The value of rows[i] is perfectly printing inside the loop, but when we print it outside it becomes undefined.I don't want to use set timeout function in my program.
function listshops(callback)
{
client.connection.query('select * from shop',function(err,rows){
if(rows.length>0)
{
for(var i=0;i<rows.length;i++)
{
(function(i){
var shopIdFetched = rows[i].shopId;
client.connection.query('select * from image where shopId=?',shopIdFetched,function(err,data){
if(data.length > 0){
var result = rows[i].image = JSON.stringify(data);
}
});
})(i);
}
console.log(rows[i]);
}
});
}
output:
First you are getting undefined cause your i in rows[i] is out of index. But solving that wont solve your problem as you are performing multiple asynchronous task within a for loop. Your rows object will not be populated by the time you print it.
Solution: You need to use async or promises for performing the task.
// Include the async package
var async = require("async");
client.connection.query('select * from shop',function(err,rows){
if(rows.length>0)
{
// 1st para in async.each() is the array of items
async.each(rows,
// 2nd param is the function that each item is passed to
function(item, callback){
// Call an asynchronous function,
var shopIdFetched = item.shopId;
client.connection.query('select * from image where shopId=?',shopIdFetched,function(err,data){
if(data.length > 0){
item.image = JSON.stringify(data);
}
callback();//required
});
},
// 3rd param is the function to call when everything's done
function(err){
if(err){
console.log('Error:' + err);
}
console.log(rows);// your result
}
);
}
});
When you exit the loop, the value of i is equal to the array's length.
Let's say the length is 10, so indexes run from 0 to 9.
So in fact, after the loop, rows[i] is rows[10], which is indeed undefined.
Related
I am new to cloud functions (node js with typescript). I am using it to fetch data from Firebase database( as the code below). But Not able to figure out how to wait until forEach loop iterates completely and use that result.
const reference = admin.database().ref('/books')
var path_key:string
var total_count:number = 0
reference .forEach(function (snapshot) {
path_key= snapshot(key).val()
ref_users_advance_bookings.child(path_key)
.once('value').then((snapshot2)=>{
if(condidtion met){
return response.send("failed")
}
else{
total_count++
}
)}
return true
})
// i want to use total_count value here
return response.send("count :"+total_count) // but every time I gets 0, as it get executed before forEach has ended
)}
My guess is that you're trying to wait for a number of items to load from the database. For that you'll want to use Promise.all(), in something like:
var promises = [];
ref.forEach(function (snapshot) {
path_key= snapshot(key).val()
promises.push(ref_users_advance_bookings.child(path_key).once('value'));
});
Promise.all(promises).then(function(snapshots) {
var failed = false;
snapshot.forEach(function(snapshot2) {
if(condition met){
failed = true;
}
else {
total_count++;
}
)}
if (failed) {
return response.status(500).send("ERROR");
}
else {
return response.send("count :"+total_count);
}
});
Little info, i have an arp.js file which takes a subnet address "192.168.2" and gets all strings returned from arp -a and stores in an array.
I can't figure out why my arpList function is returning an undefined value in my index.js file.
All the console.logs are returning the correct values in the arp.js page when called from the index.js, but the ipObj is coming up undefined. Even the console.log before i return of ipObj works.
Any help would be greatly appreciated.
var { spawn } = require('child_process');
const arpLs = spawn('arp', ['-a']);
var bufferData;
module.exports = {
arpList: function (subnet) {
arpLs.stdout.on('data', data => {
bufferData += data
})
arpLs.stderr.on('data', data => {
console.log('error: ' + data);
});
arpLs.on('exit', function (code) {
if (code != 0) {
console.log("Error exiting"); //if error occurs
}
console.log("exit start 1"); // checking internal processes at stages
var dataArray = bufferData.split(' ');
var ipArray = [];
for (i = 0; i < dataArray.length; i++) {
if (dataArray[i].includes(subnet)) {
ipArray.push(dataArray[i]);
console.log("loop working");
}
}
var ipObj = { "lanIps": ipArray };
console.log("Object is there: "+ipObj)
return ipObj; // this obj should be returned to the index.js call using
})
},
sayMyName: function () {
return "Hello";
}
}
//arpList(ipSubnet);
//INDEX.js
//the index page looks like this
//var arp = require('./arp.js);
//var ipSubnet = "192.168.2";
//var lanIps = arp.arpList(ipSubnet);
//console.log(lanIps);
I ended up adding a callback function to arpList - function (subnet, callback)
Then instead of returning the value pass it into the callback
Then on the index.js side instead of
var lanIps = arp.arpList(value)
i used
arp.arpList(value, function(res){lanIps = res}
return ipObj; // this obj should be returned to the index.js call using
It won't be returned. The reference say nothing about return value. Node-style callbacks rarely work like that because they are potentially asynchronous and returned value cannot be taken into account.
