I have a simple resizable div (achieved with jQuery's resizable) and I'm handling the resize event with a function.
So, let's say that's my code:
var a = ...
$(div).resizable({
options go here...
resize: function(e, ui){
make some operations with a
...
...
keep making operations with a
...
...
ok, we're done
}
})
Now, let's say the resize callback is called a lot of times in a short period of time. How will javascript handle that? Will callbacks overlap the use of "a"? (Note that "a" is a global var!).
Am I safe with this code or there might be a conflict of some kind?
Regards
If you're asking about thread safety, JavaScript is single-threaded. A function like that can only be run one at a time, regardless of how often it's called.
So you should be safe.
JavaScript is single-threaded so the function calls can't "overlap". If you trigger the event multiple times then the function is executed entirely (minus any asynchronous functions it may call) in response to the first triggering, before it's executed again to handle the subsequent triggers.
However, changes to a in one call will still have been made when the function is executed again, so if it relies on the value of a you may run into problems - it comes down to exactly what that function is doing with the variable.
Related
I heard that there are three queues which have tasks in Event Loop Processing Model.
MacroTaskQueue : this queue have callback functions of setTimeout, setInterval ..etc
MicroTaskQueue : this queue have callback functions of promise, mutationOberver ..etc
AnimationFrameQueue : this queue have callback functions of requestAnimationFrame.
So, what i'm wondering is that
Who fires DOMContentLoaded event ?
Where the callback function of DOMContentLoaded is queued ? MacroTaskQueue or MicroTaskQueue?
finally,
var a = 10;
console.log(a);
setTimeout(function b() { console.log('im b'); }, 1000);
in this code,
var a = 10;
console.log(a);
is this code also queued in MacroTaskQueue or MicroTaskQueue ?
or only the b is queued in MacroTaskQueue after (min) 1000ms ?
Im in black hole. Help me please :D
What you call the "MacroTaskQueue" is actually made of several task-queues, where tasks are being queued. (Note that the specs only use multiple task-sources, there could actually be a single task-queue). At the beginning of the event-loop processing, the browser will choose from which task queue it will pick the next "main" task to execute. It's important to understand that these tasks may very well not imply any JavaScript execution at all, JS is only a small part of what a browser does.
The microtask-queue will be visited and emptied several times during a single event-loop iteration. For instance every time that the JS call stack has been emptied (i.e after almost every JS callback execution) and if it wasn't enough there are fixed "Perform a microtask checkpoint" points in the event-loop processing model.
While similar to a queue, the animation frame callbacks are actually stored in an ordered map, not in a queue per se, this allows to "queue" new callbacks from one of these callbacks without it being dequeued immediately after. More importantly, a lot of other callbacks are also executed at the same time, e.g the scroll events, resize events, Web animation steps + events, ResizeObserver callbacks, etc. But this "update the rendering" step happens only once in a while, generally at the monitor refresh rate.
But, that's not saying much about DOMContentLoaded.
Who fires DOMContentLoaded event ?
This event is fired as part of the Document parsing steps, in the "the end" section. The browser has to first queue a task on the DOM manipulation task-source. This task will then eventually get selected by the browser as part of the first step of the event-loop. And once this task's steps will be executed, the event will be fired and dispatched on the Document. That's only as part of this dispatch an event algorithm that the browser will invoke and inner-invoke until it calls our listener's callback.
Note that this Document parsing step is in itself quite interesting as a task since this is the most obvious place where you will have multiple microtask-checkpoints interleaved inside the "main" task (at each <script> for instance).
Where the callback function of DOMContentLoaded is queued ?
The callback function is not queued, it is conceptually stored in the EventTarget's event listener list. In the facts, it's stored in memory, since here the EventTarget is a DOM object (Document), it's probably attached to this DOM object, though this is an implementation detail on which the specs have little to say as this is transparent to us web-devs.
MacroTaskQueue or MicroTaskQueue?
As I hope you now understand better, neither. Task queues and the microtask-queue only store tasks and microtasks, not callbacks. The callbacks are stored elsewhere, depending on what kind of callbacks they are (e.g timers and events are stored in different "conceptual" places), and some task or microtask's steps will then call them.
is this code also queued in MacroTaskQueue or MicroTaskQueue?
That depends where this script has been parsed from. If it's inline in a classic <script> tag, then that would be the special parsing task we already talked about. If it's from a <script src="url.js">, then it will be part of a task queued from fetch a classic script, but it can also be part of a microtask, e.g if after an await in a module script, or you can even force it to be if you want:
queueMicrotask(() => {
console.log("in microtask");
eval(document.querySelector("[type=myscript]").textContent);
console.log("still in microtask");
});
console.log("in parsing task");
<script type="myscript">
var a = 10;
console.log(a);
setTimeout(function b() { console.log('im b'); }, 1000);
</script>
And it is even theoretically possible by specs that a microtask becomes a "macro-"task, though no browser does implements this anymore apparently.
