I run these code on my firefox, and the console gets 3 2 1, which means statements run before the former ones end.
function test(){
setTimeout(function(){console.log(1)},1000); //statement 0
setTimeout(function(){console.log(2)},500); //statement 1
}
test();
console.log(3); //statement 2
Is it a feature of javascript, and do I have to double-check a variable after assigning it some value? Cuz' it's way too complicated.
var a;
while(a!==get('some')){
a=get('some');
if(a===get('some')){
whatShouldBeRun();afterGetSome();
break;
}
}
That is exactly what you have asked it to do. Firstly, you ask it to kick off two statements to run some point in the future. Then you write "3". The first of those statements whose time it is to run then runs, and writes "2". Finally, the third of these runs and writes "1".
If you are using asynchronous processing - settimeout - then yes you need to handle this as with any other language. But then if you need it to run in order, don't tell it to run asynchronously.
To get them to run one after the other:
function test()
{
setTimeout(function()
{
console.log(1);setTimeout(function()
{
console.log(2);console.log(3)
},500)
},1000);
}
test();
[I might have some of this wrong]
The above code starts two timers, which will fire after 500 and 1000 milliseconds. console.log(3); will run first, with no delay; after 500 milliseconds, console.log(2) will be executed, and after another 500 milliseconds, comes console.log(1). This is the general general way how timers work in any language/framework which contains timers.
Sequential code runs in order, that is, subsequent lines of code are executed in series. Event-based systems, like web browsers and their JavaScript implementation, are different in that event handlers are executed only when the event happens. The code test(); console.log(3); is executed sequentially: two timers are started by calling setTimeout with the given intervals and event handlers, and 3 is written to the console. The "tick" event of the first timer comes after 500 ms, and its event handler function will write 2 to the console. The "tick" event of the second one comes after another 500 ms, and will write 1.
Related
I'm tying to call two functions in JavaScript when a button click event happens.
<button type="submit" id="mySubmit" onClick="submitInput();getAll()">Search</button>
So I wondered what function will call first. And I have no idea.
Will the submitInput() executes first or getAll() or both executes at the same time concurrently. ?
It executes the same way as ordinary javascript. submitInput() executes first. I would not reccomend doing it this way though. It would be considered bad practice. keep your javascript out of your HTML ok.
Lastly, just because something executes first, does not mean that it will finish first.. javascript is both async and synchronous in some cases.
JavaScript is by nature mono-thread, that is to say its engine can only compute one operation at once (it is not parallel !). It means that as long as a process is not finished, the user remains stuck in front of his browser and has to wait till the end. Theoritically :)
Fortunately, JS is also asynchronous, it means that one is able to free the user thread, waiting for some other conditions to be fullfilled to continue the computation. To be more accurate, the execution of some functions can be delayed, one of the simplest examples is the use of the functions setTimeout() (once) or setInterval() (several times). A callback is a function triggered only under some conditions (i.e. a time interval expires, a script sends an answer, etc...). It prevents the browser from being "freezed", waiting for the result of a computation.
In your case, if there isn't any asynchronous call, the functions will be executed in the order you gave. Once the first is completed, the second will be triggered.
Try those two dummy functions :
function myFunction() {
for (var iter = 0; iter < 500000000; iter++) {
if (iter==499999999) {alert ("done !");}
}
}
function myFunction2() {
alert ("Hi there !");
}
Call them in this order, then change their order. The second will always be executed once the first is complete.
I'm familiar with this behavior, but don't have the vocabulary to describe (and thus google) it.
setTimeout(function () { alert("timeout!"); }, 1000);
veryLongProcess(); // lasts longer than 1000 milliseconds
I believe the result of this is that you get your alert after the long process is finished, i.e. longer than 1 second after the code was executed. I imagine this as timeouts sending functions to some separate "thread" or "stack" or "cycle" that can only start after the current one is finished, even if that current one takes longer than the timeout was originally specified for.
Is there a name for this? How can I learn more about how it works?
I believe you may be looking for the term 'synchronous' programming.
Since JavaScript is single threaded, your veryLongProcess() will in fact cause the alert to trigger after 1000ms because of something called blocking.
Be aware that blocking JavaScript can degrade the user experience significantly, such as locking up the browser, or causing it to show a 'kill script' dialog, breaking the functionality of your process.
What you're looking for is called "callback functions." You can pass functions as a variables to other functions, and then execute them whenever you want. I wrote a quick sample for how it works below (untested).
function longProcess(callback){
//a bunch of code execution goes here
var testNumber = 5;
//This portion of code happens after all desired code is run
if (callback != undefined){ //Check to see if a variable 'callback' was passed... we're assuming it's a function
callback(testNumber); //Execute the callback, passing it a value
}
}
function testCallback(number){
alert("Number: " + number); //Alert box will popup with "Number: 5"
}
longProcess(testCallback); //Call your long process, passing another function as a variable
EDIT: I would like to have one function being called and executed after one second of the last keyup event.
