Get CSS selectors from file(s) as array(s)

example.js

var manifest = require('./manifest')

manifest.selectors('./node_modules/basscss/css/basscss.css').then(function(selectors) {
  console.log(JSON.stringify(selectors, null, 2))
})

manifest.js

var fs = require('fs')
var css = require('css')

function manifest(file) {
  return new Promise(function(resolve, reject) {
    fs.readFile(file, function(error, buffer) {
      error ? reject(error) : resolve(css.parse(String(buffer), {
        source: file
      }).stylesheet)
    })
  })
}

function rules(file) {
  return new Promise(function(resolve, reject) {
    manifest(file).then(function(stylesheet) {
      resolve(stylesheet.rules)
    }).catch(reject)
  })
}

function flattener(method, key) {
  return function(file) {
    return new Promise(function(resolve, reject) {
      manifest[method](file).then(function(results) {
        resolve(results.reduce(function(flat, r) {
          return r[key] == null ? flat : flat.concat(r[key])
        }, []))
      }).catch(reject)
    })
  }
}

manifest.rules = rules
manifest.selectors = flattener('rules', 'selectors')
manifest.declarations = flattener('rules', 'declarations')
module.exports = manifest;