There is nothing in the syntax that tells you your callback is executed asynchronously. Callbacks can be asynchronous, such as:
console.log("this is async");
or it can be synchronous, such as:
console.log("this is sync");
So, how can you know if a function will invoke the callback synchronously or asynchronously? The only reliable way is to read the documentation.
You can also write a test to find out if documentation is not available:
var t = "this is async";
t = "this is sync";
How asynchronous code work
Use another asynchronous function such as
setTimeout or web workers to execute your logic.
Write it in C.
As for how the C coded functions (such as
setTimeout) implement asynchronous execution? It all has to do with the event loop (or mostly).
The Event Loop
Inside the web browser there is this piece of code that is used for networking. Originally, the networking code could only download one thing: the HTML page itself. When Mosaic invented the
<img> tag the networking code evolved to download multiple resources. Then Netscape implemented progressive rendering of images, they had to make the networking code asynchronous so that they can draw the page before all images are loaded and update each image progressively and individually. This is the origin of the event loop.
In the heart of the browser there is an event loop that evolved from asynchronous networking code. So it's not surprising that it uses an I/O primitive as its core:
select() (or something similar such as poll, epoll etc. depending on OS).
select() function in C allows you to wait for multiple I/O operations in a single thread without needing to spawn additional threads.
select() looks something like:
select (max, readlist, writelist, errlist, timeout)
To have it wait for an I/O (from a socket or disk) you'd add the file descriptor to the
readlist and it will return when there is data available on any of your I/O channels. Once it returns you can continue processing the data.
select() function. When
select() returns the interpreter figures out which callback is associated with which I/O channel and then calls it.
select() also allows you to specify a
timeout value. By carefully managing the
timeout passed to
select() you can cause callbacks to be called at some time in the future. This is how
setInterval are implemented. The interpreter keeps a list of all timeouts and calculates what it needs to pass as
select(). Then when
select() returns in addition to finding out if there are any callbacks that needs to be called due to an I/O operation the interpreter also checks for any expired timeouts that needs to be called.
Node.js also spawns threads when dealing with file/disk I/O. When the I/O operation completes it communicates back with the main event loop to cause the appropriate callbacks to execute.
Hopefully this answers your question. I've always wanted to write this answer but was to busy to do so previously. If you want to know more about non-blocking I/O programming in C I suggest you take a read this: http://www.gnu.org/software/libc/manual/html_node/Waiting-for-I_002fO.html
For more information see also: