Why doesn t JavaScript support multithreading

Question

Is it a deliberate design decision or a problem with our current day browsers which will be rectified in the coming versions

User · Answer

Multi-threading with javascript is clearly possible using webworkers bring by HTML5.

Main difference between webworkers and a standard multi-threading environment is memory resources are not shared with the main thread, a reference to an object is not visible from one thread to another. Threads communicate by exchanging messages, it is therefore possible to implement a synchronzization and concurrent method call algorithm following an event-driven design pattern.

Many frameworks exists allowing to structure programmation between threads, among them OODK-JS, an OOP js framework supporting concurrent programming https://github.com/GOMServices/oodk-js-oop-for-js

User · Answer

Traditionally  JS was intended for short  quick-running pieces of code  If you had major calculations going on  you did it on a server - the idea of a JS HTML app that ran in your browser for long periods of time doing non-trivial things was absurd   Of course  now we have that  But  it ll take a bit for browsers to catch up - most of them have been designed around a single-threaded model  and changing that is not easy  Google Gears side-steps a lot of potential problems by requiring that background execution is isolated - no changing the DOM  since that s not thread-safe   no accessing objects created by the main thread  ditto   While restrictive  this will likely be the most practical design for the near future  both because it simplifies the design of the browser  and because it reduces the risk involved in allowing inexperienced JS coders mess around with threads      marcio      Why is that a reason not to implement multi-threading in Javascript  Programmers can do whatever they want with the tools they have    So then  let s not give them tools that are so easy to misuse that every other website i open ends up crashing my browser  A naive implementation of this would bring you straight into the territory that caused MS so many headaches during IE7 development  add-on authors played fast and loose with the threading model  resulting in hidden bugs that became evident when object lifecycles changed on the primary thread  BAD  If you re writing multi-threaded ActiveX add-ons for IE  i guess it comes with the territory  doesn t mean it needs to go any further than that

User · Answer

However you can use eval function to bring concurrency TO SOME EXTENT     content of the threads to be run    var threads                            document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                    document write  Foo  lt br  gt                                        document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                    document write  Bar  lt br  gt                        window onload   function         var lines   0  quantum   3  max   0          get the longer thread length        for var i 0  i lt threads length  i              if max  lt  threads i  length                max   threads i  length                          execute them        while lines  lt  max            for var i 0  i lt threads length  i                  for var j   lines  j  lt  threads i  length  amp  amp  j  lt   lines   quantum   j                      eval threads i  j                                    lines    quantum

User · Answer

Do you mean why doesn t the language support multithreading or why don t JavaScript engines in browsers support multithreading   The answer to the first question is that JavaScript in the browser is meant to be run in a sandbox and in a machine OS-independent way  to add multithreading support would complicate the language and tie the language too closely to the OS

User · Answer

Javascript is a single-threaded language  This means it has one call stack and one memory heap  As expected  it executes code in order and must finish executing a piece code before moving onto the next  It s synchronous  but at times that can be harmful  For example  if a function takes a while to execute or has to wait on something  it freezes everything up in the meanwhile

User · Answer

Node js 10 5  supports worker threads as experimental feature  you can use it with --experimental-worker flag enabled   https   nodejs org api worker threads html  So  the rule is    if you need to do I O bound ops  then use the internal mechanism  aka callback promise async-await  if you need to do CPU bound ops  then use worker threads    Worker threads are intended to be long-living threads  meaning you spawn a background thread and then you communicate with it via message passing   Otherwise  if you need to execute a heavy CPU load with an anonymous function  then you can go with https   github com wilk microjob  a tiny library built around worker threads

User · Answer

Intel has been doing some open-source research on multithreading in Javascript  it was showcased recently on GDC 2012   Here is the link for the video  The research group used OpenCL which primarily focuses on Intel Chip sets and Windows OS  The project is code-named RiverTrail and the code is available on GitHub  Some more useful links   Building a Computing Highway for Web Applications

User · Answer

As far as I have heared Google Chrome will have multithreaded javascript  so it is a  current implementations  problem

User · Answer

JavaScript multi-threading  with some limitations  is here  Google implemented workers for Gears  and workers are being included with HTML5  Most browsers have already added support for this feature   Thread-safety of data is guaranteed because all data communicated to from the worker is serialized copied   For more info  read   http   www whatwg org specs web-workers current-work   http   ejohn org blog web-workers

User · Answer

JavaScript does not support multi-threading because the JavaScript interpreter in the browser is a single thread  AFAIK   Even Google Chrome will not let a single web page   s JavaScript run concurrently because this would cause massive concurrency issues in existing web pages  All Chrome does is separate multiple components  different tabs  plug-ins  etcetera  into separate processes  but I can   t imagine a single page having more than one JavaScript thread   You can however use  as was suggested  setTimeout to allow some sort of scheduling and    fake    concurrency  This causes the browser to regain control of the rendering thread  and start the JavaScript code supplied to setTimeout after the given number of milliseconds  This is very useful if you want to allow the viewport  what you see  to refresh while performing operations on it  Just looping through e g  coordinates and updating an element accordingly will just let you see the start and end positions  and nothing in between   We use an abstraction library in JavaScript that allows us to create processes and threads which are all managed by the same JavaScript interpreter  This allows us to run actions in the following manner    Process A  Thread 1 Process A  Thread 2 Process B  Thread 1 Process A  Thread 3 Process A  Thread 4 Process B  Thread 2 Pause Process A Process B  Thread 3 Process B  Thread 4 Process B  Thread 5 Start Process A Process A  Thread 5   This allows some form of scheduling and fakes parallelism  starting and stopping of threads  etcetera  but it will not be true multi-threading  I don   t think it will ever be implemented in the language itself  since true multi-threading is only useful if the browser can run a single page multi-threaded  or even more than one core   and the difficulties there are way larger than the extra possibilities   For the future of JavaScript  check this out   https   developer mozilla org presentations xtech2006 javascript

User · Answer

It s the implementations that doesn t support multi-threading  Currently Google Gears is providing a way to use some form of concurrency by executing external processes but that s about it   The new browser Google is supposed to release today  Google Chrome  executes some code in parallel by separating it in process   The core language  of course can have the same support as  say Java  but support for something like Erlang s concurrency is nowhere near the horizon

User · Answer

I don t know the rationale for this decision  but I know that you can simulate some of the benefits of multi-threaded programming using setTimeout   You can give the illusion of multiple processes doing things at the same time  though in reality  everything happens in one thread   Just have your function do a little bit of work  and then call something like   setTimeout function              do the rest of the work       0     And any other things that need doing  like UI updates  animated images  etc  will happen when they get a chance

User · Answer

Currently some browsers do support multithreading  So  if you need that you could use specific libraries  For example  view the next materials    https   developer mozilla org en-US docs Web API Web Workers API Using web workers  support background threads   https   keithwhor github io multithread js   the library

User · Answer

Without proper language support for thread syncronization  it doesn t even make sense for new implementations to try  Existing complex JS apps  e g  anything using ExtJS  would most likely crash unexpectedly  but without a synchronized keyword or something similar  it would also be very hard or even impossible to write new programs that behave correctly

User · Answer

Just as matt b said  the question is not very clear  Assuming that you are asking about multithreading support in the language  because it isn t needed for 99 999  of the applications running in the browser currently  If you really need it  there are workarounds  like using window setTimeout    In general multithreading is very  very  very  very  very  very hard  did I say that it is hard   to get right  unless you put in extra restrictions  like using only immutable data

[javascript] Why doesn't JavaScript support multithreading?

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