This a special case of this well-known problem. The process is asynchronous and is finished after arp.arpList(ipSubnet) call, there's nothing to assign to lanIps. This is a use case for promises. There are already third-party promisified counterparts like child-process-promise.
The problem can be also solved by moving to synchronous API. child_process functions have synchronous counterparts, including spawnSync.
In the code below, I am trying to get a value from a 'nodehun' method called spellSuggestions. In the documentation I'm told the syntax to use this method is as follows: dict.spellSuggestions(w, handleMisspellings);
where w is a list of words and handleMisspellings is a function (which is posted below). I can see the output on the console for handleMisspellings, but for the life of me, I cannot return or find a way to return a variable from this call: [dict.spellSuggestions(w, handleMisspellings);]. After setting a var equal to 'dict.spellSuggestions(w, handleMisspellings);' the return value is undefined. Please help!
var debugFlag = process.argv.indexOf('debug') > -1;
var nodehun = require('./../build/' + (debugFlag ? 'Debug' : 'Release') + '/nodehun');
var fs = require('fs');
var dict = new nodehun(fs.readFileSync(__dirname+'/dictionaries/en_US.aff'),fs.readFileSync(__dirname+'/dictionaries/en_US.dic'));
//var words = ['original', 'roach', 'erasee', 'come', 'consol', 'argumnt', 'gage',
// 'libary', 'lisence', 'principal', 'realy', 'license', 'suprise', 'writting'];
var handleMisspellings = function(err, correct, suggestions, origWord, callback) {
if (err) throw err;
if (correct) {
console.log(origWord + ' is spelled correctly!');
}
else {
console.log(origWord + ' not recognized. Suggestions: ' + suggestions);
}
var value = {
err: err,
correct: correct,
suggestions: suggestions,
origWord: origWord
};
console.log('VALUE+++++: ' + value);
return value;
}
var foo = function(words) {
words.forEach(function(w) {
dict.spellSuggestions(w, handleMisspellings);
some = dict;
console.log(JSON.stringify(some, null, 2));
});
}
module.exports = {
foo: foo
}
Thanks Dave. I eventually discovered the practical use of callback functions. For each method that contained data that I desired to access outside of the method, I declared an individual function to wrap the method. The function accepted two input arguments. The first was the input variable to drive the method call. The second was literally 'callback'. Inside the method, I would perform whatever operation I wanted to package the data into a JSON object before returning any desired data with 'return callback(var)'. In the call of the created wrapper function, I would actually call the function using the input variable of choice to drive the method in the function definition, and pass 'function(return_variable)' as the second argument. This creates a new method in which the desired data may be accessed or even again called back. The final code I desired performs a for loop on each element of a list of words, creates metadata about those words, and appends the unique data for each word to each word in a single array. The final array is a single object which contains all input words, and all data about those words. It required 4 individual functions (one of which was recursive), and a function call. Please see the code snippet of the function described above [doCall]. Note the use of the code begins at the call of 'analyze' [which is commented out here] and works its way up to each previous function declaration. I hope this helps someone else in the future to understand the functional use of 'callbacks'. Please ask if you have any questions, and Thanks again =D.