All this to say, while I personally find all this stuff fascinating, as a web-dev you shouldn't block yourself on it.
i have a little problem in understand of callbacks. I have read a lot in the last 2 days and what i have understand is the following (correct me, if i'm wrong):
JavaScript is a single thread language and you can program synchronous and asynchronous.
Synchronous means, each statement waits for the previous statement to finish before executing. That can lead into trouble, because if for instance a connection to a database needs a lot of time, the statements after the previous has to wait.
Finally that's very bad and that's why it's better to program asynchronous in Javascript, because Asynchronous code doesn't have to wait, the code can continue to run and the user don't have to wait.
To program asynchronous the Callbacks (functions of higher order) are needed.
Now i have try to program a little example by a lot of tutorials, etc.
function testCallback(a,callback){
console.log('1.function and given parameter: '+a);
callback(10);
}
testCallback(5 , function(x){
console.log("2.function and given parameter of 1. function: "+x);
});
Is that right? the output is:
1.function and given parameter: 5
2.function and given parameter of 1. function: 10
I do not understand, what the advantage is of this code, because i think that can still lead into trouble? If "console.log('1.function and....') has problems, the callback(10) function would even stop or not?
Thanks for any help!
JavaScript is a single thread language...
No, it isn't. The language says nothing about threading. Most environments give you a single thread per global environment, though (and on browsers you can create more, which interoperate through messaging). NodeJS provides just the one thread. Some environments (such as Rhino or Nashorn on the JDK) provide true multi-threading (and all the advantages and hassles that can involve).
Using a callback doesn't make code asynchronous. Your example, for instance, is not asynchronous. Consider:
function testCallback(a,callback){
console.log('1.function and given parameter: '+a);
callback(10);
}
console.log("Before the call");
testCallback(5 , function(x){
console.log("2.function and given parameter of 1. function: "+x);
});
console.log("After the call");
Note how we don't see After the call until after 2.function and given parameter of 1. function: 10. If the callback were asynchronous, we'd see it before:
function testCallback(a,callback){
console.log('1.function and given parameter: '+a);
setTimeout(function() { // Using setTimeout
callback(10); // to make this call
}, 0); // asynchronous
}
console.log("Before the call");
testCallback(5 , function(x){
console.log("2.function and given parameter of 1. function: "+x);
});
console.log("After the call");
Whether a callback is called synchronously or asynchronously depends entirely on what the function you're passing it to does. For instance, the callback used by Array#sort is called synchronously, but the callback used by setTimeout is called asynchronously.
For code to be asynchronous, it has to start an operation and then have that operation complete later, triggering a callback. setTimeout does that, as does ajax when used correctly, as do a wide range of other things.
Note that callbacks are currently how you handle asynchronous behavior (simple callbacks like the above, or promise callbacks), but the next specification (ES2017) will define built-in language semantics for dealing with asynchronousity without callbacks in the form of async and await. You can use that syntax today if you transpile with a tool like Babel.
In javascript callbacks can be synchronous or asynchronous. Synchronous callbacks can have a lot of benefits, but they don't do anything to stop your code blocking.
I think the best way to understand what asynchronous code is, and why it's beneficial, is to learn how Javascript actually evaluates your code. I recommend this video, which explains the process very clearly https://www.youtube.com/watch?v=8aGhZQkoFbQ
A JavaScript Callback Function is a function that is passed as a parameter to another JavaScript function. Callbacks can be synchronous or asynchronous. Simply passing a function as parameter will not change its current behavior.
It's behavior can be changed by the method which will execute it by calling it inside an event listener or setTimeout function etc. Basically event listener or setTimeout etc are handled by webapi in async fashion. If callback functions are inside these functions then these are moved to a queue by webapi when they are activated like button click(event listener) or time declared in setTimeout passed. They will move from queue to stack(if stack is empty) and run on stack finally.
The main advantage of using callback function can be seen from below code :-
var add = function(x,y) {
return x+y;
}
var multiply = function(x,y){
return x*y;
}
var calculate = function(x,y,callback){
return callback(x,y);
}
console.log(calculate(4,9,add));
Given the single-threadedness of javascript, when a synchronous event is fired (for DOM manipulation for example) what happens to the currently executing function block when it is interrupted? How does the browser know where/when to continue execution of the interrupted block? Is there some sort of internal addition of a "pointer" to the event loop? I ask because I'm curious whether other waiting events in the event loop can intervene between the intervention of the synchronous event handler and the continuation of the original function block being executed.
And I am very early in my understanding of asynch events/synchronous events/event loop so if I have this totally wrong please let me know. But my understanding is that synchronous events (nested?) are "fired immediately" and I have seen it happen on jsfiddle with standard cut and paste from tutorials on the subject. I'm just confused as to how javascript knows how/where to pick up where it left off since it is so asynch driven.
