Process CSSRule objects with a callback function when given a CSSStyleSheet object and function as inputs

process-rules.js

export default function(stylesheet, callback) {
  const readRules = list => Array.from(list.cssRules).forEach(rule => {
    callback(rule)
    if (rule.cssRules !== undefined) {
      readRules(rule)
    }
  })
  if (
    [stylesheet, stylesheet.cssRules, callback].every(thing => thing !== undefined)
    && callback.constructor.name === 'Function'
  ) {
    readRules(stylesheet)
    return stylesheet
  }
}