JSON logging in winston with a timestamp

index.js

const winston = require('winston');
const MESSAGE = Symbol.for('message');

const jsonFormatter = (logEntry) => {
  const base = { timestamp: new Date() };
  const json = Object.assign(base, logEntry)
  logEntry[MESSAGE] = JSON.stringify(json);
  return logEntry;
}

const logger = winston.createLogger({
  level: 'info',
  format: winston.format(jsonFormatter)(),
  transports: new winston.transports.Console(),
});

logger.info('message content', { "context": "index.js", "metric": 1 })
logger.info('message content 2')

output.txt

# npm install
# node index.js
{"timestamp":"2017-12-07T16:07:10.518Z","context":"index.js","metric":1,"level":"info","message":"message content"}
{"timestamp":"2017-12-07T16:07:10.520Z","message":"message content 2","level":"info"}

package.json

{
  "name": "winston_test",
  "main": "index.js",
  "dependencies": {
    "winston": "^3.0.0-rc1"
  }
}

Hi,
Thanks but in latest RC - this prints out an 'undefined', so does the scream example in the docs - any idea why?