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Cannot read property 'json' of undefined

See original GitHub issue

Using the example on the homepage, I get the following error:

/Users/owner/path/log.js:7
    format: winston.format.json(),
                          ^

TypeError: Cannot read property 'json' of undefined
    at Object.setupLogging (/Users/owner/path/log.js:7:27)
    at Object.<anonymous> (/Users/owner/path/server.js:35:8)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Function.Module.runMain (module.js:609:10)
    at startup (bootstrap_node.js:158:16)
    at bootstrap_node.js:578:3

const winston = require('winston');

function setupLogging(app) {
  const logger = winston.createLogger({
    level: 'info',
    format: winston.format.json(),
    transports: [
      //
      // - Write to all logs with level `info` and below to `combined.log`
      // - Write all logs error (and below) to `error.log`.
      //
      new winston.transports.File({ filename: 'error.log', level: 'error' }),
      new winston.transports.File({ filename: 'combined.log' }),
    ],
  });

  //
  // If we're not in production then log to the `console` with the format:
  // `${info.level}: ${info.message} JSON.stringify({ ...rest }) `
  //
  if (process.env.NODE_ENV !== 'production') {
    logger.add(new winston.transports.Console({
      format: winston.format.simple(),
      colorize: true,
    }));
  }

}

module.exports = {
  setupLogging,
};

package.json lists "winston": "^2.4.0"