DukPy is a simple JavaScript interpreter for Python without any external runtime dependency.

The name comes from DukPy's original Duktape-based implementation and is kept for package compatibility.

It comes with a bunch of common transpilers built-in for convenience:

• TypeScript
• JSX
• LESS

The TypeScript compiler can be used through the dukpy.typescript_compile function:

>>> import dukpy
>>> dukpy.typescript_compile('''
... class Greeter {
...     constructor(public greeting: string) { }
...     greet() {
...         return "<h1>" + this.greeting + "</h1>";
...     }
... };
...
... var greeter = new Greeter("Hello, world!");
... ''')
'var Greeter = /** @class */ (function () {\n    function Greeter(greeting) {\n        this.greeting = greeting;\n    }\n    Greeter.prototype.greet = function () {\n        return "<h1>" + this.greeting + "</h1>";\n    };\n    return Greeter;\n}());\n;\nvar greeter = new Greeter("Hello, world!");\n'

Currently the compiler has built-in options and doesn't accept additional ones.

The DukPY based TypeScript compiler also provides a WebAssets ( http://webassets.readthedocs.org/en/latest/ ) filter to automatically compile TypeScript code in your assets pipeline. You register this filter as typescript within WebAssets using:

from webassets.filter import register_filter
from dukpy.webassets import TypeScript

register_filter(TypeScript)

Which makes the filter available with the typescript name.

NOTE: When using the TypeScript compiler for code that needs to run in the browser, make sure to add https://cdnjs.cloudflare.com/ajax/libs/systemjs/0.19.24/system.js dependency. As import statements are resolved using SystemJS.

DukPy provides a built-in compiler from JSX to React, this is available as dukpy.jsx_compile:

>>> import dukpy
>>> dukpy.jsx_compile('var react_hello = <h1>Hello, world!</h1>;')
'"use strict";\n\nvar react_hello = React.createElement(\n  "h1",\n  null,\n  "Hello, world!"\n);'

The DukPY based JSX compiler also provides a WebAssets ( http://webassets.readthedocs.org/en/latest/ ) filter to automatically compile JSX+ES6 code in your assets pipeline. You register this filter as babeljsx within WebAssets using:

from webassets.filter import register_filter
from dukpy.webassets import BabelJSX

register_filter(BabelJSX)

Which makes the filter available with the babeljsx name. The BABEL_MODULES_LOADER option accepts systemjs or umd to compile modules for SystemJS or UMD instead of CommonJS.

DukPy provides a built-in distribution of the less compiler available through dukpy.less_compile:

>>> import dukpy
>>> dukpy.less_compile('.class { width: (1 + 1) }')
'.class {\n  width: 2;\n}\n'

The DukPY based LESS compiler also provides a WebAssets ( http://webassets.readthedocs.org/en/latest/ ) filter to automatically compile LESS code in your assets pipeline. You register this filter as lessc within WebAssets using:

from webassets.filter import register_filter
from dukpy.webassets import CompileLess

register_filter(CompileLess)

Which makes the filter available with the lessc name.

Using dukpy is as simple as calling the dukpy.evaljs function with the javascript code:

>>> import dukpy
>>> dukpy.evaljs("var o = {'value': 5}; o['value'] += 3; o")
{'value': 8}

The evaljs function executes the javascript and returns the resulting value as far as it is possible to encode it in JSON.

If execution fails a dukpy.JSRuntimeError exception is raised with the failure reason.

Use dukpy.run or, when reusing an interpreter, JSInterpreter.run to run a JavaScript file entrypoint:

>>> import dukpy
>>> dukpy.run("./main.mjs")
{}

evaljs always evaluates source text as a script. run reads a file and classifies its entrypoint:

• .mjs files are native ES modules.
• .cjs files are CommonJS.
• .js files follow the nearest package.json type of module or commonjs.

A .js file without that package metadata is ambiguous. For both entrypoints and dependencies, DukPy asks QuickJS to compile the CommonJS wrapper first; if that fails, it runs the file as a native ES module. DukPy never scans source text for import, export, await, comments, strings, or identifiers.

Entrypoints use the loader's canonical module ID. For a file under a registered loader path, import.meta.url and CommonJS module.id may therefore be relative to that path.

When ES modules import files classified as CommonJS, DukPy exposes a minimal synthetic namespace: default is module.exports, and the only named exports are module, exports, and require. DukPy does not infer named exports from CommonJS source; use a default import for CommonJS API objects.

Any argument passed to evaljs is available in JavaScript inside the dukpy object in javascript. It must be possible to encode the arguments using JSON for them to be available in Javascript:

>>> import dukpy
>>>
>>> def sum3(value):
...     return dukpy.evaljs("dukpy['value'] + 3", value=value)
...
>>> sum3(7)
10

The evaljs function supports providing multiple source codes to be executed in the same context.

Multiple script can be passed in a list or tuple:

>>> import dukpy
>>> dukpy.evaljs(["var o = {'value': 5}",
...               "o['value'] += 3",
...               "o"])
{'value': 8}

This is useful when your code requires dependencies to work, as you can load the dependency and then your code.

The evaljs function creates a new interpreter on each call, this is usually convenient and avoid errors due to dirt global variables or unexpected execution status.

In some cases you might want to run code that has a slow bootstrap, so it's convenient to reuse the same interpreter between two different calls so that the bootstrap cost has already been paid during the first execution.

This can be achieved by using the dukpy.JSInterpreter object.

Creating a dukpy.JSInterpreter permits to evaluate code inside that interpreter and multiple eval calls will share the same interpreter and global status:

>>> import dukpy
>>> interpreter = dukpy.JSInterpreter()
>>> interpreter.evaljs("var o = {'value': 5}; o")
{'value': 5}
>>> interpreter.evaljs("o.value += 1; o")
{'value': 6}

When using the dukpy.JSInterpreter object it is possible to use the require('modulename') instruction to load a module inside javascript.

Modules are looked up in all directories registered with dukpy.JSInterpreter.loader.register_path:

>>> import dukpy
>>> jsi = dukpy.JSInterpreter()
>>> jsi.loader.register_path('./js_modules')
>>> jsi.evaljs("isEmpty = require('fbjs/lib/isEmpty'); isEmpty([1])")
False

When using the persistent javascript interpreter it is also possible to install packages from npmjs.org through the dukpy.install_jspackage function:

>>> import dukpy
>>> jsi = dukpy.JSInterpreter()
>>> dukpy.install_jspackage('promise', None, './js_modules')
Packages going to be installed: promise->7.1.1, asap->2.0.3
Fetching https://registry.npmjs.org/promise/-/promise-7.1.1.tgz..........................
Fetching https://registry.npmjs.org/asap/-/asap-2.0.3.tgz............
Installing promise in ./js_modules Done!

The same functionality is also provided by the dukpy-install shell command:

$ dukpy-install -d ./js_modules promise
Packages going to be installed: promise->7.1.1, asap->2.0.3
Fetching https://registry.npmjs.org/promise/-/promise-7.1.1.tgz..........................
Fetching https://registry.npmjs.org/asap/-/asap-2.0.3.tgz............
Installing promise in ./js_modules Done!

Please note that currently install_jspackage is not able to resolve conflicting dependencies.