A pure Python JavaScript sandbox engine based on MQuickJS
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Project description
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
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 JSContext
# Create a context with optional limits
ctx = JSContext(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
}
""")
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.
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