[javascript] JavaScript and Threads

Is there some way to do multi-threading in JavaScript?

The new v8 engine which should come out today supports it (i think)

Different way to do multi-threading and Asynchronous in JavaScript

Before HTML5 JavaScript only allowed the execution of one thread per page.

There was some hacky way to simulate an asynchronous execution with Yield, setTimeout(), setInterval(), XMLHttpRequest or event handlers (see the end of this post for an example with yield and setTimeout()).

But with HTML5 we can now use Worker Threads to parallelize the execution of functions. Here is an example of use.

Real multi-threading

Multi-threading: JavaScript Worker Threads

HTML5 introduced Web Worker Threads (see: browsers compatibilities)
Note: IE9 and earlier versions do not support it.

These worker threads are JavaScript threads that run in background without affecting the performance of the page. For more information about Web Worker read the documentation or this tutorial.

Here is a simple example with 3 Web Worker threads that count to MAX_VALUE and show the current computed value in our page:

//As a worker normally take another JavaScript file to execute we convert the function in an URL: http://stackoverflow.com/a/16799132/2576706_x000D_
function getScriptPath(foo){ return window.URL.createObjectURL(new Blob([foo.toString().match(/^\s*function\s*\(\s*\)\s*\{(([\s\S](?!\}$))*[\s\S])/)[1]],{type:'text/javascript'})); }_x000D_
_x000D_
var MAX_VALUE = 10000;_x000D_
_x000D_
/*_x000D_
 * Here are the workers_x000D_
 */_x000D_
//Worker 1_x000D_
var worker1 = new Worker(getScriptPath(function(){_x000D_
    self.addEventListener('message', function(e) {_x000D_
        var value = 0;_x000D_
        while(value <= e.data){_x000D_
            self.postMessage(value);_x000D_
            value++;_x000D_
        }_x000D_
    }, false);_x000D_
}));_x000D_
//We add a listener to the worker to get the response and show it in the page_x000D_
worker1.addEventListener('message', function(e) {_x000D_
  document.getElementById("result1").innerHTML = e.data;_x000D_
}, false);_x000D_
_x000D_
_x000D_
//Worker 2_x000D_
var worker2 = new Worker(getScriptPath(function(){_x000D_
    self.addEventListener('message', function(e) {_x000D_
        var value = 0;_x000D_
        while(value <= e.data){_x000D_
            self.postMessage(value);_x000D_
            value++;_x000D_
        }_x000D_
    }, false);_x000D_
}));_x000D_
worker2.addEventListener('message', function(e) {_x000D_
  document.getElementById("result2").innerHTML = e.data;_x000D_
}, false);_x000D_
_x000D_
_x000D_
//Worker 3_x000D_
var worker3 = new Worker(getScriptPath(function(){_x000D_
    self.addEventListener('message', function(e) {_x000D_
        var value = 0;_x000D_
        while(value <= e.data){_x000D_
            self.postMessage(value);_x000D_
            value++;_x000D_
        }_x000D_
    }, false);_x000D_
}));_x000D_
worker3.addEventListener('message', function(e) {_x000D_
    document.getElementById("result3").innerHTML = e.data;_x000D_
}, false);_x000D_
_x000D_
_x000D_
// Start and send data to our worker._x000D_
worker1.postMessage(MAX_VALUE); _x000D_
worker2.postMessage(MAX_VALUE); _x000D_
worker3.postMessage(MAX_VALUE);

<div id="result1"></div>_x000D_
<div id="result2"></div>_x000D_
<div id="result3"></div>

We can see that the three threads are executed in concurrency and print their current value in the page. They don't freeze the page because they are executed in the background with separated threads.

Multi-threading: with multiple iframes

Another way to achieve this is to use multiple iframes, each one will execute a thread. We can give the iframe some parameters by the URL and the iframe can communicate with his parent in order to get the result and print it back (the iframe must be in the same domain).

This example doesn't work in all browsers! iframes usually run in the same thread/process as the main page (but Firefox and Chromium seem to handle it differently).

Since the code snippet does not support multiple HTML files, I will just provide the different codes here:

index.html:

//The 3 iframes containing the code (take the thread id in param)
<iframe id="threadFrame1" src="thread.html?id=1"></iframe>
<iframe id="threadFrame2" src="thread.html?id=2"></iframe>
<iframe id="threadFrame3" src="thread.html?id=3"></iframe>

//Divs that shows the result
<div id="result1"></div>
<div id="result2"></div>
<div id="result3"></div>


<script>
    //This function is called by each iframe
    function threadResult(threadId, result) {
        document.getElementById("result" + threadId).innerHTML = result;
    }
</script>

thread.html:

//Get the parameters in the URL: http://stackoverflow.com/a/1099670/2576706
function getQueryParams(paramName) {
    var qs = document.location.search.split('+').join(' ');
    var params = {}, tokens, re = /[?&]?([^=]+)=([^&]*)/g;
    while (tokens = re.exec(qs)) {
        params[decodeURIComponent(tokens[1])] = decodeURIComponent(tokens[2]);
    }
    return params[paramName];
}

//The thread code (get the id from the URL, we can pass other parameters as needed)
var MAX_VALUE = 100000;
(function thread() {
    var threadId = getQueryParams('id');
    for(var i=0; i<MAX_VALUE; i++){
        parent.threadResult(threadId, i);
    }
})();

Simulate multi-threading

Single-thread: emulate JavaScript concurrency with setTimeout()

The 'naive' way would be to execute the function setTimeout() one after the other like this:

setTimeout(function(){ /* Some tasks */ }, 0);
setTimeout(function(){ /* Some tasks */ }, 0);
[...]

