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

Runner.prototype.runTests = function(callback) {
  function next() {
    // grab next test
    test = tests.shift()
    // no tests left, run callback running next suite
    if (!test) callback()
    // run beforeEach hooks
    hooks(HOOK_TYPE_BEFORE_EACH, () => {
      runTest(err => {
        if (err) {
          // test failure
          this.emit(EVENT_TEST_FAIL, test, err)
          self.emit(EVENT_TEST_END, test)
          // run after each hook
          return hooks(HOOK_TYPE_AFTER_EACH, next)
        }
        // test pass
        self.emit(EVENT_TEST_PASS, test)
        self.emit(EVENT_TEST_END, test)
        // run after each hook
        hooks(HOOK_TYPE_AFTER_EACH, next)
      })
    })
  }
  // run next test
  next()
}