var ext = path.replace(/.*[\.\/\\]/, '').toLowerCase();
                 ^

TypeError: path.replace is not a function
    at Mime.lookup (/Users/devOps/Desktop/_devOps/once-rest/node_modules/mime/mime.js:70:18)
    at GoogleToken.getToken (/Users/devOps/Desktop/_devOps/once-rest/node_modules/gtoken/lib/index.js:55:33)
    at /Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auth-library/lib/auth/jwtclient.js:137:21
    at JWT._createGToken (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auth-library/lib/auth/jwtclient.js:225:12)
    at JWT.refreshToken_ (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auth-library/lib/auth/jwtclient.js:133:15)
    at JWT.OAuth2Client.refreshAccessToken (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auth-library/lib/auth/oauth2client.js:203:8)
    at JWT.OAuth2Client.getAccessToken (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auth-library/lib/auth/oauth2client.js:237:10)
    at /Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auto-auth/index.js:118:12
    at addScope (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auto-auth/index.js:74:5)
    at Auth.getAuthClient (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auto-auth/index.js:49:5)
    at Auth.getToken (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auto-auth/index.js:112:8)
    at Auth.authorizeRequest (/Users/devOps/Desktop/_devOps/once-rest/node_modules/google-auto-auth/index.js:16:8)
    at Storage.makeAuthenticatedRequest (/Users/devOps/Desktop/_devOps/once-rest/node_modules/gcloud/lib/common/util.js:371:18)
    at Storage.Service.request (/Users/devOps/Desktop/_devOps/once-rest/node_modules/gcloud/lib/common/service.js:117:15)
    at Storage.getBuckets (/Users/devOps/Desktop/_devOps/once-rest/node_modules/gcloud/lib/storage/index.js:399:8)
    at makeRequest (/Users/devOps/Desktop/_devOps/once-rest/node_modules/gcloud/lib/common/stream-router.js:202:5)

var gcloud = require('gcloud')({
  keyFilename: key,
  projectId: 'xxxxx-projectId'
});

var gcs = gcloud.storage();
var bucket = gcs.bucket('xxxxxx-bucket');
gcs.getBuckets(function(err, buckets) {
  if (!err) {
    // buckets is an array of Bucket objects.
    console.log(buckets);
  }
});