But this method does not work because each task will be executed one after the other.

We can simulate asynchronous execution by calling the function recursively like this:

var MAX_VALUE = 10000;_x000D_
_x000D_
function thread1(value, maxValue){_x000D_
    var me = this;_x000D_
    document.getElementById("result1").innerHTML = value;_x000D_
    value++;_x000D_
  _x000D_
    //Continue execution_x000D_
    if(value<=maxValue)_x000D_
        setTimeout(function () { me.thread1(value, maxValue); }, 0);_x000D_
}_x000D_
_x000D_
function thread2(value, maxValue){_x000D_
    var me = this;_x000D_
    document.getElementById("result2").innerHTML = value;_x000D_
    value++;_x000D_
 _x000D_
    if(value<=maxValue)_x000D_
        setTimeout(function () { me.thread2(value, maxValue); }, 0);_x000D_
}_x000D_
_x000D_
function thread3(value, maxValue){_x000D_
    var me = this;_x000D_
    document.getElementById("result3").innerHTML = value;_x000D_
    value++;_x000D_
 _x000D_
    if(value<=maxValue)_x000D_
        setTimeout(function () { me.thread3(value, maxValue); }, 0);_x000D_
}_x000D_
_x000D_
thread1(0, MAX_VALUE);_x000D_
thread2(0, MAX_VALUE);_x000D_
thread3(0, MAX_VALUE);

<div id="result1"></div>_x000D_
<div id="result2"></div>_x000D_
<div id="result3"></div>

As you can see this second method is very slow and freezes the browser because it uses the main thread to execute the functions.

Single-thread: emulate JavaScript concurrency with yield

Yield is a new feature in ECMAScript 6, it only works on the oldest version of Firefox and Chrome (in Chrome you need to enable Experimental JavaScript appearing in chrome://flags/#enable-javascript-harmony).

The yield keyword causes generator function execution to pause and the value of the expression following the yield keyword is returned to the generator's caller. It can be thought of as a generator-based version of the return keyword.

A generator allows you to suspend execution of a function and resume it later. A generator can be used to schedule your functions with a technique called trampolining.

Here is the example:

var MAX_VALUE = 10000;_x000D_
_x000D_
Scheduler = {_x000D_
 _tasks: [],_x000D_
 add: function(func){_x000D_
  this._tasks.push(func);_x000D_
 }, _x000D_
 start: function(){_x000D_
  var tasks = this._tasks;_x000D_
  var length = tasks.length;_x000D_
  while(length>0){_x000D_
   for(var i=0; i<length; i++){_x000D_
    var res = tasks[i].next();_x000D_
    if(res.done){_x000D_
     tasks.splice(i, 1);_x000D_
     length--;_x000D_
     i--;_x000D_
    }_x000D_
   }_x000D_
  }_x000D_
 } _x000D_
}_x000D_
_x000D_
_x000D_
function* updateUI(threadID, maxValue) {_x000D_
  var value = 0;_x000D_
  while(value<=maxValue){_x000D_
 yield document.getElementById("result" + threadID).innerHTML = value;_x000D_
 value++;_x000D_
  }_x000D_
}_x000D_
_x000D_
Scheduler.add(updateUI(1, MAX_VALUE));_x000D_
Scheduler.add(updateUI(2, MAX_VALUE));_x000D_
Scheduler.add(updateUI(3, MAX_VALUE));_x000D_
_x000D_
Scheduler.start()

<div id="result1"></div>_x000D_
<div id="result2"></div>_x000D_
<div id="result3"></div>

In raw Javascript, the best that you can do is using the few asynchronous calls (xmlhttprequest), but that's not really threading and very limited. Google Gears adds a number of APIs to the browser, some of which can be used for threading support.

There's no true threading in JavaScript. JavaScript being the malleable language that it is, does allow you to emulate some of it. Here is an example I came across the other day.

If you can't or don't want to use any AJAX stuff, use an iframe or ten! ;) You can have processes running in iframes in parallel with the master page without worrying about cross browser comparable issues or syntax issues with dot net AJAX etc, and you can call the master page's JavaScript (including the JavaScript that it has imported) from an iframe.

E.g, in a parent iframe, to call egFunction() in the parent document once the iframe content has loaded (that's the asynchronous part)

parent.egFunction();

Dynamically generate the iframes too so the main html code is free from them if you want.

