under-the-hood of test runner: runner.run.js

Runner.prototype.run = function(fn) {
  var self = this
  var rootSuite = this.suite
  fn = fn || function() {}
  function start() {
    self.started = true
    self.emit(Runner.constants.EVENT_RUN_BEGIN)

    self.runSuite(rootSuite, function() {
      self.emit(Runner.constants.EVENT_RUN_END)
    })
  }
  // callback
  this.on(Runner.constants.EVENT_RUN_END, function() {
    fn(self.failures)
  })

  start()

  return this
}