winston-format-bug

run-with-bug.js

const winston = require('winston');
const { combine, printf, splat } = winston.format;

const customFormat = combine(
  splat(),
  printf((info) => JSON.stringify(info))
);

const logger = winston.createLogger({
  transports: [
    new winston.transports.Console({
      format: customFormat,
    }),
    new winston.transports.Console({
      format: customFormat,
    }),
  ]
});

logger.info('foo', 'bar', 'ham');
logger.info('with interpolated strings: %s and %s', 'bar', 'ham');

run-without-bug.js

const winston = require('winston');
const { combine, printf, splat } = winston.format;

const customFormat = combine(
  splat(),
  printf((info) => JSON.stringify(info))
);

const logger = winston.createLogger({
  format: customFormat,
  transports: [
    new winston.transports.Console(),
    new winston.transports.Console(),
  ]
});

logger.info('foo', 'bar', 'ham');
logger.info('with interpolated strings: %s and %s', 'bar', 'ham');

$ node run-with-bug.js 
{"level":"info","message":"foo","meta":["bar","ham"]}
{"level":"info","message":"foo"}
{"level":"info","message":"with interpolated strings: bar and ham"}
{"level":"info","message":"with interpolated strings: bar and ham"}

$ node run-without-bug.js 
{"level":"info","message":"foo","meta":["bar","ham"]}
{"level":"info","message":"foo","meta":["bar","ham"]}
{"level":"info","message":"with interpolated strings: bar and ham"}
{"level":"info","message":"with interpolated strings: bar and ham"}