I have a few lines of code that I want to run asynchronously in Javascript so that it doesn't slow down my main algorithm. See this pseudo code:
//main algorithm with critical code that should run as soon as possible
...
...
runInParallel(function(){
//time consuming unimportant code to shows some progress feedback to user
...
}
//the rest of the time critical algorithm
...
...
runInParallel(function(){
//time consuming unimportant code to shows some progress feedback to user
...
}
//and so on and so forth
I searched Stackoverflow for how to write asynchronous code in Javascript but the following questions are not similar to mine:
how to run a javascript function asynchronously, without using setTimeout?: it's about server side
Loading javascript asynchronously - How to do callbacks?: it's about loading source code
I guess I can use timers for this purpose. All I want is the body of the function runInParallel() that runs a code efficiently in parallel with my main algorithm with lower priority if possible. Anyone?
Javascript has no synchronization / thread management. If you wish to execute something asynchronously, you can use setTimeout combined with a callback to be notified when the function 's finished.
var asyncHandle = setTimeout(function () { asyncCode(); callback(); }, 10);
The asyncHandle can be used to cancel the timeout prior to the function being called.
If you're targeting HTML5 supporting browsers, go with HTML5 Web Workers.
You can also try this interesting, but quite old JavaScript compiler that allows a language extension for this purpose.
Related
is there any way to create new process and make it execute some code while main process execute some code too.
in my code,i used something like this:
<script>
function f1(){//some code
}
setTimeout(f1,delay);
</script>
my question is: the main process execute f1 function or a new one?,and if it is the main how can i execute f1 function with a new process.
i had tried setTimeout function and i see that the main process execute the function after a delay time.
any help please.
Threads and processes are two different things.
Until recently JavaScript had neither. You could emulate multi-threading using a timeout of 0. You can still do that.
For true multi-threading now you can use Web Workers. I don't think you can have multiple process except if you are using some plugin (Java Applets for example).
Just search in Wikipedia for thread, process and WebWorkers.
I'm using a framework which features auto-connecting to server on page load. I can disable it by passing options arguments, but the line that confuses me is this:
You can prevent this initial socket from connecting automatically by disabling io.sails.autoConnect before the first cycle of the event loop elapses.
My questions are:
When does the first cycle of the event loop elapses?
Is this behaviour the same across ALL modern (IE9+) browsers?
I have a bunch of scripts (in <body>) loading between the lib and my entry file. Does this affect when the first cycle elapses? EDIT: Yes, it does.
How can I ensure my code runs before the first cycle elapses?
Is this kind of implementation of auto-connect considered good practice?
The documentation for the source file is a little more explicit; it says "This can be disabled or configured by setting io.socket.options within the first cycle of the event loop."
Basically what's happening is that there exists within the library a setTimeout(fn, 0) call, which is idiomatic for starting a parallel process. However, in the JS standards it's explicitly stated that JS is single-threaded: in other words, even though setTimeout and setInterval are asynchronous they are not actually parallel in the sense that any of their code will be executing simultaneously with any other code. So they wait until the current function is over before they execute. This queueing mechanism is known as the JavaScript event loop.
I believe that what you are asked to do by the script author is to modify the source to include the relevant change, perhaps at the bottom of the file for your convenience.
It is also likely that a similar effect will be achieved by putting a <script> tag underneath the one that loads the given JS. This has not been explicitly standardized by HTML 4, but may be implicitly standardized in the new HTML 5 spec (it's a complicated interaction between different parts of the specs).
In terms of HTML5, it looks like the current specs say that there is a afterscriptexecute event and a load event which occur immediately after any remote script is loaded (or, if it's an inline script, the load event is scheduled as a task -- I am not sure when those occur). So you might be able to guarantee it without modifying the script by instead doing:
<script>
function do_not_autoload() { /* ... */ }
</script>
<script onload="do_not_autoload()" src="./path/to/sails.io.js"></script>
but I'm not sure what the compatibility table for script#onload is going to look like.
