under-the-hood of test runner: mocha run().js

Mocha.prototype.run = function (fn) {
  var suite = this.suite;
  var options = this.options;
  options.files = this.files;
  var runner = new Runner(suite, options.delay);
  createStatsCollector(runner);
  var reporter = new this._reporter(runner, options);

  const noop = () => "";
  function done(failures) {
    fn = fn || noop;
    if (reporter.done) {
      reporter.done(failures, fn);
    } else {
      fn(failures);
    }
  }

  return runner.run(done);
};