Asynchronous call to synchronous function

From your comment on Quentin's answer

How to create a function which has some manipulation inside it which is a normal synchronous function but the overall flow doesn't wait for this function to complete rather moves to the next function.

JavaScript, the language, doesn't have a feature to do that. So you look to the environment in which it's running to see if the environment has that feature.

NodeJS does. In fact, it has several of them: setTimeout (which is a bit like what browsers give you), setImmediate, and process.nextTick. For your use case, setImmediate is probably the best choice, but see this question and its answers for more on that. setImmediate looks like this:

setImmediate(functionToCall);

(It's a global, you don't have to require anything.)

That does exactly what your title asks for: An asynchronous call to a synchronous function.

---

From your comment below:

I just need to create an async function inside which I call a normal synchronous function. When I mean asynchronous function I mean that when I call this function it doesn't obstruct the flow and the flow goes to the next function and this function can do something(like logging) after completion

That's not quite the same thing your question asks. :-)

First off, there are no asynchronous functions (in JavaScript). When you call a function in JavaScript, that function is executed synchronously. So that means you have two choices:

1. The calling code can use setImmediate to tell Node to call the function asynchronously, or

2. The function can do that for itself, probably using a nested function.

Here's #1:

function foo() {
    console.log("A");
    setImmediate(bar);
    console.log("B");
}
function bar() {
    console.log("C");
}
foo();

which outputs:

A
B
C

Here's #2:

function foo() {
    console.log("A");
    bar("C");
    console.log("B");
}
function bar(arg) {

    setImmediate(run);

    function run() {
        console.log(arg);
    }
}
foo();

...which also outputs

A
B
C

Note how "C" was passed as an argument to bar. bar does nothing but schedule the callback to its nested function run. run uses the argument.

How is that possible?

It isn't. Everything it does is a synchronous operation.

You use asynchronous code when dealing with things which are intrinsically asynchronous. This is limited, in JS, almost entirely to event handling.

Is the function below be considered an asynchronous function?

No. It has a callback, but everything it does is synchronous.

I'm not completely sure that I understand what you need, but I believe that what you're looking for is to create concurrent function calls, which means you probably need to run a potentially computationally intensive code, and you don't want it to block other code from executing while this computation runs. Node.js is usually used in a single-threaded process, which means that only a single piece of code can run at a given time, but there are still some ways to do it.

One way to do it, as stated in @T.J. Crowder 's answer, is to queue your function call with process.nextTick so it'll run on the next cycle of the event-loop. This can help break down intensive computation into iterations and that each iteration will be queued to run on the next event-loop cycle after the last one. This can help in some cases, but not always you can break down computations into iterations that run async from each other. If you're looking for a more detailed explanation into how Javascript runs asynchronously, I suggest reading this book.

There are at least two other ways (that I know of) to achieve some sort of concurrency with Node.js- Workers/Threads, and child processes. Child processes in short are just another Node.js process that is run by your main application. You can use the native *child_process* module of Node.js to run and manage your child processes if you want to go this way. Look at the official docs for more info.

The third way to do it is to use Workers in a very similar manner to how Web Workers are used in the browser. A Worker is a snippet of code that is run in a different OS thread than your main application. To me it seems that this is the easiest way to achieve concurrency, although some will argue it is not the most robust way of doing it, and that creating child processes is better.

There a very good explanation of how Workers function on MDN. To use Workers in Node.js you will need an external module such as webworker-threads.

Here's a code sample created from your sample with a snippet from their documentation:

var Worker = require('webworker-threads').Worker;

var worker = new Worker(function(){
    onmessage = function(event) {
        result = sum(event.data.a, event.data.b);
        result = sum(result, event.data.c);
        result = sum(result, event.data.d);
        result = sum(result, event.data.e);
        result = sum(result, event.data.f);
        postMessage(result);
        self.close();
    };
});
worker.onmessage = function(event) {
    console.log("The result is: " + event.data);
};
worker.postMessage({a: 1, b: 2, c: 3, d: 4, e: 5, f: 6});

// Do some other stuff that will happen while your Worker runs...

With all that said, most people will try to avoid running concurrent code in Node.js. Javascript's async nature makes it easy to write simple and concise back-end code, and usually you would want to keep it that way by not trying to introduce concurrency into your application, unless it is absolutely necessary.

In Javascript (leaving node aside for a moment) An asynchronous function should be called via setTimeout(). SetTimeout will not block the script execution while waiting:

function someThingThatTakesAWhile() { 
    for (var i = 0; i < someVeryLargeNumber; i++) {
        //...
    };
}
setTimeout(function () {
    someThingThatTakesAWhile();
}, 0);

(haven't tested that, please forgive typos, &c)

But if you're wanting to do this in node, I don't think this is what you really want.