I made you a jsfiddle which can be used to grab a 'fingerprint' for different browsers to get an idea of what evaluation orders are out there in the wild. The * is the document.body.onload event. On my system it produces:
Firefox 32.0.3 : cafdbe*
Chrome 37.0.2062 : cafd*be
IE 11.0.9600 : cafd*be
In other words,
At a certain point on my javascript I have the following (using Modernizr 2.6.2):
Modernizr.load([{
load: '/path/file.js',
complete: function () {
//do stuff
}
}]);
It works great most of the time, except on IE8, about 1 time out of 3 it executes the callback first then the script being loaded. I am adding a break point on the callback and another inside file.js. Both scripts are being executed, just swapping the order at times.
I tried to isolate and reproduce the error, but I couldn't. It works fine every time when I have only this code on the page (using the exact same Modernizr file). For that reason I believe something else on my project could be interfering, but I have no clue on what to look next along the 10000 (really!) lines of js. Has anyone experienced anything similar or has any idea on what could cause this sort of behaviour?
I am not looking for a workaround for this specific case. This is one of many instances where I really would like to keep as is.
EDIT 1 - NEW INFO:
Using the network panel, I can see that two requests are being done (which is expected), but the first request always returns blank. The second request brings up the correct content. That works fine when the callback is executed after both requests are complete, but sometimes the callback is executed between the first (empty) and second request. That is when it crashes!
EDIT 2 - NEW INFO: Ok seems like I can reproduce it now. To be 100% sure on my assumptions I would have to know the javascript engine in deep, which I don't, so I could be dead wrong, but here is what I found:
Request file1.js and file2.js. If file2 finishes loading while the engine is busy executing file1's callback, file2 execution goes somewhere at the end of the queue, which means after file2's callback.
Try waiting for all of the files to load before firing the callbacks. Expand on the pattern below to suit your needs.
Modernizr.load([{
load: '/path/file.js',
complete: function () {
filesLoaded++;
checkStatus();
}
}]);
function checkStatus(){
if(filesLoaded == 2){
//do stuff
}else{
setTimeout(checkStatus, 100);
}
}
After doing some research on the subject, I think this issue may be more common than it seems at first, but has been overlooked because it is context dependent, and has different ways of being described:
Modernizr load [A, B] seems to execute B before A, how is this possible?
YepNopeJS: callback triggered before tiny JS file is fully loaded
https://github.com/SlexAxton/yepnope.js/issues/195
After reading this nice article (referenced on yepnope page), about the inconsistencies experienced on IE, I came up with a fix, which has been sent to yepnope maintainers via github:
http://github.com/SlexAxton/yepnope.js/pull/196
I don't know whether it is going to be accepted or not, but it solved my problem.
As editing a library may not always be a good idea, I would suggest the following workaround:
//instead of
complete:function(){
doStuff()
}
//try
complete:function(){
setTimeout(doStuff, 0);
}
Here's my issue - I need to dynamically download several scripts using jQuery.getScript() and execute certain JavaScript code after all the scripts were loaded, so my plan was to do something like this:
function GetScripts(scripts, callback)
{
var len = scripts.length
for (var i in scripts)
{
jQuery.getScript(scripts[i], function()
{
len --;
// executing callback function if this is the last script that loaded
if (len == 0)
callback()
})
}
}
This will only work reliably if we assume that script.onload events for each script fire and execute sequentially and synchronously, so there would never be a situation when two or more of the event handlers would pass check for (len == 0) and execute callback method.
So my question - is that assumption correct and if not, what's the way to achieve what I am trying to do?
No, JavaScript is not multi-threaded. It is event driven and your assumption of the events firing sequentially (assuming they load sequentially) is what you will see. Your current implementation appears correct. I believe jQuery's .getScript() injects a new <script> tag, which should also force them to load in the correct order.
Currently JavaScript is not multithreaded, but the things will change in near future. There is a new thing in HTML5 called Worker. It allows you to do some job in background.
But it's currently is not supported by all browsers.
The JavaScript (ECMAScript) specification does not define any threading or synchronization mechanisms.