A snippet:
<script>
var button = document.body.children[0]
var text = document.body.children[1]
button.onclick = function() {
alert('in onclick')
text.focus()
alert('out onclick')
}
text.onfocus = function() {
alert('onfocus')
text.onfocus = null //(*)
}
</script>
will produce 'in onclick', 'onfocus', 'out onclick'. The focus is a synchronous event. How does js know to pick back up with next statement after "text.focus()"? Is it something as simple as the something being done with the frame stack?
All the built-in events are asynchronous, but if you were to fire a synchronous event in your code, it would work like any regular function call that returns back once it's done. No event suddenly interrupts executing code - well pressing "stop execution" in developer tools actually does because the compiled code is littered with checks for that in every function and loop.
I have a function which is an event handler for websocket.onmessage, now since the server can send multiple messages (one after another) and each message will trigger that event, and since the function block may take a few seconds (a lot of rendering going on inside), the function may be called again while the first function call is still running.
I need a critical block in this function in some cases so that the second call will only start the critical section when the first call ends, what's considered a 'best practice' for implementing locks in JavaScript?
Since js is single-threaded, you can't really do locks. Well, you can but you shouldn't.
One idea might be to keep a status variable.
Your function will be called on each onmessage, but you only do something if the variable is set to false. If so, you set it to true and when its done, set it back to false.
var handler; //expose outside the closure
(function(){
var busy = false;
handler = function(){
if( !busy ){
busy = true;
//do rendering stuff
busy = false;
}
}
})();
Obviously, adapt this idea to your own needs.
You could use jQuery Socket https://github.com/flowersinthesand/jquery-socket as it has a callback for the message event.
message(data, [callback])
This means you can get the next message after the first has completed.
EXAMPLE:
websocket.onmessage(data, function(){
//get next message
});
JS is multithreaded only if you use webworkers. I don't know if websockets are even allowed on worker threads because of the lack of synchronization available, but if you set up all your websockets from the main thread, the events will all fire in order on the main thread, so you do not have to perform any blocking or synchronization yourself (see this thread on synchronization in JS)
I am quiet new to javascript, can not understand that why all javascript calls are asynchronous,
for example, we have calling with order like
call_function1;
call_function2:
if function2 is depend on the results of function1, that can not be ensured, because the execution is asynchronous. Is that true ? And why ?
If true, how to ensure they are synchronous.
If this is duplicated question, I am sorry, because it is quiet new for me.
Thanks fo your answer first.
JavaScript calls are synchonous. If second function depends on results of first function, you might use callbacks model.
function foo(callback) {
var results = // get some results;
callback(results);
}
function boo(results) {
// do something with results here..
}
foo(boo);
No Javascript has functions that guarantee order and behave much like other languages. For example:
function f1() {
alert(1);
}
function f2() {
alert(2);
}
f1();
f2();
You will always get 1 and then 2. What's more is AFAIK javascript runs on one thread so you don't have to worry about race conditions either.
The asynchronous part of javascript comes from waiting on events. For example if you make 2 ajax requests (the a in ajax standing for asynchronous is a hint), you cannot guarantee which will come back first and thus if you have different callbacks for the two requests, you can't guarantee which will be called first.
Calling two functions in a row is definitely not asynchronous.
In fact.. javascript is 100% synchronous. Unless you use 'web workers' all javascript will always run in a single thread, in a single process. There is never, ever a situation where 2 scripts are running at the same time.
Even if you handle an event, coming from an XMLHTTPRequest for instance, the event will only be triggered after all other javascript code has stopped executing.
As mentioned in the comments to your question, JavaScript function calls are usually not asynchronous. The following will be executed in order:
function sayHello() {
console.log("Hi");
}
function sayBye() {
console.log("Bye");
}
sayHello();
sayBye();
The above code will first print "Hi", then "Bye", as the calls to sayHello and sayBye are in that order.
However, if a function does perform some action asynchronously, and you have another function that relies on the result of that, you can supply the second function as a callback to the asynchronous request. For example:
xmlHttpRequestObj.onreadystatechange = function() {
if(xmlHttpRequestObj.readyState == 4 && xmlHttpRequestObj.status == 200) {
//Asynchronous call returned successfully. Do something that relies on that here.
}
}
I don't know how you say that "Javascript is generally said to be asynchronous". It is generally the opposite: Javascript is almost always not asynchronous.
So generally (no ajax, no callback, no event handling), the following functions calls will execute sequentially.
function1();
function2();
If you are assigning both these functions to a single event, both will be called concurrently, but will not be executed in parallel. Same if both are called at the same time (using setTimeout), or as an Ajax callback. See Is JavaScript guaranteed to be single-threaded? for a very good explanation.
You could have heard that Ajax is asynchronous. That is true when you consider the request to servers without interfering with the user, "in the background". But this does not mean that Javascript is asynch. In fact even with Ajax calls, the javascript part is single threaded.