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This project was created by a developer from Ukraine.

Russia has invaded Ukraine and already killed tens of thousands of civilians,

with many more raped or tortured. It's a genocide. We need your help.

Support Ukraine



Bindings to use Crystal compiled to WebAssembly in a JavaScript environment, such as Node.js, Deno or the Web.


This library provides bindings to run a Crystal application in a JavaScript environment, such as Node.js or the Web. Using this it is possible to consume existing JavaScript API's inside Crystal when compiling for the WebAssembly target. It is similar to wasm-bindgen from Rust.

This library and WebAssembly with Crystal is highly experimental and still in the early days. Please report bugs.

For a Web APIs bindings see crystal-web.


  1. Add the dependency to your shard.yml:

        github: lbguilherme/crystal-js
  2. Run shards install

  3. Build your project with lib/js/scripts/ src/ Use the --release flag for an optimized build. This will produce two files: a main.wasm and a main.js.

You can run your project:

  • with Deno: deno run --allow-read main.js
  • with Node.js: node main.js
  • on the Web: <script defer src="main.js"></script>

See crystal-web-demo for an example project.


For basic usage you can require "js" and then use common JavaScript methods and classes from the JS module.

require "js"

JS.console.log "Hello from the JavaScript!"

Defining raw JavaScript methods

You can define special methods that run JavaScript code from Crystal. They can take parameters but their body must be a single string literal using interpolation to receive arguments:

require "js"

module Test
  # You need to include this module. It won't add any new methods, all it will do
  # is expanding JavaScript methods you define.
  include JS::ExpandMethods

  # Mark JavaScript methods with this annotation. They must be fully typed and
  # be aware that not all types are supported yet.
  def self.add(first : Int32, second : Int32) : Int32
    # This is NOT a raw string interpolation. The notation here is used to pass
    # valid values to JavaScript land.
      return #{first} + #{second}; // This is JavaScript!

five = Test.add(2, 3) # Returns 5.

JavaScript methods are always defined in a class or module that includes JS::ExpandMethods. Their body must be a single string literal that interpolates values. The string is raw JavaScript code that will be used as is, and the interpolations are placeholders to receive values from Crystal land. The bridging managed by this library handles converting the values to the appropriate type based on the annotations. Splat arguments are supported. The following types are supported:

  • Int, they become number on JavaScript land.
  • String, they are converted to string. Only proper UTF-8 is supported.
  • Bool, they become boolean.
  • Enumerable(T), such as Array, Tuple, Hash, etc. They are converted to an array of T's conversion.

The return type restriction is used to convert the JavaScript return value to the corresponding Crystal type. Only Int, String, and Bool are supported for now.

Unions are not supported yet.

In some cases it may be better to keep the value on JavaScript land instead of converting it, and instead hold a reference to the value that can be used later. The JS::Reference class provides this interface. When used as an argument type or a return type, it does no conversion and returns a reference to the value. This class is abstract, you must define a subclass to handle your custom types. Here is a more complex example:

require "js"

class MyObj < JS::Reference
  def : String) : MyObj
      const pieces = #{data}.split(', ')

      return {                    // The value returned here will be held by MyObj
        pieces,                   // and can later be accessed using `#{self}`.
        length: pieces.length

  def size : Int32
      return #{self}.length;

  def add_piece(piece : String)
      #{self}.pieces.push(#{}); // You can use complex Crystal expressions,
      #{self}.length += 1;                            // as long as you add `.as(type)` to it.

obj ="a, b, c")
p obj.size # => 3
p obj.size # => 4

Only the methods that are actually called will be generated in the output .js file, thus it is fine to define lots of unused methods.

Additionally, there are some helper functions to facilitate creating bindings to existing JavaScript APIs. Use js_getter, js_setter, js_property, and js_method to define matching methods. For example:

require "js"

class XMLHttpRequest < JS::Reference
  def : self
      return new XMLHttpRequest();

  js_getter readyState : Int32    # defines a `ready_state` method.
  js_getter responseText : String
  js_getter status : Int32

  # defines `response_type` and `response_type=` methods.
  js_property responseType : String

  # methods can be overloaded
  js_method send
  js_method send(body : String)
  js_method open(method : String, url : String)

  # for the return type use a comma, not a colon.
  js_method getResponseHeader(header : String), String

req ="GET", "")
p req.response_text
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Mon, 08 Aug 2022 21:34:17 GMT