Moreover, the JavaScript engines in our browsers are deliberately single-threaded, in part because allowing more than one UI thread to operate concurrently would open an enormous can of worms. So your assumption and implementation are correct.
As a sidenote, another commenter alluded to the fact that any JavaScriptengine vendor could add threading and synchronization features, or a vendor could enable users to implement those features themselves, as described in this article: Multi-threaded JavaScript?
JavaScript is absolutely not multithreaded - you have a guarantee that any handler you use will not be interrupted by another event. Any other events, like mouse clicks, XMLHttpRequest returns, and timers will queue up while your code is executing, and run one after another.
No, all the browsers give you only one thread for JavaScript.
To be clear, the browser JS implementation is not multithreaded.
The language, JS, can be multi-threaded.
The question does not apply here however.
What applies is that getScript() is asynchronous (returns immediately and get's queued), however, the browser will execute DOM attached <script> content sequentially so your dependent JS code will see them loaded sequentially. This is a browser feature and not dependent on the JS threading or the getScript() call.
If getScript() retrieved scripts with xmlHTTPRequest, setTimeout(), websockets or any other async call then your scripts would not be guaranteed to execute in order. However, your callback would still get called after all scripts execute since the execution context of your 'len' variable is in a closure which persists it's context through asynchronous invocations of your function.
JS in general is single threaded. However HTML5 Web workers introduce multi-threading. Read more at http://www.html5rocks.com/en/tutorials/workers/basics/
Thought it might be interesting to try this out with a "forced", delayed script delivery ...
added two available scripts from
google
added delayjs.php as the 2nd
array element. delayjs.php sleeps
for 5 seconds before delivering an empty js
object.
added a callback that
"verifies" the existence of the
expected objects from the script
files.
added a few js commands that
are executed on the line after the
GetScripts() call, to "test" sequential js commands.
The result with the script load is as expected; the callback is triggered only after the last script has loaded. What surprised me was that the js commands that followed the GetScripts() call triggered without the need to wait for the last script to load. I was under the impression that no js commands would be executed while the browser was waiting on a js script to load ...
var scripts = [];
scripts.push('http://ajax.googleapis.com/ajax/libs/prototype/1.6.1.0/prototype.js');
scripts.push('http://localhost/delayjs.php');
scripts.push('http://ajax.googleapis.com/ajax/libs/scriptaculous/1.8.3/scriptaculous.js');
function logem() {
console.log(typeof Prototype);
console.log(typeof Scriptaculous);
console.log(typeof delayedjs);
}
GetScripts( scripts, logem );
console.log('Try to do something before GetScripts finishes.\n');
$('#testdiv').text('test content');
<?php
sleep(5);
echo 'var delayedjs = {};';
You can probably get some kind of multithreadedness if you create a number of frames in an HTML document, and run a script in each of them, each calling a function in the main frame that should make sense of the results of those functions.
I'd like to measure how long it takes to run the whole $().ready() scope in each of page.
For profiling specific functions I just set a new Date() variable at the beginning of the relevant part and then check how long it takes to get to the end of the relevant part.
The problem with measuring the whole $().ready scope is that it can sometimes run some of the code asynchronously and then I can not wait for it all to finish and see how long it has taken.
Is there any event which is fired once the page has completely finished running all $().ready code?
EDIT: Using Firebug or other client debuggers are not an option since I also need to collect this profiling information from website users for monitoring and graphing our web site's page load speeds
Thanks!
There will be no event fired because its virtually impossible for ready() to know when any asynchronous functions are done processing. Thus, you'll need to bake this functionality in yourself; you could use jQuery's custom events, or perhaps set a function to run on setInterval() that can introspect the environment and deduce whether or not everything else is done.
Swap out the jQuery ready function with a function that does your start and finish tracking, and calls the original method.
jQuery.ready = (function() {
var original = jQuery.ready;
return function() {
alert('starting profiler');
original();
alert('ending profiler');
};
})();
$(function() {
alert('this message will appear between the profiler messages above...');
});
Have you tried using Profiler in Firebug?