return;
}
// read file contents into variable to be manipulated
var fcnts = data;
let icnt = 0;
for (var x = 0; x < fcnts.length; x++) {
var c = fcnts.charAt(x);
//find the comma
if (c == ',') {
// found the comma, count it so we know where we are.
icnt++;
if (icnt == 1) {
// the first param
udGasPrice = fcnts.slice(0, x);
console.log(`udGasPrice = ` + udGasPrice);
} else if (icnt == 2) {
// second param
udGaslimit = fcnts.slice(udGasPrice.length + 1, x);
console.log(`udGaslimit = ` + udGaslimit);
} else {
udSlippage = fcnts.slice(udGaslimit.length + udGasPrice.length + 2, x);
console.log(`udSlippage = ` + udSlippage);
}
}
}
callback()
})
}
fs.readFileSync(path[, options]) - it perform same operation in sync - you still need to edit your code accordingly
Also, it's advisable that you don't edit global variables in the function and return updated variables from the function.
I need to read a grid and take that data and call a $getJSON url. The grid could have over 100 lines of data. The getJSON returns a list of comma separated values that I add to an array. Once the loop is finished I take the array and process it for the duplicates. I need to use the duplicates in another process. I know that I can't determine the order of the data that is coming back but I need to know that all of the calls have been make.
for (let i = 0; i < rowscount; i++){
$.getJSON(
"https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&retmode=json&retmax=500&term=" +
terms,
function (data) {
var pmids = data.esearchresult.idlist;
var pmidlist = pmids.join();
pmid_List.push(pmidlist);
if (i == rowscount - 1) {
// call the related function
}
});
}
I can't figure out how to be sure that the process has finished. The call to the related function has been done early at times.
Well if we keep track of how many have completed we can fire off the code when the last one is done.
let complete = 0;
for (let i = 0; i < rowscount; i++){
$.getJSON(
"https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&retmode=json&retmax=500&term=" +
terms,
function (data) {
var pmids = data.esearchresult.idlist;
var pmidlist = pmids.join();
pmid_List.push(pmidlist);
complete += 1;
if (complete == rowscount) {
// call the related function
}
});
}
I'd use fetch and Promise.all
const link = "https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&retmode=json&retmax=500&term=";
Promise.all(Array.from({
length: 3
}, () => fetch(link + 'foo').then(e => e.json()))).then(e => {
//called when all requests are done
console.log(e);
})
Try this
function getJson(url, i) {
return $.getJSON(url, function (data) {
//var pmids = data.esearchresult.idlist;
//var pmidlist = pmids.join();
//pmid_List.push(pmidlist);
console.log('completed', i)
return data;
});
}
function run() {
let promises = []
for (let i = 0; i < rowscount; i++) {
const terms = 'foot';
const url = "https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&retmode=json&retmax=500&term=" + terms;
promises.push(getJson(url, i));
}
return promises;
}
Promise.all(run()).then(() => console.log('All are completed'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
I have this code :
exports.formatDepArrTables = function(jsonReturn, type, placeName, callback) {
let toSend = "";
if (type == 'departure') {
for (let j = 0; j < jsonReturn.departures.length; j++) {
console.log("DEPARTURES LOOP j = " + j + "/" + jsonReturn.departures.length);
if(currentDeparture.display_informations.links[0] === undefined) {
toSend += ""; // setting some string informations
}
else {
let oceTrainId = ""; // random number given by a json
_this.getDisruptionFromDisruptionId(oceTrainId, function(data) {
for(let i = 0; i < data.disruptions[0].impacted_objects[0].impacted_stops.length; i++) {
if(currentImpactedStop.stop_point.label == placeName) {
toSend += "string";
}
}
});
}
}
console.log("End of the first loop");
return callback(toSend);
} else if (type == 'arrival') {
// copy&paste
} else {
throw new Error("not defined");
}
};
When I run this code (I use NodeJS), it makes 1/10, 2/10... from the first loop but it didn't iterate the second loop on the 1st iteration of the first loop (and it shows "End of the first loop" and starts the second loop).
getDisruptionFromDisruptionId is a custom method which makes a request (with 'request' NodeJS module) on an API.
Of course, I need to have informations given by getDisruptionFromDisruptionId to run my next loop...
Parent function of this code part is returning a callback that needs to be "filled" at the end of the both two loops.
Any ideas ?
request is async function, you need to add async / await to your code or use recursion
My Requirement:
I want to get the list of values using for loops. In for loop one iteration completed one time then the callback will send that list of values(array).
Once the first iteration completed second time loop value should be get incremented value.
For example : 5 values
after 5th iteration then loop is over. then second time loop should start with '0' but here it's starting with last incremented value. please help me to achieve this.
Below code is working fine for the first time.
Callback function:
$inventoryManagement.getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId(objectId,attributeId, function(objectAttributeBlockElement) {
//$scope.val = myOwnJ;
console.log(objectAttributeBlockElement);
});
Function:
var myOwnJ = 0;
// Getting ObjectId And AttributeId Using CellId For Normal Controls
var getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId = function(objectId,attributeId, callback) {
var objectAttributeBlockElement = [];// one array
try {
// iterate over the objectAttributes
for (var i = 0; i < pageObject.objects.length; i++) {
if (pageObject.objects[i].id == objectId) {
var name = "";
var labelName = "";
var dataTypeId = "";
for (;myOwnJ < pageObject.objects[i].objectAttribute.length;) {
name = pageObject.objects[i].objectAttribute[myOwnJ].name;// got the current label name
labelName = pageObject.objects[i].objectAttribute[myOwnJ].labelName;// got the current name
dataTypeId = pageObject.objects[i].objectAttribute[myOwnJ].dataTypeId;// got the current dataTypeId
objectAttributeBlockElement.push(name,labelName,dataTypeId);
callback(objectAttributeBlockElement, myOwnJ++);
return;
}
}
}
throw {
message: "objectId not found: " + objectId
};
} catch (e) {
console.log(e.message + " in getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId");
}
};
You could pass j as an additional function parameter, such as
var getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId = function(objectId, attributeId, j, callback) {
so it won't be a local variable. Then, instead of declaring it locally, use the following:
for (j = ((j === null) ? 0 : j); j < pageObject.objects[i].objectAttribute.length; j++) {
That way, if you call your function with j, you'll get it incremented after each call.
