ExeJS

---

Run JavaScript code from Python.

• Supported Runtime
• Installation
• Getting Started
• Reference
• Why
• Improvement & Change
• Notable Usage

Supported Runtime

ID	Runtime	Browser Engine	Team
1	Node	Chrome	Google
2	JavaScriptCore	Safari	Apple
3	SpiderMonkey	Firefox	Mozilla
4	JScript	IE	Microsoft
5	PhantomJS	Webkit*	Apple
6	SlimerJS	Gecko*	Mozilla
7	Nashorn	Java*	Oracle

Installation

# PYPI
pip install --upgrade exejs

# Conda
conda install conda-forge::exejs

# Source
git clone https://github.com/UlionTse/exejs.git
cd exejs
pip install .

Getting Started

import asyncio
import exejs

# evaluate (one-shot expression):
print(exejs.evaluate("'red yellow blue'.split(' ')"))
# ['red', 'yellow', 'blue']

# execute (one-shot statements; use `return` to send a value back):
print(exejs.execute('var x = 40 + 2; return x;'))
# 42

# compile + call (reuse a JS context across multiple calls):
ctx = exejs.compile('function add(x, y) { return x + y; }')
print(ctx.call('add', 1, 2))   # 3

# call an object method (key may be a property path):
ctx = exejs.compile('var calc = { mul: function(a, b) { return a * b; } };')
print(ctx.call('calc.mul', 3, 4))  # 12

# call with structured arguments (auto JSON-serialized):
ctx = exejs.compile('function greet(u) { return "hi " + u.name + ", age " + u.age; }')
print(ctx.call('greet', {'name': 'Tom', 'age': 18}))  # hi Tom, age 18

# async evaluate:
print(asyncio.run(exejs.evaluate_async("'red yellow blue'.split(' ')")))

# timeout (kill a runaway script instead of hanging forever):
try:
    exejs.execute('while (true) {}', timeout=2.0)
except exejs.ExejsTimeoutError as e:
    print('killed:', str(e))

Reference

PyExecJS (EOL)

Why

1. We need to run JavaScript from Python, but PyExecJS has been EOL since 2018. Issue#1
2. Package builds that rely on PyExecJS fail or get cancelled. Issue#2
3. PyExecJS writes compiled code to a temp file by default, which triggers antivirus alerts and blocks execution. Issue#3

Improvement and Change

1. Stop writing compiled code to a temp file (except JScript); compile and run just-in-time via stdin.
2. Drop Python 2 support.
3. Add an async API.

Notable Usage

ExeJS is a core dependency of Translators, where it handles the JavaScript execution layer that translation engines rely on.