Jint

Jint is a Javascript interpreter for .NET which can run on any modern .NET platform as it supports .NET Standard 2.0 and .NET 4.6.1 targets (and up). Because Jint neither generates any .NET bytecode nor uses the DLR it runs relatively small scripts really fast.

💡 You should prefer 3.x beta over the 2.x legacy version as all new features and improvements are targeted against version 3.x.

ECMAScipt Features

Version 2.x

• ✔ Full support for ECMAScript 5.1 (ES5)
• .NET Interoperability

Version 3.x

The entire execution engine was rebuild with performance in mind, in many cases at least twice as fast as the old engine. All the features of 2.x and more:

ECMAScript 2015 (ES6)

• ✔ Arrow function expression
• ✔ Class support
• ✔ Enhanced object literals
• ✔ Template strings
• ✔ Destructuring
• ✔ Default, rest and spread
• ✔ Lexical scoping of variables (let and const)
• ✔ for...of
• ✔ Map and Set
• ✔ Proxies
• ✔ Symbols
• ✔ Reflect
• ✔ Binary and octal literals
• ❌ Generators
• ❌ Unicode
• ❌ Modules and module loaders
• ❌ Weakmap and Weakset
• ❌ Promises
• ❌ Tail calls

ECMAScript 2016

• ✔ Block-scoping of variables and functions
• ✔ Destructuring patterns (of variables)
• ✔ Exponentiation operator **
• ✔ Array.prototype.includes
• ❌ await, async

ECMAScript 2017

• ✔ Object.values, Object.entries and Object.getOwnPropertyDescriptors

ECMAScript 2018

• ✔ Rest/spread operators for object literals (...identifier),
• ❌ Promise.prototype.finally

ECMAScript 2019

• ✔ Array.prototype.flat, Array.prototype.flatMap

ECMAScript 2020

• ✔ Nullish coalescing operator (??)
• ✔ globalThis object
• ❌ BigInt

Other

• Further refined .NET CLR interop capabilities
• Constraints for execution (recursion, memory usage, duration)

Follow new features as they are being implemented, see sebastienros#343

Discussion

Join the chat on Gitter or post your questions with the jint tag on stackoverflow.

Video

Here is a short video of how Jint works and some sample usage

https://channel9.msdn.com/Shows/Code-Conversations/Sebastien-Ros-on-jint-a-Javascript-Interpreter-for-NET

Examples

This example defines a new value named log pointing to Console.WriteLine, then executes a script calling log('Hello World!').

var engine = new Engine()
    .SetValue("log", new Action<object>(Console.WriteLine));
    
engine.Execute(@"
    function hello() { 
        log('Hello World');
    };
 
    hello();
");

Here, the variable x is set to 3 and x * x is executed in JavaScript. The result is returned to .NET directly, in this case as a double value 9.

var square = new Engine()
    .SetValue("x", 3) // define a new variable
    .Execute("x * x") // execute a statement
    .GetCompletionValue() // get the latest statement completion value
    .ToObject(); // converts the value to .NET

You can also directly pass POCOs or anonymous objects and use them from JavaScript. In this example for instance a new Person instance is manipulated from JavaScript.

var p = new Person {
    Name = "Mickey Mouse"
};

var engine = new Engine()
    .SetValue("p", p)
    .Execute("p.Name = 'Minnie'");

Assert.AreEqual("Minnie", p.Name);

You can invoke JavaScript function reference

var add = new Engine()
    .Execute("function add(a, b) { return a + b; }")
    .GetValue("add");

add.Invoke(1, 2); // -> 3

or directly by name

var engine = new Engine()
   .Execute("function add(a, b) { return a + b; }");

engine.Invoke("add", 1, 2); // -> 3

Accessing .NET assemblies and classes

You can allow an engine to access any .NET class by configuring the engine instance like this:

var engine = new Engine(cfg => cfg.AllowClr());

Then you have access to the System namespace as a global value. Here is how it's used in the context on the command line utility:

jint> var file = new System.IO.StreamWriter('log.txt');
jint> file.WriteLine('Hello World !');
jint> file.Dispose();

And even create shortcuts to common .NET methods

jint> var log = System.Console.WriteLine;
jint> log('Hello World !');
=> "Hello World !"

