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 4.6.2 targets (and later).

Use cases and users

• Run JavaScript inside your .NET application in a safe sand-boxed environment
• Expose native .NET objects and functions to your JavaScript code (get database query results as JSON, call .NET methods, etc.)
• Support scripting in your .NET application, allowing users to customize your application using JavaScript (like Unity games)

Some users of Jint include RavenDB, EventStore, OrchardCore, ELSA Workflows, docfx, JavaScript Engine Switcher, and many more.

Supported features

ECMAScript 2015 (ES6)

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

ECMAScript 2016

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

ECMAScript 2017

• ✔ Object.values, Object.entries and Object.getOwnPropertyDescriptors
• ✔ Shared memory and atomics

ECMAScript 2018

• ✔ Asynchronous iteration
• ✔ Promise.prototype.finally
• ✔ RegExp named capture groups
• ✔ Rest/spread operators for object literals (...identifier)
• ✔ SharedArrayBuffer

ECMAScript 2019

• ✔ Array.prototype.flat, Array.prototype.flatMap
• ✔ String.prototype.trimStart, String.prototype.trimEnd
• ✔ Object.fromEntries
• ✔ Symbol.description
• ✔ Optional catch binding

ECMAScript 2020

• ✔ BigInt
• ✔ export * as ns from
• ✔ for-in enhancements
• ✔ globalThis object
• ✔ import
• ✔ import.meta
• ✔ Nullish coalescing operator (??)
• ✔ Optional chaining
• ✔ Promise.allSettled
• ✔ String.prototype.matchAll

ECMAScript 2021

• ✔ Logical Assignment Operators (&&= ||= ??=)
• ✔ Numeric Separators (1_000)
• ✔ AggregateError
• ✔ Promise.any
• ✔ String.prototype.replaceAll
• ✔ WeakRef
• ✔ FinalizationRegistry

ECMAScript 2022

• ✔ Class Fields
• ✔ RegExp Match Indices
• ✔ Top-level await
• ✔ Ergonomic brand checks for Private Fields
• ✔ .at()
• ✔ Accessible Object.prototype.hasOwnProperty (Object.hasOwn)
• ✔ Class Static Block
• ✔ Error Cause

ECMAScript 2023

• ✔ Array find from last
• ✔ Change Array by copy
• ✔ Hashbang Grammar
• ✔ Symbols as WeakMap keys

ECMAScript 2024

• ✔ ArrayBuffer enhancements - ArrayBuffer.prototype.resize and ArrayBuffer.prototype.transfer
• ✔ Atomics.waitAsync
• ✔ Ensuring that strings are well-formed - String.prototype.ensureWellFormed and String.prototype.isWellFormed
• ✔ Grouping synchronous iterables - Object.groupBy and Map.groupBy
• ✔ Promise.withResolvers
• ❌ Regular expression flag /v

ECMAScript 2025

• ✔ 16-bit floating point numbers (float16), Requires NET 8 or higher, Float16Array, Math.f16round()
• ✔ Array.fromAsync
• ✔ Import attributes
• ✔ Iterator helper methods
• ✔ JSON modules
• ✔ Promise.try
• ✔ RegExp.escape()
• ❌ Regular expression pattern modifiers (inline flags)
• ❌ Duplicate named capture groups
• ✔ Set methods (intersection, union, difference, symmetricDifference, isSubsetOf, isSupersetOf, isDisjointFrom)

ECMAScript proposals (no version yet)

• ✔ Await Dictionary (Promise.allKeyed, Promise.allSettledKeyed)
• ✔ Error.isError
• ✔ Explicit Resource Management (using and await using)
• ✔ Immutable Arraybuffers
• ✔ Import Bytes (import x from './file' with { type: 'bytes' })
• ✔ Iterator Sequencing
• ✔ Joint Iteration
• ✔ JSON.parse source text access
• ✔ Math.sumPrecise
• ✔ ShadowRealm
• ✔ Temporal
• ✔ Uint8Array to/from base64
• ✔ Upsert

Other

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

Performance

• Because Jint neither generates any .NET bytecode nor uses the DLR it runs relatively small scripts really fast
• If you repeatedly run the same script, you should cache the Script or Module instance produced by Esprima and feed it to Jint instead of the content string
• You should prefer running engine in strict mode, it improves performance

You can check out the engine comparison results, bear in mind that every use case is different and benchmarks might not reflect your real-world usage.

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://docs.microsoft.com/shows/code-conversations/sebastien-ros-on-jint-javascript-interpreter-net

Thread-safety

Engine instances are not thread-safe and they should not accessed from multiple threads simultaneously.

Examples

This example defines a new value named log pointing to Console.WriteLine, then runs 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 evaluated 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
    .Evaluate("x * x") // evaluate a statement
    .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 result = new Engine()
    .Execute("function add(a, b) { return a + b; }")
    .Invoke("add",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<TheType>(engine));

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.
• Sc