index.js

;(function (global, factory) {
  if (typeof define === 'function' && define.amd) {
    define([], factory)
  } else if (typeof exports === 'object') {
    module.exports = factory()
  } else {
    global.Index = factory()
  }
}(this, function () {
  var config = null
  var directAccess = false
  var gapiUrl = 'https://apis.google.com/js/api:client.js'

  var gAuth = {
    install: function (Vue, options) {
      Vue.googleAuth = googleAuth
      Vue.prototype.$googleAuth = googleAuth

      if (typeof options === 'object') {
        config = Object.assign(options, { scope: 'profile email https://www.googleapis.com/auth/plus.login' })
      }
    }
  }

  function googleAuth () {
    return {
      load: function (options) {
        return new Promise(function (resolve, reject) {
          if (window.gapi === undefined) {
            installClient().then(function () {
              return initClient(options)
            }).then(function () {
              resolve()
            })
          } else if (window.gapi !== undefined && window.gapi.auth2 === undefined) {
            initClient(options).then(function () {
              resolve()
            })
          } else {
            resolve()
          }
        })
      },

      directAccess: function () {
        directAccess = true
      },

      signIn: function (successCallback, errorCallback) {
        if (directAccess) {
          window.gapi.auth2.getAuthInstance().signIn().then(function (googleUser) {
            successCallback(googleUser)
          }, function (error) {
            errorCallback(error)
          })
        } else {
          window.gapi.auth2.getAuthInstance().grantOfflineAccess({'redirect_uri': 'postmessage'}).then(function (response) {
            successCallback(response.code)
          }, function (error) {
            errorCallback(error)
          })
        }
      },

      signOut: function (successCallback, errorCallback) {
        window.gapi.auth2.getAuthInstance().signOut().then(function () {
          successCallback()
        }, function (error) {
          errorCallback(error)
        })
      }
    }
  }

  function installClient () {
    return new Promise(function (resolve, reject) {
      var script = document.createElement('script')
      script.src = gapiUrl
      script.onreadystatechange = script.onload = function () {
        if (!script.readyState || /loaded|complete/.test(script.readyState)) {
          setTimeout(function () {
            resolve()
          }, 500)
        }
      }
      document.getElementsByTagName('head')[0].appendChild(script)
    })
  }

  function initClient (options) {
    if (typeof options === 'object') {
        config = Object.assign(options, { scope: 'profile email https://www.googleapis.com/auth/plus.login' })
    }

    return new Promise(function (resolve, reject) {
      window.gapi.load('auth2', function () {
        window.gapi.auth2.init(config)
        resolve()
      })
    })
  }

  return gAuth
}))