const transform = require('babel-transform-dir')

// Transform all javascript files in `./src` and write the result to `./lib`
transform('./src', './lib')
.then(() => {
  console.log('done')
})

// Or explicitly specify babel config
readJson('./.babelrc')
.then((babel_config) => {
  return transform('./src', './lib', {
    babel: babel_config,
    // Invokes whenever a file is transformed and written.
    onFile: (file) => {
      console.log(`src/${file} -> lib/${file}`)
    }
  })
})
.then(() => {
  console.log('done')
})

// src/a.js -> lib/a.js
// src/b.js -> lib/b.js
// done