https://codesandbox.io/s/ywl4jjjw5j

BEM.js

export default styles => Object.keys(styles)
  .filter((key) => !/^_/.test(key))
  .reduce((prev, next) => ({...prev, [next]:styles[next]}), {});

example.js

import React from 'react';
import ReactDOM from "react-dom";
import BEM from './BEM';

const styles = {
    favorites: {
        color: 'green',
        _list: {
            listStyle: 'none',
            padding: 0,
        },
        _listItem: {
            color: 'red',
            __active: {
                color: 'brown',
            }
        },
    }
};

const Foo = () => (
  <div style={BEM(styles.favorites)}>
    Foo
    <ul style={BEM(styles.favorites._list)}>
      <li style={BEM(styles.favorites._listItem)}>Bar</li>
      <li
        style={{
          ...BEM(styles.favorites._listItem),
          ...BEM(styles.favorites._listItem.__active)
        }}
      >
        Tar
      </li>
    </ul>
  </div>
);

const rootElement = document.body;
ReactDOM.render(<Foo />, rootElement);