Programming language: Crystal
License: MIT License
Tags: Low Level Bindings    
Latest version: v1.0.1

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Duktape.cr provides Crystal bindings to the Duktape javascript engine.


Duktape.cr is best installed using Shards.

Add this to your shard.yml:

name: example  # your project's name
version: 1.0.0 # your project's version

    github: jessedoyle/duktape.cr
    version: ~> 1.0.0

then execute:

shards install

Shards will automatically make the native library. You can make the library manually by calling make libduktape.


You must first create a Duktape context:

require "duktape"

sbx = Duktape::Sandbox.new

sbx.eval! <<-JS
  var birthYear = 1990;

  function calcAge(birthYear){
    var current = new Date();
    var year = current.getFullYear();
    return year - birthYear;

  print("You are " + calcAge(birthYear) + " years old.");

An overwhelming majority of the Duktape API has been implemented. You can call the API functions directly on a Duktape::Sandbox or Duktape::Context instance:

sbx = Duktape::Sandbox.new
sbx.push_global_object   # [ global ]
sbx.push_string "Math"   # [ global "Math" ]
sbx.get_prop -2          # [ global Math ]
sbx.push_string "PI"     # [ global Math "PI" ]
sbx.get_prop -2          # [ global Math PI ]
pi = sbx.get_number -1
puts "PI: #{pi}"         # => PI: 3.14159

Eval vs Eval!

All of the evaluation API methods have a corresponding bang-method (!). The bang method calls will raise when a javascript error occurs, the non-bang methods will not raise on invalid javascript.

For example:

sbx = Duktape::Context.new
sbx.eval <<-JS
  var a =

will not raise any errors, but will return a non-zero error code.

The following code:

sbx = Duktape::Context.new
sbx.eval! <<-JS

will raise Duktape::SyntaxError.

Sandbox vs Context

You should only execute untrusted javascript code from within a Duktape::Sandbox instance. A sandbox isolates code from insecure operations such as Duktape's internal require mechanism and the Duktape global javascript object.

Creating a Duktape::Context gives code access to internal Duktape properties:

ctx = Duktape::Context.new
ctx.eval! <<-JS

Setting a Timeout

Duktape::Sandbox instances may optionally take an execution timeout limit in milliseconds. This provides protection against infinite loops when executing untrusted code.

A Duktape::RangeError exception is raised when the following code executes for longer than specified:

sbx = Duktape::Sandbox.new 500 # 500ms execution time limit
sbx.eval! "while (true) {}"    # => RangeError


An alternative interface for evaluating JS code is available via the Duktape::Runtime class. This class provides a streamlined evaluation API (similar to ExecJS) that allows easier access to javascript values without the need to call many low-level Duktape API functions.

The entire Runtime API is as follows:

  • call(property, *args) - Call the property or function with the given arguments and return the result.
  • call([properties], *args) - Call the property that is nested within an array of string property names.
  • eval(source) - Evaluate the javascript source and return the last value.
  • exec(source) - Evaluate the javascript source and always return nil.

Duktape::Runtime instances can also be provided an initialization block when created.

Here's an example:

  require "duktape/runtime"

  # A Runtime (optionally) accepts an initialization block
  rt = Duktape::Runtime.new do |sbx|
    sbx.eval! <<-JS
      function test(a, b, c) { return a + b + c; }

  rt.call("test", 3, 4, 5) # => 12.0 (same as test(3, 4, 5);)
  rt.call(["Math", "PI"])  # => 3.14159
  rt.eval("1 + 1")         # => 2.0
  rt.exec("1 + 1")         # => nil

Note that duktape/runtime is not loaded by the base duktape require, and may be used standalone if necessary (ie. replace your require "duktape" calls with require "duktape/runtime" if you want this functionality).

Calling Crystal Code from Javascript

Note: This functionality is considered experimental and syntax/functionality may change dramatically between releases.

It is possible to call Crystal code from your javascript:

  sbx = Duktape::Sandbox.new

  # Push a global function named "add_together"
  # that accepts two arguments.
  sbx.push_global_proc("add_together", 2) do |ptr|
    env = Duktape::Sandbox.new ptr

    # Get the two arguments
    # from the stack
    a = env.require_number 0
    b = env.require_number 1

    env.push_number a + b # Push the return value to the stack
    env.call_success      # call_success -> stack top is value returned

  sbx.eval! "print(add_together(2, 3));" # => 5

The proc object that is pushed to the Duktape stack accepts a pointer to a Context instance. We must wrap this pointer by calling env = Duktape::Sandbox.new ptr. The proc must also return an Int32 status code - env.call_failure and env.call_success will provide the proper integer values.

Note: Because it is currently not possible to pass closures to C bindings in Crystal, one must be careful that any variables used in the proc must not be referenced or initialized outside the scope of the proc. This is why variable names such as env are used.


The following exceptions may be thrown at runtime and may be rescued normally:

  • Duktape::Error
  • Duktape::EvalError
  • Duktape::RangeError
  • Duktape::ReferenceError
  • Duktape::SyntaxError
  • Duktape::TypeError
  • Duktape::URIError

These exceptions all inherit from Duktape::Error, so it may be used as a catch-all for runtime errors.

The following exceptions represent errors internal to the Duktape engine and are generally not recoverable when thrown from a context:

  • Duktape::InternalError
  • Duktape::HeapError

These exceptions all inherit from Duktape::InternalError.


I'll accept any pull requests that are well tested for bugs/features with Duktape.cr.

You should fork the main repo, create a feature branch, write tests and submit a pull request.


Duktape.cr is licensed under the MIT License. Please see LICENSE for details.

*Note that all licence references and agreements mentioned in the duktape.cr README section above are relevant to that project's source code only.