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Pooling of reusable JavaScript engines for fast, performant scripting integration in in .NET applications


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JSPool facilitates easy integration of JavaScript scripting into a .NET application in a scalable and performant manner. It does so by creating a pool of engines that can be reused multiple times. This makes it easy to use JavaScript libraries that have long initialisation times without having to create a new engine and load the script again every time.

It is powered by the JavaScriptEngineSwitcher library.


  • Supports any JavaScript engine supported by JavaScriptEngineSwitcher (including V8, MSIE, Jint, and Jurassic).
  • Pools are thread-safe.
  • Automatically calls an initialisation callback when engines are created. This can be used to load JavaScript libraries.
  • Pools have a configurable number of maximum engines.
  • There is no global state; pools are instantiated so you can have multiple pools in your application, each with their own configuration.


Installation can be done either through NuGet (Install-Package JSPool) or by cloning the Git repository and running dev-build.bat. In addition to JSPool, you will need to have at least one engine supported by JavaScriptEngineSwitcher installed (eg. V8 or MSIE). V8 is recommended.

var pool = new JsPool(new JsPoolConfig
  Initializer = initEngine =>
    // This initializer will be called whenever a new engine is created. In a 
    // real app you'd commonly use ExecuteFile and ExecuteResource to load
    // libraries into the engine.
      function sayHello(name) {
        return 'Hello ' + name + '!';

// Get an engine from the pool. The engine will be returned to the pool when
// disposed. In this case, the using() block will automatically return the
// engine to the pool when it falls out of scope.
using (var engine = pool.GetEngine()) {
  var message = engine.CallFunction<string>("sayHello", "Daniel");
  Console.WriteLine(message); // "Hello Daniel!"

// Disposing the pool will also dispose all its engines. Always dispose the pool
// when it is no longer required.


The following configuration settings are available for JSPool:

  • StartEngines: Number of engines to initially start when a pool is created. Defaults to 10.
  • MaxEngines: Maximum number of engines that will be created in the pool. If an engine is requested but all the current engines are busy, a new engine will be created unless the maximum has been reached. Defaults to 25.
  • Initializer: Action called when a new engine is created. This should configure the environment and load any required JavaScript libraries. The engine will not be available for use until this method has completed.
  • MaxUsagesPerEngine: The maximum number of times an engine can be reused before it is disposed. 0 is unlimited. Defaults to 100.
  • GarbageCollectionInterval: The number of times an engine can be reused before its garbage collector is ran. Only affects engines that support garbage collection (V8). Defaults to 20.
  • GetEngineTimeout: If all engines in the pool are currently busy and MaxEngines has been reached, the call to GetEngine will block for this period of time waiting for an engine to become free. If an engine can not be acquired in this timeframe, throws a JsPoolExhaustedException. Set this to -1 to wait indefinitely. Defaults to 5 seconds.
  • WatchPath: Path to watch for file changes. If any files in this path change, all engines in the pool will be recycled.
  • WatchFiles: Used in combination with WatchPath. If specified, only these particular files within the path will be watched. If not specified, all files within the path will be watched.
  • EngineFactory: Method used to create new JavaScript engines. Defaults to the default factory method in JavaScriptEngineSwitcher (JsEngineSwitcher.Current.CreateDefaultJsEngineInstance)


4.0 - 4th January 2019

  • Updated to JavaScriptEngineSwitcher 3.0. Upgrade instructions can be found in the How to upgrade applications to version 3.X document.
  • #37 - Fix JsPool recycling problem in a concurrent environment. Thanks @benokit
  • #34 - Made properties of the PooledJsEngine class virtual, to ease unit testing. Thanks @Taritsyn

4.0-beta1 - 30th June 2018

JavaScriptEngineSwitcher has been upgraded to version 3.0.0 beta. Upgrade instructions can be found in the How to upgrade applications to version 3.X document.

Other changes:

  • #30 - Added net471 and netstandard2.0 targets
  • #32 - Downgraded net451 target to net45

All changes in this release were contributed by Andrey Taritsyn. Thanks for your contributions!

3.0 - 1st July 2017

  • #7 - Breaking API change: Engines should now be returned to the pool by disposing them, rather than using ReturnEngineToPool:

Old API:

var engine = pool.GetEngine();

New API:

// with a "using" block
using (var engine = pool.GetEngine()) {

// without a "using" block
var engine = pool.GetEngine();
var message = engine.CallFunction("doStuff");

The old ReturnEngineToPool method still works, but it now takes an instance of PooledJsEngine rather than IJsEngine.

  • #20 - Remove JsEngineWithOwnThread as it's no longer needed in MsieJsEngine 2.1.0 and above
  • Upgrade to latest JavaScriptEngineSwitcher

2.0 - 25th September 2016

  • #17 - Upgrade to JavaScriptEngineSwitcher 2.0
  • #16 - Add support for .NET Core
  • #10 - Add DebounceTimeout configuration option for specifying the number of milliseconds to wait after a file is changed before recycling the JavaScript engines.

0.4.0 - 9th April 2016

  • Upgrade to latest JavaScriptEngineSwitcher

0.3.2 - 9th April 2016

  • #13 - Fix concurrency issue

0.3.1 - 15th November 2015

  • #8 - Do not throw if WatchPath is used without WatchFiles
  • Fire Recycled event when pool is recycled

0.3 - 12th April 2015

  • Added Recycle method to dispose all current engines and create new ones. This is essentially the same as disposing the whole pool and creating a new pool
  • Added the ability to recycle the pool when any watched files are modified

0.2 - 21st February 2015

  • #2 - Collect garbage when engine returned to pool
  • #3 - Recycle workers after a certain number of usages
  • #4 and #5 - Upgrade JavaScriptEngineSwitcher to latest version to allow settings such as max heap size in Web.config or App.config

0.1 - 28th November 2014

  • Initial release


Pooling of reusable JavaScript engines for fast, performant scripting integration in in .NET applications







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