Is Javascript synchronous(blocking) or Asynchronous(nonblocking) by default

问题

I am trying to grasp on Javascript Asynchronous functions and callbacks.

I got stuck on the concept of callback functions, where I am reading on some places: they are use to have sequential execution of code (mostly in context of jquery e.g animate)and some places specially in the context of Nodejs; they are use to have a parallel execution Asynchronous and avoid blocking of code.

So can some expert in this topic please shed light on this and clear this fuzz in my mind (examples??). so I could make my mind for the usage of callback function

or that is solely depends on the place of where you are calling/placing a callback function in your code? .

Thanks,

P.S: I am scared that this question would be close as subjective but still I could expect concrete answer for this (perhaps some examples)

Edit: actually this is the example from internet which makes me ambigous:

function do_a(){
  // simulate a time consuming function
  setTimeout( function(){
    console.log( '`do_a`: this takes longer than `do_b`' );
  }, 1000 );
}

function do_b(){
  console.log( '`do_b`: this is supposed to come out after `do_a` but it comes out before `do_a`' );
}

do_a();
do_b();

Result

`do_b`: this is supposed to come out after `do_a` but it comes out before `do_a`
`do_a`: this takes longer than `do_b`

when JS is sequential then do_b should always come after do_a according to my understanding.

回答1:

The core of JavaScript is largely synchronous, in that functions complete their task fully, before completing. Prior to the advent of AJAX, it was really only setTimeout and setInterval that provided asynchronous behavior.

However, it's easy to forget that event handlers are, effectively async code. Attaching a handler does not invoke the handler code and that code isn't executed until some unknowable time in the future.

Then came AJAX, with its calls to the server. These calls could be configured to be synchronous, but developers generally preferred async calls and used callback methods to implement them.

Then, we saw the proliferation of JS libraries and toolkits. These strove to homogenize different browsers' implementations of things and built on the callback approach to async code. You also started to see a lot more synchronous callbacks for things like array iteration or CSS query result handling.

Now, we are seeing Deferreds and Promises in the mix. These are objects that represent the value of a long running operation and provide an API for handling that value when it arrives.

NodeJS leans towards an async approach to many things; that much is true. However this is more a design decision on their part, rather than any inherent async nature of JS.

回答2:

Javascript is always a synchronous(blocking) single thread language but we can make Javascript act Asynchronous through programming.

Synchronous code:

console.log('a');
console.log('b');

Asynchronous code:

console.log('a');
setTimeout(function() {
    console.log('b');
}, 1000);
setTimeout(function() {
    console.log('c');
}, 1000);
setTimeout(function() {
    console.log('d');
}, 1000);
console.log('e');

This outputs: a e b c d

回答3:

In node long running processes use process.nextTick() to queue up the functions/callbacks. This is usually done in the API of node and unless your programming(outside the api) with something that is blocking or code that is long running then it doesn't really effect you much. The link below should explain it better then I can.

howtonode process.nextTick()

jQuery AJAX also takes callbacks and such as it its coded not to wait for server responses before moving on to the next block of code. It just rememebers the function to run when the server responds. This is based on XMLHTTPRequest object that the browsers expose. The XHR object will remember the function to call back when the response returns.

setTimeout(fn, 0) of javascript will run a function once the call stack is empty (next available free tick) which can be used to create async like features.setTimeout(fn, 0) question on stackoverflow

To summerise the async abilities of javascript is as much to do with the environments they are programmed in as javascript itself. You do not gain any magic by just using lots of function calls and callbacks unless your using some API/script.

Jquery Deferred Object Is another good link for async capabilities of jQuery. Googling might find you information on how jQuery Deferred works also for more insight.

来源：https://stackoverflow.com/questions/16523870/is-javascript-synchronousblocking-or-asynchronousnonblocking-by-default