build.js

var fs = require('fs')
var path = require('path')
var rollup = require('rollup')
var babel = require('rollup-plugin-babel')
var uglify = require('rollup-plugin-uglify')
var version = process.env.VERSION || require('./package.json').version

var banner =
  '/*!\n' +
  ' * Vue-Lazyload.js v' + version + '\n' +
  ' * (c) ' + new Date().getFullYear() + ' Awe <hilongjw@gmail.com>\n' +
  ' * Released under the MIT License.\n' +
  ' */\n'

rollup.rollup({
    entry: path.resolve(__dirname, 'src/index.js'),
    plugins: [
      babel(),
      uglify()
    ]
})
.then(bundle => {
  return write(path.resolve(__dirname, 'vue-lazyload.js'), rewriteVersion(bundle.generate({
      format: 'umd',
      moduleName: 'VueLazyload'
  }).code))
})
.then(() => {
    console.log('Vue-Lazyload.js v' + version + ' builded')
})
.catch(console.log)

function rewriteVersion (code) {
  return code.replace('__VUE_LAZYLOAD_VERSION__', version)
}

function getSize (code) {
  return (code.length / 1024).toFixed(2) + 'kb'
}

function blue (str) {
  return '\x1b[1m\x1b[34m' + str + '\x1b[39m\x1b[22m'
}

function write (dest, code) {
  return new Promise(function (resolve, reject) {
    code = banner + code
    fs.writeFile(dest, code, function (err) {
      if (err) return reject(err)
      console.log(blue(dest) + ' ' + getSize(code))
      resolve()
    })
  })
}