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));
Related
I have a doubt the JavaScript Is a single-threaded synchronous programming Language. and it has only One Callstack .then How the Callback and promise Achieve Asycrones property?
They don't. Callbacks and Promises do not make anything asynchronous. Instead callbacks and Promises are design patterns allowing asynchronous functions to interact with the rest of your code.
The following are two example of callbacks, one is synchronous, one is asynchronous:
function a (input, callback) {
callback(input * 2);
}
function b (input, callback) {
setTimeout(() => callback(input *2), 100);
}
console.log('start!');
a(100, x => console.log(x));
b(1000, x => console.log(x));
console.log('end!');
The output should be:
start!
200
end!
2000
Callbacks don't make anything asynchronous but is a technique of allowing asynchronous code to interact with other parts of your code.
Promises do re-schedule your .then() callback because that's the way it was designed. But all it does is execute the .then() at the end of your code. That's all it does, it does not execute the .then() callback in a separate thread.
How does it execute your code later? Well, your code is a function. It simply calls it later. How does it call it later? It simply adds your function to a list of things that needs to be done later. At the end of your code the interpreter will check this list (or lists) to see if it needs to do anything. This part of code that checks if anything needs to be executed is called the event loop. The things in the list/lists that needs to be checked are called "tasks" or "microtasks" in the documentation.
Asynchronous functions are asynchronous. They are implemented using C/C++. So if a piece of C/C++ code uses threads or signals or async I/O it can let the javascript interpreter wait for the asynchronous results using javascript callbacks (it simply accepts a javascript function that it will call later) or promises (it returns a javascript Promise object).
I know JavaScript is synchronous by nature. So that when I call a function with web API It performs synchronously:
setTimeout(function(){
console.log('1');
}, 2000);
console.log('2');
it will print '2' then '1'.
But when I run a loop like for loop and increase the iteration it executes synchronously:
var first = function(){
for(var i=0;i<=100000000;i++){
if(i==100000000){
console.log('first')
};
};
};
var second = function() {
console.log('second')
};
var a = function() {
first();
second();
}
a();
It will print the first second respectively.
So, is JavaScript performing synchronously with native code?
The first example is asynchronous because you've explicitly asked it to be asynchronous by using setTimeout. setTimeout (and its relation setInterval) explicitly set up an asynchronous timed callback to the function you pass them.
The remaining examples don't use anything that creates asynchronousness like setTimeout (and ajax and such) do, so naturally it's synchronous.
I know javaScript is asynchronous by nature
No, JavaScript (the language) is synchronous by nature. Literally the only asynchronous aspect of JavaScript was added in ES2015 and relates to when the callback passed into a promise's then or catch is called. That's it. setTimeout, for instance, is not part of JavaScript; it's part of the host environment where JavaScript is largely used (browsers). (It's also part of a couple of non-browser host environments, like NodeJS.)
JavaScript is primarily used in an environment that encourages asynchronous operations (browsers) where it's used to respond to user events (asynchronous), ajax completions (asynchronous), and timer callbacks (asynchronous), but the language is almost entirely synchronous.
JS is event-driven, this is why you think its async.. But it only has async features..
for loop does not have any event-callbacks, so its just sync
I got to know that
JavaScript is sync and async functionality depends on your functions implementation.
It has an event loop and it executes synchronously but whenever it will get the async function( the function which takes callback i.e setTimeout ,fs.readFile() etc) which is not the part of the javaScript then,
It put the function into the queue and invoke the queue function after all the native code(which is in current scope) execute then it pop the queue and invoke the functions.
and setTimeout explicitly set up an asynchronously timed callback to the function.
check this javaScript Async behavior ;
I am not an experienced developer and I have spent the last couple of days trying to understand few fundamental concepts about Javascript.
Single thread, blocking vs non-blocking and callback.
I have read a lot but I am still confused.
My understanding is that in a simple JS code all the statements are executed sequentially within a single thread.
