Node should be added to the requirements and note that it is the only runtime that works for Windows 7.

[CookieOfFortune]

Windows 7 64-bit
pyexecjs 1.1.0
pyv8 1.0
NodeJS 4.0.0

Using execjs with pyv8 (it seems that PyV8 takes precedence):

import execjs

execjs.get().name
Out[2]: u'PyV8'

from react import jsx
jsx.transform('helloworld.jsx')
Traceback (most recent call last):

 File "<ipython-input-4-ba80c49e788e>", line 1, in <module>
 jsx.transform('helloworld.jsx')

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 64, in transform
 return JSXTransformer().transform(jsx_path, **opts)

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 51, in transform
 js = self.transform_string(i.read(), **kwargs)

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 45, in transform_string
 raise TransformError(e.message[7:])

TransformError: ()

'JScript' was my original runtime:

import os
os.environ['EXECJS_RUNTIME'] = 'JScript'
jsx.transform('helloworld.jsx')
Traceback (most recent call last):

 File "<ipython-input-7-ba80c49e788e>", line 1, in <module>
 jsx.transform(helloworld.jsx')

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 64, in transform
 return JSXTransformer().transform(jsx_path, **opts)

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 51, in transform
 js = self.transform_string(i.read(), **kwargs)

 File "C:\Python27\python-2.7.6.amd64\lib\site-packages\react\jsx.py", line 45, in transform_string
 raise TransformError(e.message[7:])

TransformError: or: 'undefined' is null or not an object

But installing NodeJS and using it as the runtime works:

os.environ['EXECJS_RUNTIME'] = 'Node'

In [9]: jsx.transform('helloworld.jsx')
Out[9]: u'var HelloMessage = React.createClass({displayName: \'HelloMessage\',\n render: function() {\n return React.createElement("div", null, "Hello ", this.props.name);\n }\n});\n\nReact.render(React.createElement(HelloMessage, {name: "John"}), mountNode);'

So I guess that should be added to the readme. I have uninstalled PyV8 and now Node is the default runtime under execjs.