How could I run a function after the completion of a for loop in Node.js?

Question

Say if I have a structure in Node.js shown below:

for (i = 0; i < 50; i++) {
    //Doing a for loop.
}

function after_forloop() {
    //Doing a function.
}

after_forloop();

So how could I make sure the after_forloop() function is fired after the forloop is completed?

In case if you want to see what I am actually working on:

var http = require('http');
http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

var proxyChecker = require('proxy-checker');
var fs = require('fs');

function get_line(filename, line_no, callback) {
    fs.readFile(filename, function (err, data) {
        if (err) throw err;
        var lines = data.toString('utf-8').split("\n");
        var firstLineBreak = data.toString('utf-8').indexOf("\n");
        var originalText = data.toString('utf-8');
        var newText = originalText.substr(firstLineBreak + 1);
        if(+line_no > lines.length){
            return callback('File end reached without finding line', null);
        }

        callback(null, lines[+line_no], newText);
    });
}


for (i = 0; i < 50; i++) {
    get_line('proxy_full.txt', i, function(err, line, newText){
        fs.appendFile('proxy.txt', line + '\n', function (err) {
            if (err) throw err;         
        });     
        fs.writeFile('proxy_full.txt', newText, function (err) {
            if (err) throw err;
        });
    })
}

after_forloop();

function after_forloop() {
    proxyChecker.checkProxiesFromFile(
        // The path to the file containing proxies
        'proxy.txt',
        {
            // the complete URL to check the proxy
            url: 'http://google.com',   
            // an optional regex to check for the presence of some text on the page
            regex: /.*/
        },
        // Callback function to be called after the check
        function(host, port, ok, statusCode, err) {
            if (ok) {
                console.log(host + ':' + port);
                fs.appendFile('WorkingProxy.txt', host + ':' + port + '\n', function (err) {
                    if (err) throw err;             
                });
            }                   
        }
    );

    setTimeout(function(){
        fs.writeFile('proxy.txt', "", function (err) {
            if (err) throw err;
        });
        console.log('Proxy Check Completed.')
        process.exit(1); 
    }, 5000);
}

Basically I like to allow the node server run 50 test on a list proxy servers at a time (within five seconds). And then the server should save the working proxies to a new file.

By "after the for loop is completed" you probably mean "after all the asynchronous activity started in the for loop is completed", right? — Pointy
– Pointy, Commented Jan 13, 2015 at 2:51
I'm not getting what special here, just call that after for loop finished. — Mritunjay
– Mritunjay, Commented Jan 13, 2015 at 2:51
code usually runs top-down, but if it's not, you can make sure i is 50... — dandavis
– dandavis, Commented Jan 13, 2015 at 2:55

Ethan Lynn · Accepted Answer · 2015-01-13 20:14:57Z

10

Maybe this helps:

var operationsCompleted = 0;
function operation() {
    ++operationsCompleted;
    if (operationsCompleted === 100) after_forloop(); 
}
for (var i = 0; i < 50; i++) {
    get_line('proxy_full.txt', i, function(err, line, newText){
        fs.appendFile('proxy.txt', line + '\n', function (err) {
            if (err) throw err;
            operation();
        });     
        fs.writeFile('proxy_full.txt', newText, function (err) {
            if (err) throw err;
            operation();
        });
    })
}

Admittedly this isn't an elegant solution. If you're doing a whole lot of this you might want to check out something like Async.js.

edited Jan 13, 2015 at 20:14

answered Jan 13, 2015 at 3:54

Ethan Lynn

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Comments

Parth Vyas · Accepted Answer · 2016-04-09 12:05:41Z

10

If you're doing any async operation in the for loop, you can simply keep it like

function after_forloop() {
// task you want to do after for loop finishes it's execution
}


 for (i = 0; i < 50; i++) {
 //Doing a for loop.

 if(i == 49) {
  after_forloop() // call the related function
 }
}

Still in node.js, instead of making use of for loops with asynchronous functions, you should see the async module. Here's Async.js or you can consider using recursion too.

answered Apr 9, 2016 at 12:05

Parth Vyas

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Comments

Mritunjay · Accepted Answer · 2015-01-13 03:00:50Z

8

If there is no magic happening then it should be straight forword.

Function:-

function after_forloop() {
    //Doing a function.
}

For Loop:-

for (i = 0; i < 50; i++) {
    //Doing a for loop.
}
for (i = 0; i < 50; i++) {
    //Doing another for loop.
}
after_forloop();

This will call after_forloop just after both for loops finishes. Because for loop is a blocking call, calling of after_forloop() has to wait.

Note:- If you are doing some async task in for loops then the function will be called after loop finished, but the work you are doing in loops might not finished by the time of calling function.

edited Jan 13, 2015 at 3:00

answered Jan 13, 2015 at 2:55

Mritunjay

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6 Comments

Aero Wang Over a year ago

Great! What happen if I have multiple for loops, will it wait for all the for loops to complete?

Mritunjay Over a year ago

@AeroWindwalker Yes the line will be called after for loop finished, because for loop is a sync task, it will blocking.

Aero Wang Over a year ago

I think Nodejs isn't letting for loop running as a sync task. For example if I let the for loop to create a file with 50 lines, then having a function output the file, it will only output 1 or 2 lines (sometimes no line at all).

Mritunjay Over a year ago

@AeroWindwalker you should probably forgot to read the note part of answer. I told you if you are doing some async task in for loop that might not finished.

Aero Wang Over a year ago

Interesting! What would be the solution to it if I am doing asyn tasks in the forloop?

|

sonEtLumiere · Accepted Answer · 2020-08-18 02:45:53Z

4

You can handle the last iteration of the for loop with a conditional checking index.

for(let i = 0; i < 10; i++){
  if(i == 10 - 1){
    your_function();
  }
}

edited Aug 18, 2020 at 2:45

sonEtLumiere

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answered Feb 15, 2020 at 18:16

Smit Luvani

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1 Comment

Izzi Over a year ago

I use this pattern, it works well. Also, I can make it dynamic with array.length.

Amit Rana · Accepted Answer · 2019-09-10 06:59:44Z

2

use the Async module, Its easy and best.

async.each(array, function (element, callback) {
                        callback()
                    }, async function (err) {
                       console.log("array ends")
                    });

answered Sep 10, 2019 at 6:59

Amit Rana

438 bronze badges

Comments

Rohit Nishad · Accepted Answer · 2021-08-24 03:26:47Z

1

Simplest one!

try {
  for (i = 0; i < 50; i++) {
    //Doing a for loop.
  }
} finally {
  after_forloop();
}

answered Aug 24, 2021 at 3:26

Rohit Nishad

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Comments

L.Gaunt · Accepted Answer · 2024-09-08 14:45:26Z

0

I had this problem and created a function using setTimeout to repeatedly check every half second whether the for loop was completed before calling the function to run after the for loop completed:

function forLoopFunction() {
    // any other function code
    for loop {
      // body of for loop
    }
    checkForLoopComplete();
}
function checkForLoopComplete() {
  if (condition which indicates for loop is complete) {
    // call function to run on for loop complete
    functionOnForLoopComplete();
  } else {
    setTimeout(checkForLoopComplete, 500);
  }
}

function functionOnForLoopComplete() {
    // function code
}

answered Sep 8, 2024 at 14:45

L.Gaunt

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