CODEWITHSUNDEEP
node.js
Carlos Navarro Astiasarán
Carlos Navarro Astiasarán

Reputation: 567

How Nodejs knows if sync or async

I understand what a callback is and what asynchronous means, what I don't get is how to run asynchronous functions in node.

For example, how is this

var action = (function(data,callback) {
    result = data+1;
    callback(result);
});

http.createServer(function (req, res) {
    action(5, function(r){
        res.end(r.toString());
    });
}).listen(80);

different from this

var action = (function(data) {
    result = data+1;
    return result;
});

http.createServer(function (req, res) {
    var r = action(5);
    res.end(r.toString());
}).listen(80);

?

I guess in the first example I'm doing it asynchronously, yet I don't know how Node knows when to do it sync or async... is it a matter of the return? or the fact that in the sync mode we're doing var x = func(data);?

And also: when to use sync or async? Because obviously you don't want to use it when adding +1... is it OK to use async just when performing IO tasks, such as reading from DB?

For example, I'm using the library crypto to encrypt a short string (50 chars at most), is this case a good example where I should already be using async?

Upvotes: 2

Views: 920

Answers (3)

Mustapha AJEGHRIR
Mustapha AJEGHRIR

Reputation: 71

if you want a function to work asynchronously, you can do that using promises, look at the code below :

function is_asynch(){
   return new Promise((resolve,reject)=>{
      resolve( here_your_synch_function() )
})
}

Upvotes: 0

robertklep
robertklep

Reputation: 203231

I guess in the first example I'm doing it asynchronously...

Your first example isn't async :) Merely passing a callback and calling it when you're done doesn't make a function asynchronous.

Asynchronous means that, basically, you're telling Node: "here, do this for me, and let me know when you're done while I continue doing other stuff".

Your example is not handing anything to Node for future completion. It's doing a calculation and calling the callback immediately after that. That's functionally the same as your second example, where you return the result of the calculation.

However, you can change your first example to something that is asynchronous:

var action = (function(data,callback) {
  setTimeout(function() {
    result = data + 1;
    callback(result);
  }, 1000);
});

Here, you're telling Node to delay calling the callback for one second, by using setTimeout. In the mean time, Node won't get stuck waiting for a second; it will happily accept more HTTP requests, and each one will be delayed one second before the response is sent.

When to use sync or async?

Asynchronous code is "viral": if you rely on functions that are async, your own code that uses those functions will also have to be async (generally by accepting a callback, or using another mechanism to deal with asynchronicity, like promises).

For example, I'm using the library crypto to encrypt a short string (50 chars at most), is this case a good example where I should already be using async?

This depends on which function you're using. AFAIK, most encryption functions in crypto aren't asynchronous, so you can't "make" them asynchronous yourself.

Upvotes: 3

Lazyexpert
Lazyexpert

Reputation: 3154

Both examples will work synchronous. Simple async operations are setTimout and setInterval.

Node actually doesn't care what code are you running. You can block or not (blocking/non-blocking).

In other words - you have event loop. If your process is async he will pass the program control to the event loop, so it can execute any other action node needs to be done. If not - he wont.

Upvotes: 0

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