Here's my code: http://jsfiddle.net/gKkAQ/
JS:
function aFunction() {
setTimeout(function () {
console.log("1");
}, 1000);
}
$(document).ready(function () {
$("#Input").keyup(function () {
aFunction();
});
});
HTML:
<input type="text" id="Input"></input>
You can easily run the JSFiddle and see in your console that no matter how fast you type, the function will be executed regardless of its previous execution status (in this case, setTimeout is not done yet in 1 second, but if you keep typing, ALL function calls will be executed)
"you will see the function being executed every second. I would like it to be executed after one second of the last keyup action"
The setTimeout() function returns an id that you can pass to clearTimeout() to prevent the timeout from occurring - assuming of course that you call clearTimeout() before the time is up.
var timeoutId;
function aFunction() {
clearTimeout(timeoutId);
timeoutId = setTimeout(function () {
console.log("1");
}, 1000);
}
$(document).ready(function () {
$("#Input").keyup(function () {
aFunction();
});
});
There is no harm in calling clearTimeout() with the id of a timeout that did already happen, in that case it has no effect.
So the above code clears the previous timeout (if there is one) and then creates a new one, with the effect that the anonymous function with the console.log() will only be execute 1000ms after the user stops typing. If the user starts typing again after waiting more than 1000ms that will queue up another timeout.
NOTE: THIS ANSWER CORRESPONDS TO THE INITIAL, VERY GENERAL QUESTION, BEFORE IT WAS SUBJECT TO EXTENSIVE EDITING AND TRANSFORMED IN A VERY SPECIFIC ONE.
It depends on your exact execution environment. Theoretically, JavaScript is single-threaded. The situation you describe should never happen. In practice, however, it does happen (especially inside browsers), and there is no way to fully control it.
The closest alternative to "control" is to start by checking the value of a global "activation counter" variable. If it is 0, immediately increment it and proceed with execution. Decrement it when exiting.
Why this doesn't work? Because 2 activations can reach the test at the same time. As the variable is 0, both tests will succeed and both will procceed to increment the variable and execute. As with most syncronization/concurrency issues, problems will not occurr systematically, but randomly every now and then. This makes things difficult to detect, reproduce and correct.
You can try to ask two times for the variable, as in:
if( activationCounter <= 0 ) {
if( activationCounter <= 0 ) {
activationCounter++;
// Execution
activationCounter--;
}
}
In many circles this has been described as a solution to the problem, but it only reduces the probability of conflict (by quite a bit, but not to zero). It's just the result of a bad understanding of the "double-checked locking" pattern. Problems will still occur, but with much less frequency. I'm not sure if this is good or bad, as they will be even more difficult to detect, reproduce and correct.
So I know that there are differences between setTimeout and setInterval, but consider these two code examples:
function myFunction(){
setTimeout('myFunction();', 100);
doSomething();
}
setTimeout('myFunction();', 100);
and
function myFunction(){
doSomething();
}
setInterval('myFunction();', 100);
Note that in the first example I call setTimeout at the begining of the function and then I doSomething. Therefore there is no extra delay from doSomething(). Does that mean that those two examples do exactly the same? Or is there even more subtle difference?
They're functionally about the same, but there are differences. One difference is in how browsers handle it if doSomething takes longer than the interval. With setInterval, at least some browsers will just skip the next interval if doSomething is still running. So if you use 100ms as you have, and doSomething takes 110 ms to run, the next run won't happen until 90ms later (of course, all of these times are approximate).
Another difference is that with the setTimeout, you'll get a new handle every time, whereas with setInterval you get one handle.
Another difference with your examples as given is that in the setTimeout example, you're firing up a JavaScript parser/compiler every time, whereas with setInterval you're only firing up the parser/compiler once. But this difference shouldn't matter, because you shouldn't be doing that at all — see below.
But subtleties aside, what you have there is functionally the same.
Side note: It's not best practice to pass strings into either setTimeout or setInterval. Instead, pass in a function reference:
// setTimeout
function myFunction(){
setTimeout(myFunction, 100);
doSomething();
}
setTimeout(myFunction, 100);
// setInterval
function myFunction(){
doSomething();
}
setInterval(myFunction, 100);
Passing in a string fires up a JavaScript parser and does the same thing as eval. It should be avoided whenever possible (and it's almost always possible).
T.J. Crowder explained the main differences, one other more subtle could appear (I change the time scale as it's easier to explain) :
Lets plot the difference with a very big timeout time : 1 Day. You call both methods at 00:00 on Day 1 and let it run for 1 year...
1 Year latter your method called by setInterval will execute at 00:00 + some milliseconds (because you may not be the only one asking for the processors to do things at this exact moment and the OS timers have granularity anyway).
But your setTimeout method will occur latter, maybe around 00:01 because each day it would have been called a little after the requested time and requested to be called the next day at the same time...