Another approach, which I won't recommend, would be making j a global variable by declaring it ouside your function instead of passing it as a parameter. That way you don't have to modify your function declaration at all. If you're up to that, I strongly suggest modifying the variable name cause j would be too generic for a global scope variable and it will cause trouble sooner or later: use something like myOwnJ and you'll be fine.
EDIT: Full source code (as requested by the OP):
var myOwnJ = 0;
// Getting ObjectId And AttributeId Using CellId For Normal Controls
var getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId = function(objectId,attributeId, callback) {
var objectAttributeBlockElement = [];// one array
try {
// iterate over the objectAttributes
for (var i = 0; i < pageObject.objects.length; i++) {
if (pageObject.objects[i].id == objectId) {
var name = "";
var labelName = "";
var dataTypeId = "";
if(myOwnJ < pageObject.objects[i].objectAttribute.length) {
name = pageObject.objects[i].objectAttribute[myOwnJ].name;// got the current label name
labelName = pageObject.objects[i].objectAttribute[myOwnJ].labelName;// got the current name
dataTypeId = pageObject.objects[i].objectAttribute[myOwnJ].dataTypeId;// got the current dataTypeId
objectAttributeBlockElement.push(name,labelName,dataTypeId);
callback(objectAttributeBlockElement, myOwnJ++);
return;
}
else {
myOwnJ = 0;
}
}
}
throw {
message: "objectId not found: " + objectId
};
} catch (e) {
console.log(e.message + " in getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId");
}
};
What you are looking for is a global variable for 'j'. Although this is discouraged to be used.
var j=0;
var getObjectNameAndAttributeAndDataTypeIdUsingObjectAndAttributeId =
function(objectId, attributeId, callback) {
//do your stuff
//increment j
j++;
}
Basically what I am trying to do here is , do nmap scan on 7 machines using nmap/nodejs and then when the response comeback, I am writing it to the client page.
When I run this it works fine.Its using closure, to wrap the variable count, so every callback gets count = 0.
Now what I don't understand is, why is the value of count increasing from 1 to 7 as per the console.log. As per my understanding, every method gets a count.
Can someone please explain why this is
Output from nodejs ::
server is listening
8.8.8.1
8.8.8.2
8.8.8.3
8.8.8.4
8.8.8.5
8.8.8.6
8.8.8.8
count = 1
count = 2
count = 3
count = 4
count = 5
count = 6
count = 7
ending the response
Code ::
var http = require('http');
var cb = function(httpRequest, httpResponse){
var count = 0;
var ips = [];
ips.push('8.8.8.1');
ips.push('8.8.8.2');
ips.push('8.8.8.3');
ips.push('8.8.8.4');
ips.push('8.8.8.5');
ips.push('8.8.8.6');
ips.push('8.8.8.8');
var exec = require('child_process').exec;
for(var i =0; i< ips.length ; i++)
{
exec('nmap.exe -v ' + ips[i], function(error, stdout, stderr) {
if(error)
{
httpResponse.write(stderr);
httpResponse.write("</br>")
httpResponse.write("*************************");
}
else
{
httpResponse.write(stdout);
httpResponse.write("</br>")
httpResponse.write("*************************");
}
count = count + 1;
console.log('count = ' + count);
if (count === 7)
{
console.log('ending the response');
httpResponse.end();
}
});
console.log(ips[i]);
}
}
var server = http.createServer(cb);
server.listen(8080, function(){
console.log('server is listening');
});
-Thanks
Because count is declared in the global space in line 3 of your code, each loop is increasing the outer count by one. This will not cause your count to reset. If you want to instantiate a new count variable, you need to declare var count within your loop instead.
count is increasing from 1 to 7 because you're increasing it by 1 on every iteration of the loop which loops over the array of ips which contain 7 elements. Every new request gets the count reset to 0. Every request you are iterating through all 7 elements.
It looks like you are wanting to use closures on the count variable. However, you did not implement closures on the count variable. In order to pass a copy of count to each asynchronous function you should wrap the corresponding blocks of code in a function call that takes count as an argument. For Example:
var exec = require('child_process').exec;
for(var i =0; i< ips.length ; i++)
{
(function(count) { //Start of closure function storing a local copy of count
exec('nmap -v ' + ips[i], function(error, stdout, stderr) {
if(error)
{
httpResponse.write(stderr);
httpResponse.write("</br>")
httpResponse.write("*************************");
}
else
{
httpResponse.write(stdout);
httpResponse.write("</br>")
httpResponse.write("*************************");
}
count = count + 1;
console.log('count = ' + count);
if (count === 7)
{
console.log('ending the response');
httpResponse.end();
}
});
})(count); // end of closure function
console.log(ips[i]);
}
Running this will get you the output:
count = 1
count = 1
count = 1
count = 1
count = 1
count = 1
count = 1