When allowing the CLR, you can optionally pass custom assemblies to load types from.

var engine = new Engine(cfg => cfg
    .AllowClr(typeof(Bar).Assembly)
);

and then to assign local namespaces the same way System does it for you, use importNamespace

jint> var Foo = importNamespace('Foo');
jint> var bar = new Foo.Bar();
jint> log(bar.ToString());

adding a specific CLR type reference can be done like this

engine.SetValue("TheType", TypeReference.CreateTypeReference(engine, typeof(TheType)))

and used this way

jint> var o = new TheType();

Generic types are also supported. Here is how to declare, instantiate and use a List<string>:

jint> var ListOfString = System.Collections.Generic.List(System.String);
jint> var list = new ListOfString();
jint> list.Add('foo');
jint> list.Add(1); // automatically converted to String
jint> list.Count; // 2

Internationalization

You can enforce what Time Zone or Culture the engine should use when locale JavaScript methods are used if you don't want to use the computer's default values.

This example forces the Time Zone to Pacific Standard Time.

var PST = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
var engine = new Engine(cfg => cfg.LocalTimeZone(PST));
    
engine.Execute("new Date().toString()"); // Wed Dec 31 1969 16:00:00 GMT-08:00

This example is using French as the default culture.

var FR = CultureInfo.GetCultureInfo("fr-FR");
var engine = new Engine(cfg => cfg.Culture(FR));
    
engine.Execute("new Number(1.23).toString()"); // 1.23
engine.Execute("new Number(1.23).toLocaleString()"); // 1,23

Execution Constraints

Execution constraints are used during script execution to ensure that requirements around resource consumption are met, for example:

• Scripts should not use more than X memory.
• Scripts should only run for a maximum amount of time.

You can configure them via the options:

var engine = new Engine(options => {

    // Limit memory allocations to MB
    options.LimitMemory(4_000_000);

    // Set a timeout to 4 seconds.
    options.TimeoutInterval(TimeSpan.FromSeconds(4));

    // Set limit of 1000 executed statements.
    options.MaxStatements(1000);

    // Use a cancellation token.
    options.CancellationToken(cancellationToken);
}

You can also write a custom constraint by implementing the IConstraint interface:

public interface IConstraint
{
    /// Called before a script is executed and useful when you us an engine object for multiple executions.
    void Reset();

    // Called before each statement to check if your requirements are met.
    void Check();
}

For example we can write a constraint that stops scripts when the CPU usage gets too high:

class MyCPUConstraint : IConstraint
{
    public void Reset()
    {
    }

    public void Check()
    {
        var cpuUsage = GetCPUUsage();

        if (cpuUsage > 0.8) // 80%
        {
            throw new OperationCancelledException();
        }
    }
}

var engine = new Engine(options =>
{
    options.Constraint(new MyCPUConstraint());
});

When you reuse the engine you want to use cancellation tokens you have to reset the token before each call of Execute:

var constraint = new CancellationConstraint();

var engine = new Engine(options =>
{
    options.Constraint(constraint);
});

for (var i = 0; i < 10; i++) 
{
    using (var tcs = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
    {
        constraint.Reset(tcs.Token);

        engine.SetValue("a", 1);
        engine.Execute("a++");
    }
}

.NET Interoperability

• Manipulate CLR objects from JavaScript, including:
  • Single values
  • Objects
    • Properties
    • Methods
  • Delegates
  • Anonymous objects
• Convert JavaScript values to CLR objects
  • Primitive values
  • Object -> expando objects (IDictionary<string, object> and dynamic)
  • Array -> object[]
  • Date -> DateTime
  • number -> double
  • string -> string
  • boolean -> bool
  • Regex -> RegExp
  • Function -> Delegate
• Extensions methods

Security

The following features provide you with a secure, sand-boxed environment to run user scripts.

• Define memory limits, to prevent allocations from depleting the memory.
• Enable/disable usage of BCL to prevent scripts from invoking .NET code.
• Limit number of statements to prevent infinite loops.
• Limit depth of calls to prevent deep recursion calls.
• Define a timeout, to prevent scripts from taking too long to finish.

Continuous Integration kindly provided by AppVeyor

Branches and releases

• The recommended branch is dev, any PR should target this branch
• The dev branch is automatically built and published on Myget. Add this feed to your NuGet sources to use it: https://www.myget.org/F/jint/api/v3/index.json
• The dev branch is occasionally merged to master and published on NuGet
• The 3.x releases have more features (from es6) and is faster than the 2.x ones. They run the same test suite so they are as reliable. For instance RavenDB is using the 3.x version.
• The 3.x versions are marked as beta as they might get breaking changes while es6 features are added.