PS: It could also be called before the requested time in some cases but more often than not it run after :-D
I thought I would try and be clever and create a Wait function of my own (I realise there are other ways to do this). So I wrote:
var interval_id;
var countdowntimer = 0;
function Wait(wait_interval) {
countdowntimer = wait_interval;
interval_id = setInterval(function() {
--countdowntimer <=0 ? clearInterval(interval_id) : null;
}, 1000);
do {} while (countdowntimer >= 0);
}
// Wait a bit: 5 secs
Wait(5);
This all works, except for the infinite looping. Upon inspection, if I take the While loop out, the anonymous function is entered 5 times, as expected. So clearly the global variable countdowntimer is decremented.
However, if I check the value of countdowntimer, in the While loop, it never goes down. This is despite the fact that the anonymous function is being called whilst in the While loop!
Clearly, somehow, there are two values of countdowntimer floating around, but why?
EDIT
Ok, so I understand (now) that Javascript is single threaded. And that - sort of - answers my question. But, at which point in the processing of this single thread, does the so called asynchronous call using setInterval actually happen? Is it just between function calls? Surely not, what about functions that take a long time to execute?
There aren't two copies of the variable lying around. Javascript in web browsers is single threaded (unless you use the new web workers stuff). So the anonymous function never has the chance to run, because Wait is tying up the interpreter.
You can't use a busy-wait functions in browser-based Javascript; nothing else will ever happen (and they're a bad idea in most other environments, even where they're possible). You have to use callbacks instead. Here's a minimalist reworking of that:
var interval_id;
var countdowntimer = 0;
function Wait(wait_interval, callback) {
countdowntimer = wait_interval;
interval_id = setInterval(function() {
if (--countdowntimer <=0) {
clearInterval(interval_id);
interval_id = 0;
callback();
}
}, 1000);
}
// Wait a bit: 5 secs
Wait(5, function() {
alert("Done waiting");
});
// Any code here happens immediately, it doesn't wait for the callback
Edit Answering your follow-up:
But, at which point in the processing of this single thread, does the so called asynchronous call using setInterval actually happen? Is it just between function calls? Surely not, what about functions that take a long time to execute?
Pretty much, yeah — and so it's important that functions not be long-running. (Technically it's not even between function calls, in that if you have a function that calls three other functions, the interpreter can't do anything else while that (outer) function is running.) The interpreter essentially maintains a queue of functions it needs to execute. It starts starts by executing any global code (rather like a big function call). Then, when things happen (user input events, the time to call a callback scheduled via setTimeout is reached, etc.), the interpreter pushes the calls it needs to make onto the queue. It always processes the call at the front of the queue, and so things can stack up (like your setInterval calls, although setInterval is a bit special — it won't queue a subsequent callback if a previous one is still sitting in the queue waiting to be processed). So think in terms of when your code gets control and when it releases control (e.g., by returning). The interpreter can only do other things after you release control and before it gives it back to you again. And again, on some browsers (IE, for instance), that same thread is also used for painting the UI and such, so DOM insertions (for instance) won't show up until you release control back to the browser so it can get on with doing its painting.
When Javascript in web browsers, you really need to take an event-driven approach to designing and coding your solutions. The classic example is prompting the user for information. In a non-event-driven world, you could do this:
// Non-functional non-event-driven pseudo-example
askTheQuestion();
answer = readTheAnswer(); // Script pauses here
doSomethingWithAnswer(answer); // This doesn't happen until we have an answer
doSomethingElse();
That doesn't work in an event-driven world. Instead, you do this:
askTheQuestion();
setCallbackForQuestionAnsweredEvent(doSomethingWithAnswer);
// If we had code here, it would happen *immediately*,
// it wouldn't wait for the answer
So for instance, askTheQuestion might overlay a div on the page with fields prompting the user for various pieces of information with an "OK" button for them to click when they're done. setCallbackForQuestionAnswered would really be hooking the click event on the "OK" button. doSomethingWithAnswer would collect the information from the fields, remove or hide the div, and do something with the info.
Most Javascript implementation are single threaded, so when it is executing the while loop, it doesn't let anything else execute, so the interval never runs while the while is running, thus making an infinite loop.
There are many similar attempts to create a sleep/wait/pause function in javascript, but since most implementations are single threaded, it simply doesn't let you do anything else while sleeping(!).
The alternative way to make a delay is to write timeouts. They can postpone an execution of a chunk of code, but you have to break it in many functions. You can always inline functions so it makes it easier to follow (and to share variables within the same execution context).
There are also some libraries that adds some syntatic suggar to javascript making this more readable.
EDIT:
There's an excelent blog post by John Resig himself about How javascript timers work. He pretty much explains it in details. Hope it helps.
Actually, its pretty much guaranteed that the interval function will never run while the loop does as javascript is single-threaded.
There is a reason why no-one has made Wait before (and so many have tried); it simply cannot be done.
You will have to resort to braking up your function into bits and schedule these using setTimeout or setInterval.
//first part
...
setTimeout(function(){
//next part
}, 5000/*ms*/);
Depending on your needs this could (should) be implemented as a state machine.
Instead of using a global countdowntimer variable, why not just change the millisecond attribute on setInterval instead? Something like:
var waitId;
function Wait(waitSeconds)
{
waitId= setInterval(function(){clearInterval(waitId);}, waitSeconds * 1000);
}