micro-javascript

A pure Python JavaScript engine, inspired by MicroQuickJS.

Overview

This project provides a JavaScript execution environment with:

• Memory limits - Configurable maximum memory usage
• Time limits - Configurable execution timeout
• Pure Python - No C extensions or external dependencies
• Broad ES5+ support - Variables, functions, closures, classes, iterators, promises, regex, and more

[!WARNING] The sandbox is not production ready. Malicious code can still exhaust memory and has not been fully audited against escapes that could allow JavaScript to run arbitrary Python. See the sandbox issue label for more.

Interactive demos

Try playground.html in your browser to execute JavaScript using this Python library run via Pyodide (so JavaScript in Python in WebAssembly in JavaScript).

Use parser-playground.html to see how the micro-javascript tokenizer and parser works with different JavaScript code.

How it was built

Most of this library was built using Claude Code for web - here is the 15+ hour transcript.

Installation

pip install micro-javascript

Usage

from microjs import Context

# Create a context with optional limits
ctx = Context(memory_limit=1024*1024, time_limit=5.0)

# Evaluate JavaScript code
result = ctx.eval("1 + 2")  # Returns 3

# Functions and closures
ctx.eval("""
    function makeCounter() {
        var count = 0;
        return function() { return ++count; };
    }
    var counter = makeCounter();
""")
assert ctx.eval("counter()") == 1
assert ctx.eval("counter()") == 2

# Regular expressions
result = ctx.eval('/hello (\\w+)/.exec("hello world")')
# Returns ['hello world', 'world']

# Error handling with line/column tracking
ctx.eval("""
try {
    throw new Error("oops");
} catch (e) {
    // e.lineNumber and e.columnNumber are set
}
""")

Setting and Getting Variables

Use set() and get() to pass values between Python and JavaScript:

ctx = Context()

# Set a Python value as a JavaScript global variable
ctx.set("x", 42)
ctx.set("name", "Alice")
ctx.set("items", [1, 2, 3])

# Use the variable in JavaScript
result = ctx.eval("x * 2")  # Returns 84
result = ctx.eval("'Hello, ' + name")  # Returns 'Hello, Alice'
result = ctx.eval("items.map(n => n * 2)")  # Returns [2, 4, 6]

# Get a JavaScript variable back into Python
ctx.eval("var total = items.reduce((a, b) => a + b, 0)")
total = ctx.get("total")  # Returns 6

Exposing Python Functions to JavaScript

You can expose Python functions to JavaScript by setting them as global variables:

ctx = Context()

# Define a Python function
def add(a, b):
    return a + b

# Expose it to JavaScript
ctx.set("add", add)

# Call it from JavaScript
result = ctx.eval("add(2, 3)")  # Returns 5

Primitive values (numbers, strings, booleans) are passed directly as Python types, and primitives returned from Python functions work in JavaScript.

Arrays and objects are passed as internal JavaScript types (JSArray, JSObject). To return objects that JavaScript can use, return JSObject instances:

from microjs.values import JSObject, JSArray

ctx = Context()

# Access array elements via ._elements
def sum_array(arr):
    return sum(arr._elements)

ctx.set("sumArray", sum_array)
result = ctx.eval("sumArray([1, 2, 3, 4, 5])")  # Returns 15

# Return a JSObject for JavaScript to use
def make_point(x, y):
    obj = JSObject()
    obj.set("x", x)
    obj.set("y", y)
    return obj

ctx.set("makePoint", make_point)
result = ctx.eval("var p = makePoint(10, 20); p.x + p.y;")  # Returns 30

Supported Features

• Core: variables, operators, control flow, functions, closures
• Objects: object literals, prototypes, getters/setters, JSON
• Arrays: literals, methods (map, filter, reduce, etc.), typed arrays
• Functions: arrow functions, rest/spread, default parameters
• Classes: class syntax, inheritance, static methods
• Iteration: for-of, iterators, generators
• Async: Promises, async/await
• Regex: Full regex support with capture groups, lookahead/lookbehind
• Error handling: try/catch/finally with stack traces

Known Limitations

See open-problems.md for details on:

• Deep nesting limits (parser uses recursion)
• Some regex edge cases with optional lookahead captures
• Error constructor location tracking

Development

This project uses uv for dependency management.

# Run tests
uv run pytest

License

MIT License - see LICENSE file.

Based on MicroQuickJS by Fabrice Bellard.