In a more sophisticated situation where there are long running statements (db calls, network interaction, etc) this can be a huge problem.
The program execution (next statement) is BLOCKED until the current (long running) statement is completed.
Solution is to implement an asynchronous pattern, where the long running statement is executed in back ground (I can be brutally wrong here!) while the next statements are executed and when the long running statement has done, it passes back its result using a callback function provided in the calling statement.
Copy and pasting code from here and there I have written a trivial piece of code where I have a call back function
// function with callback
function doSomething(callback) {
callback();
}
// callback function implementation
function doSomethingElse() {
document.write("<p>Second element in the page</p>");
}
doSomething(doSomethingElse);
document.write("<p>First element in the page</p>");
The result of this code is actually what I would have expected before starting reading about non-blocking and callback (sequential code execution):
- Second element
- First element
So my question is, what's the magic fairy dust that transforms my code into asynchronous non-blocking one?
Cheers, Giovanni
what's the magic fairy dust that transforms my code into asynchronous non-blocking one?
In general, most long running operations will be handled APIs which handle asynchronous operations at a lower level than JavaScript (XMLHttpRequest for making HTTP requests being a prime example).
If you have need to implement a long running function yourself (maybe you want to do some heavy number crunching on the client) then you can use Web Workers.
var myWorker = new Worker("find_prime_numbers.js");
myWorker.onmessage = function(e) {
console.log('Next prime number is ' + e.data);
}
and in find_prime_numbers.js:
// Calculate some primes
postMessage(my_prime_number);
You have used callbacks, but it doesn't mean that this call is asynchronous.
It synchronously runs doSomething, which runs callback and outputs "Second element".
It would be asynchronous, if you had an asynchronous call there - AJAX call, file system access or a simple setTimeout:
function doSomething(callback) {
setTimeout(callback, 1000);
}
// callback function implementation
function doSomethingElse() {
document.write("<p>Second element in the page</p>");
}
doSomething(doSomethingElse);
document.write("<p>First element in the page</p>");
Now, it does the following: runs doSomething, which runs setTimeout and runs doSomethingElse right after that. At the same time, setTimeout asynchronously waits for 1 second and calls the function.
In other cases, it could be any other asynchronous operation, which requires time to complete.
Promises in JS allow you to do async programming, as follows:
DoSomething().then(success, failure);
DoSomethingElse();
whenever i write the previous code it reaches DoSomethingElse() before it reaches success.
How is that possible? Isn't JS a single threaded environment (not including web-workers)? is it done with setTimeout?
Yes, JavaScript is single-threaded, which means you should never block this single thread. Any long-running, waiting operation (typically AJAX calls or sleeps/pauses) are implemented using callbacks.
Without looking at the implementation here is what happens:
DoSomething is called and it receives success and failure functions as arguments.
It does what it needs to do (probably initiating long-running AJAX call) and returns
DoSomethingElse() is called
...
Some time later AJAX response arrives. It calls previously defined success and failure function
See also (similar problems)
JavaScript equivalent of SwingUtilities.invokeLater()
Are there any atomic javascript operations to deal with Ajax's asynchronous nature?
jqGrid custom edit rule function using Ajax displays "Custom Function should return array!"
Promises in JavaScript usually involve some kind of call chains or fluent method call APIs, where function results usually provide continuation methods like with, then, when, whenAll etc plus some status flags that indicate if the result is in fact available. Functions with input parameters can also support promised values detecting that the input is a promise and encapsulation their functionality into a thunk that can be chained when the promised value is ready.
With these you can provide an environment where promises simulate a parallel language like this:
MyApi.LongRunningTask().then( function(result) { MyAppi.LongOtherTask(result); }).then
or a sequential use case where long running calls are not dependant:
var value1 = MyApi.LongRunningTask();
var value2 = MyApi.LongRunningOtherTask();
MyApi.DoSomeFunction( value1, value2).then ==> DoSomeFunction can check if values are ready and if not chains their then/when function to execute its logic.
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.