This question might be possible duplication. I am a noob to node.js and asynchronous programming palindrome. I have google searched and seen a lot of examples on this, but I still have bit confusion.

OK, from google search what I understand is that all the callbacks are handled asynchronous. for example, let's take readfile function from node.js api

fs.readFile(filename, [options], callback) // callback here will be handled asynchronously fs.readFileSync(filename, [options])

  var fs = require('fs');

    fs.readFile('async-try.js' ,'utf8' ,function(err,data){
        console.log(data);   })

    console.log("hii");

The above code will first print hii then it will print the content of the file.

So, my questions are:

1. Are all callbacks handled asynchronously?

2. The below code is not asynchronous, why and how do I make it?

   function pute(callback){
    for(var i =0; i < 1000 ; i++){}
    callback(i);
   
   }
   
   function print(num){
    console.log("value of i is:" + num);
   }
   
   
   pute(print);
   console.log("hii");
   

This question might be possible duplication. I am a noob to node.js and asynchronous programming palindrome. I have google searched and seen a lot of examples on this, but I still have bit confusion.

OK, from google search what I understand is that all the callbacks are handled asynchronous. for example, let's take readfile function from node.js api

fs.readFile(filename, [options], callback) // callback here will be handled asynchronously fs.readFileSync(filename, [options])

  var fs = require('fs');

    fs.readFile('async-try.js' ,'utf8' ,function(err,data){
        console.log(data);   })

    console.log("hii");

The above code will first print hii then it will print the content of the file.

So, my questions are:

1. Are all callbacks handled asynchronously?

2. The below code is not asynchronous, why and how do I make it?

   function pute(callback){
    for(var i =0; i < 1000 ; i++){}
    callback(i);
   
   }
   
   function print(num){
    console.log("value of i is:" + num);
   }
   
   
   pute(print);
   console.log("hii");
   

• javascript
• node.js
• asynchronous

Share Improve this question Follow asked Jun 13, 2013 at 14:50 Rushabh RajeshKumar PadaliaRushabh RajeshKumar Padalia 1,58122 gold badges1414 silver badges2828 bronze badges

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4 Answers 4

Sorted by: Reset to default 4

Are all callbacks handled asynchronously?

Not necessarily. Generally, they are, because in NodeJS their very goal is to resume execution of a function (a continuation) after a long running task finishes (typically, IO operations). However, you wrote yourself a synchronous callback, so as you can see they're not always asynchronous.

The below code is not asynchronous, why and how do I make it?

If you want your callback to be called asynchronously, you have to tell Node to execute it "when it has time to do so". In other words, you defer execution of your callback for later, when Node will have finished the ongoing execution.

function pute(callback){
 for (var i = 0; i < 1000; i++);

 // Defer execution for later
 process.nextTick(function () { callback(i); });
}

Output:

hii
value of i is:1000

For more information on how asynchronous callbacks work, please read this blog post that explains how process.nextTick works.

No, that is a regular function call.

A callback will not be asynchronous unless it is forced to be. A good way to do this is by calling it within a setTimeout of 0 milliseconds, e.g.

setTimeout(function() { 
   // Am now asynchronous
}, 0);

Generally callbacks are made asynchronous when the calling function involves making a new request on the server (e.g. Opening a new file) and it doesn't make sense to halt execution whilst waiting for it to plete.

The below code is not asynchronous, why and how do I make it?

function pute(callback){
  for(var i =0; i < 1000 ; i++){}
  callback(i);
}

I'm going to assume your code is trying to say, "I need to do something 1000 times then use my callback when everything is plete".

Even your for loop won't work here, because imagine this:

function pute(callback){
  for(var i =0; i < 1000 ; i++){
    DatabaseModel.save( function (err, result) {
      // ^^^^^^ or whatever, Some async function here.
      console.log("I am called when the record is saved!!");
    });
  }
  callback(i);
}

In this case your for loop will execute the save calls, not wait around for them to be pleted. So, in your example, you may get output like (depending on timing)

I am called when the record is saved
hii
I am called when the record is saved
...

For your pute method to only call the callback when everything is truely plete - all 1000 records have been saved in the database - I would look into the async Node package, which can do this easily for you, and provide patterns for many async problems you'll face in Node.

So, you could rewrite your pute function to be like:

function pute(callback){
  var count = 0
  async.whilst(
     function() { return count < 1000 },
     function(callback_for_async_module) {
        DatabaseModel.save( function (err, result) {
            console.log("I am called when the record is saved!!");
            callback_for_async_module();
            count++;
        });
     },
     function(err) {
       // this method is called when callback_for_async_module has
       // been called 1000 times

       callback(count);
  );
  console.log("Out of pute method!");
}

Note that your pute function's callback parameter will get called sometime after console.log("Out of pute method"). This function is now asynchronous: the rest of the application does not wait around for pute to plete.

You can put every callback call inside a timeout with one milisecond, that way they will be executed first when there are a thread free and all synchron tasks are done, then will the processor work through the stack of timeouts that want to be executet.