log4js-node/lib/appenders/logLevelFilter.js

function logLevelFilter(minLevelString, maxLevelString, appender, levels) {
  const minLevel = levels.getLevel(minLevelString);
  const maxLevel = levels.getLevel(maxLevelString, levels.FATAL);
  return (logEvent) => {
    const eventLevel = logEvent.level;
    if (minLevel.isLessThanOrEqualTo(eventLevel) && maxLevel.isGreaterThanOrEqualTo(eventLevel)) {
      appender(logEvent);
    }
  };
}

function configure(config, layouts, findAppender, levels) {
  const appender = findAppender(config.appender);
  return logLevelFilter(config.level, config.maxLevel, appender, levels);
}

module.exports.configure = configure;