With the HTML5 "side-specs" no need to hack javascript anymore with setTimeout(), setInterval(), etc.

HTML5 & Friends introduces the javascript Web Workers specification. It is an API for running scripts asynchronously and independently.

Links to the specification and a tutorial.

Another possible method is using an javascript interpreter in the javascript environment.

By creating multiple interpreters and controlling their execution from the main thread, you can simulate multi-threading with each thread running in its own environment.

The approach is somewhat similar to web workers, but you give the interpreter access to the browser global environment.

I made a small project to demonstrate this.

A more detailed explanation in this blog post.

You could use Narrative JavaScript, a compiler that will transforms your code into a state machine, effectively allowing you to emulate threading. It does so by adding a "yielding" operator (notated as '->') to the language that allows you to write asynchronous code in a single, linear code block.

Here is just a way to simulate multi-threading in Javascript

Now I am going to create 3 threads which will calculate numbers addition, numbers can be divided with 13 and numbers can be divided with 3 till 10000000000. And these 3 functions are not able to run in same time as what Concurrency means. But I will show you a trick that will make these functions run recursively in the same time : jsFiddle

This code belongs to me.

Body Part

    <div class="div1">
    <input type="button" value="start/stop" onclick="_thread1.control ? _thread1.stop() : _thread1.start();" /><span>Counting summation of numbers till 10000000000</span> = <span id="1">0</span>
</div>
<div class="div2">
    <input type="button" value="start/stop" onclick="_thread2.control ? _thread2.stop() : _thread2.start();" /><span>Counting numbers can be divided with 13 till 10000000000</span> = <span id="2">0</span>
</div>
<div class="div3">
    <input type="button" value="start/stop" onclick="_thread3.control ? _thread3.stop() : _thread3.start();" /><span>Counting numbers can be divided with 3 till 10000000000</span> = <span id="3">0</span>
</div>

Javascript Part

var _thread1 = {//This is my thread as object
    control: false,//this is my control that will be used for start stop
    value: 0, //stores my result
    current: 0, //stores current number
    func: function () {   //this is my func that will run
        if (this.control) {      // checking for control to run
            if (this.current < 10000000000) {
                this.value += this.current;   
                document.getElementById("1").innerHTML = this.value;
                this.current++;
            }
        }
        setTimeout(function () {  // And here is the trick! setTimeout is a king that will help us simulate threading in javascript
            _thread1.func();    //You cannot use this.func() just try to call with your object name
        }, 0);
    },
    start: function () {
        this.control = true;   //start function
    },
    stop: function () {
        this.control = false;    //stop function
    },
    init: function () {
        setTimeout(function () {
            _thread1.func();    // the first call of our thread
        }, 0)
    }
};
var _thread2 = {
    control: false,
    value: 0,
    current: 0,
    func: function () {
        if (this.control) {
            if (this.current % 13 == 0) {
                this.value++;
            }
            this.current++;
            document.getElementById("2").innerHTML = this.value;
        }
        setTimeout(function () {
            _thread2.func();
        }, 0);
    },
    start: function () {
        this.control = true;
    },
    stop: function () {
        this.control = false;
    },
    init: function () {
        setTimeout(function () {
            _thread2.func();
        }, 0)
    }
};
var _thread3 = {
    control: false,
    value: 0,
    current: 0,
    func: function () {
        if (this.control) {
            if (this.current % 3 == 0) {
                this.value++;
            }
            this.current++;
            document.getElementById("3").innerHTML = this.value;
        }
        setTimeout(function () {
            _thread3.func();
        }, 0);
    },
    start: function () {
        this.control = true;
    },
    stop: function () {
        this.control = false;
    },
    init: function () {
        setTimeout(function () {
            _thread3.func();
        }, 0)
    }
};

_thread1.init();
_thread2.init();
_thread3.init();

I hope this way will be helpful.

With HTML5 specification you do not need to write too much JS for the same or find some hacks.

One of the feature introduced in HTML5 is Web Workers which is JavaScript running in the background,independently of other scripts, without affecting the performance of the page.

It is supported in almost all browsers :

Chrome - 4.0+

IE - 10.0+

Mozilla - 3.5+

Safari - 4.0+

Opera - 11.5+

Javascript doesn't have threads, but we do have workers.

Workers may be a good choice if you don't need shared objects.

Most browser implementations will actually spread workers across all cores allowing you to utilize all cores. You can see a demo of this here.

I have developed a library called task.js that makes this very easy to do.

task.js Simplified interface for getting CPU intensive code to run on all cores (node.js, and web)

A example would be

function blocking (exampleArgument) {
    // block thread
}

// turn blocking pure function into a worker task
const blockingAsync = task.wrap(blocking);

// run task on a autoscaling worker pool
blockingAsync('exampleArgumentValue').then(result => {
    // do something with result
});

There is no true multi-threading in Javascript, but you can get asynchronous behavior using setTimeout() and asynchronous AJAX requests.

What exactly are you trying to accomplish?