add_flow_to_project_files.js

const fs = require('fs')
const path = require('path')
const os = require('os')

const walk = function (dir, done) {
  let results = []
  fs.readdir(dir, function (err, list) {
    if (err) return done(err)
    let pending = list.length
    if (!pending) return done(null, results)
    list.forEach(function (file) {
      file = path.resolve(dir, file)
      fs.stat(file, function (err, stat) {
        if (stat && stat.isDirectory()) {
          walk(file, function (err, res) {
            results = results.concat(res)
            if (!--pending) done(null, results)
          })
        } else {
          results.push(file)
          if (!--pending) done(null, results)
        }
      })
    })
  })
}

walk('./src', function (err, results) {
  if (err) throw err
  console.log(results)

  // add flow
  results.forEach((file) => {
    let data = fs.readFileSync(file) ///read existing contents into data
    let fd = fs.openSync(file, 'w+')
    let buffer = new Buffer('// @flow \r\n')
    const flowReg = /@flow/
    if (data.toString().split(os.EOL)[0] && !flowReg.test(data.toString().split(os.EOL)[0])) {
      fs.writeSync(fd, buffer, 0, buffer.length, 0) //write new data
      fs.writeSync(fd, data, 0, data.length, buffer.length) //append old data
      fs.close(fd)
    } else {
      fs.writeSync(fd, data, 0, data.length, 0) //append old data
      fs.close(fd)
    }

  })
  console.log('Finish')
})