text: const { exec } = require('child_process')
  const fs = require('fs')
  
  let building = false
  fs.watch('./src', {
    recursive: true
  }, (event, filename) => {
    if (building) {
      return
    } else {
      building = true
      console.log('start: building ...')
      exec('npm run build', (err, stdout, stderr) => {
        if (err) {
          console.log(err)
        } else {
          console.log('end: building: ', stdout)
        }
        building = false
      })
    }
  })
  