function doCall(word, callback) {
dict.spellSuggestions(word, function(err, correct, suggestions, origWord) {
if (err) throw err;
// if (correct)
// console.log(origWord + ' is spelled correctly!');
// else
// console.log(origWord + ' not recognized. Suggestions: ' + suggestions);
val = {
err: err,
correct: correct,
origWord: origWord,
suggestions: suggestions
}
return callback(val);
});
}
function addMember(array, index, callback){
doCall(array[index], function(val){
// console.log(val);
// console.log(index);
// console.log(array[index]);
// console.log(val.origWord);
array[val.origWord] = val;
// console.log(array[val.origWord]);
index = index + 1;
return callback(array, index);
});
}
function Loop(array, index, callback) {
addMember(array, index, function(array2, index2){
// console.log(index);
// console.log(index2);
if(index2 === array2.length) {
return callback(array2);
}
else{
Loop(array2, index2, callback);
}
});
}
function analyze(array, index, callback){
Loop(array, index, function(complete_array){
console.log('!!!!!!!!!!!!!!!!!' + complete_array);
return callback(complete_array);
});
}
/*
analyze(words, 0, function(complete_array){
// for(i = 0; i < complete_array.length; i++) {
console.log(complete_array);
// }
});
*/
module.exports = {
analyze
}
Is there a way to make Node.js stream as coroutine.
Example
a Fibonacci numbers stream.
fibonacci.on('data', cb);
//The callback (cb) is like
function cb(data)
{
//something done with data here ...
}
Expectation
function* fibonacciGenerator()
{
fibonacci.on('data', cb);
//Don't know what has to be done further...
};
var fibGen = fibonacciGenerator();
fibGen.next().value(cb);
fibGen.next().value(cb);
fibGen.next().value(cb);
.
.
.
Take desired numbers from the generator. Here Fibonacci number series is just an example, in reality the stream could be of anything a file, mongodb query result, etc.
Maybe something like this
Make the 'stream.on' function as a generator.
Place yield inside the callback function.
Obtain generator object.
Call next and take the next value in stream.
Is it at-least possible if yes how and if not why? Maybe a dumb question :)
If you don't want to use a transpiler (e.g. Babel) or wait until async/await make it to Node.js, you can implement it yourself using generators and promises.
The downside is that your code must live inside a generator.
First, you can make a helper that receives a stream and returns a function that, when called, returns a promise for the next "event" of the stream (e.g. data).
function streamToPromises(stream) {
return function() {
if (stream.isPaused()) {
stream.resume();
}
return new Promise(function(resolve) {
stream.once('data', function() {
resolve.apply(stream, arguments);
stream.pause();
});
});
}
}
It pauses the stream when you're not using it, and resumes it when you ask it the next value.
Next, you have a helper that receives a generator as an argument, and every time it yields a promise, it resolves it and passes its result back to the generator.
function run(fn) {
var gen = fn();
var promise = gen.next().value;
var tick = function() {
promise.then(function() {
promise = gen.next.apply(gen, arguments).value;
}).catch(function(err) {
// TODO: Handle error.
}).then(function() {
tick();
});
}
tick();
}
Finally, you would do your own logic inside a generator, and run it with the run helper, like this:
run(function*() {
var nextFib = streamToPromises(fibonacci);
var n;
n = yield nextFib();
console.log(n);
n = yield nextFib();
console.log(n);
});
Your own generator will yield promises, pausing its execution and passing the control to the run function.
The run function will resolve the promise and pass its value back to your own generator.
That's the gist of it. You'd need to modify streamToPromises to check for other events as well (e.g. end or error).
class FibonacciGeneratorReader extends Readable {
_isDone = false;
_fibCount = null;
_gen = function *() {
let prev = 0, curr = 1, count = 1;
while (this._fibCount === -1 || count++ < this._fibCount) {
yield curr;
[prev, curr] = [curr, prev + curr];
}
return curr;
}.bind(this)();
constructor(fibCount) {
super({
objectMode: true,
read: size => {
if (this._isDone) {
this.push(null);
} else {
let fib = this._gen.next();
this._isDone = fib.done;
this.push(fib.value.toString() + '\n');
}
}
});
this._fibCount = fibCount || -1;
}
}
new FibonacciGeneratorReader(10).pipe(process.stdout);
Output should be:
1
1
2
3
5
8
13
21
34
55
Hi I have a backbone web app using Jquery and NodeJs/mongo as the server side framework. I'm having problems with making a http get call with a foreah loop and the results of the get call being iteratively added to each row of the loop.
