Duktape.rb is a C extension for the Duktape JavaScript interpreter.


$ rake
$ ruby example.rb


require 'duktape'

# Create a new context
ctx = Duktape::Context.new

## Evaluate a string
p ctx.eval_string('1 + 1')  # => 2


Creating a context creates a fresh evaluation environment with no global variables or functions defined.

A common pattern is to create a new context, define static functions once, and reuse the context to invoke the function many times with call_prop.

ctx = Duktape::Context.new

ctx.exec_string <<-JS
  function process(str, options) {
    // ...

ctx.call_prop('process', 'some data', a: 1, b: 2)

Call APIs

  • exec_string - Evaluate a JavaScript String on the context and return nil.
  • eval_string - Evaluate a JavaScript String expression and return the result as a Ruby Object.
  • get_prop - Access the property of the global object and return the value as a Ruby Object.
  • call_prop - Call a defined function with the given parameters and return the value as a Ruby Object.

Defining functions

You can define simple functions in Ruby that can be called from JavaScript:

ctx.define_function("leftpad") do |str, n, ch=' '|
  str.rjust(n, ch)


Executing JS may raise two classes of errors: Duktape::Error and Duktape::InternalError.

Any JS runtime error that is thrown in the interpreter is converted to a Ruby Duktape::Error. Specific error subclasses, such as SyntaxError and TypeError, are mapped from JS to the Ruby equivalent of the same name.

ctx = Duktape::Context.new
ctx.exec_string <<JS
  (function() {
    throw new Error("fail");
# raises Duktape::Error: fail

The second error hierarchy, Duktape::InternalError, is reserved for errors in the Duktape interpreter itself. It may be an indication of a bug in this library.