// karma.conf.js
module.exports = function(config) {
  config.set({
    "logReporter": {
      "outputPath": "test/some/path/", // default name is current directory
      "logFileName": "logfile.log" // default name is logFile_month_day_year_hr:min:sec.log
    },
    "plugins": [
      "karma-log-reporter"
    ],
    "reporters": ["log-reporter"]
  });
};

// karma.conf.js
module.exports = function(config) {
  config.set({
    "logReporter": {
      "outputPath": "test/some/path/",
      "logFileName": "logfile.log",
      "filter_key": "log-filter"
    },

	...
  });
};

\\ this will be logged.
console.log(JSON.stringify({filter_key: 'log-filter', message: 'this message will be logged to file.'}));

\\ this will not be logged.
console.log('this message will not be logged to file')