var eventid = this.model.get("_id");
var inPromise = $.get("/registrants/list?eventid="+eventid,null,null,"json").then(
function (result){
var temp;
var finalVal = '';
var tempfinalVal = "";
var loop = 0
percentage = 0;
$.each(result.registrants,function(index,registrant){
temp = JSON.parse(registrant.fields);
for (var key in temp) {
if(key =="Email"){
if(temp[key] != ""){
$.get("/stats/registrant?userid="+temp[key]+"&eventid="+eventid,null,null,"json").then(function(result2){
percentage = (result2.Stats.type ===undefined || result2.Stats.type ==null) ? "0": result2.Stats.type;
finalVal +=percentage+"\n";
}).fail(function(){
percentage = "0";
});
}
}else if(key =="eventid"){
loop++;
finalVal = finalVal.slice(0, - 1);
finalVal +='\n';
}
finalVal +=temp[key] + ',';
}
});
//promises.push(inPromise);
}
).done(function(finalVal){
$("#webcast-download-registrants-tn").attr("href",'data:text/csv;charset=utf-8;filename=registration.csv",'+encodeURIComponent(finalVal));
console.log("DONE");
}).fail(function(){
console.log("fail");
});
// promise.done(function () {
// console.log(" PROMISE DONE");
// });
So I have the loop through a collection and the last item of the docuemnt gets a content froma nother http call and when all is fone it will create a CSV file. The problem is that THE "DONE" text echos firts then the "CALL" text is displayed
Rick, your problem is not the simplest due to :
the need for nested asynchronous gets
the need to build each CSV data row partly synchronously, partly asynchronously.
the need for a mechanism to handle the fulfilment of multiple promises generated in the inner loop.
From what you've tried, I guess you already know that much.
One important thing to note is that you can't rely on for (var key in temp) to deliver properties in any particular order. Only arrays have order.
You might try something like this :
var url = "/stats/registrant",
data = { 'eventid': this.model.get('_id') },
rowTerminator = "\n",
fieldNames = ['firstname','lastname','email','company','score'];
function getScore(email) {
return $.get(url, $.extend({}, data, {'userid':email}), null, "json").then(function(res) {
return res.Stats ? res.Stats.type || 0 : 0;
}, function() {
//ajax failure - assume score == 0
return $.when(0);
});
}
$.get("/registrants/list", data, null, "json").then(function(result) {
var promises = [];//An array in which to accumulate promises of CSV rows
promises.push($.when(fieldNames)); //promise of CSV header row
if(result.registrants) {
$.each(result.registrants, function(index, registrant) {
if(registrant.fields) {
// Synchronously initialize row with firstname, lastname, email and company
// (omitting score for now).
var row = fieldNames.slice(0,-1).map(function(fieldName, i) {
return registrant.fields[fieldName] || '';
});
//`row` remains available to inner functions due to closure
var promise;
if(registrant.fields.Email) {
// Fetch the registrant's score ...
promise = getScore(registrant.fields.Email).then(function(score) {
//... and asynchronously push the score onto row
row.push(score);
return row;
});
} else {
//or synchronously push zero onto row ...
row.push(0);
//... and create a resolved promise
promise = $.when(row);
}
promises.push(promise);//Accumulate promises of CSV data rows (still in array form), in the correct order.
}
});
}
return $.when.apply(null, promises).then(function() {
//Join all the pieces, in nested arrays, together into one long string.
return [].slice.apply(arguments).map(function(row) {
return row.join(); //default glue is ','
}).join(rowTerminator);
});
}).done(function(str) {
$("#webcast-download-registrants-tn").attr("href",'data:text/csv;charset=utf-8;filename=registration.csv",'+encodeURIComponent(str));
console.log("DONE");
}).fail(function() {
console.log("fail");
});
partially tested
See comments in code for explanation and please ask if there's anything you don't follow.