under-the-hood of test runner: stats collector.js

// lib/stats-collector.js
function createStatsCollector(runner) {
  var stats = {
    suites: 0,
    tests: 0,
    passes: 0,
    pending: 0,
    failures: 0,
  };

  if (!runner) {
    throw new TypeError("Missing runner argument");
  }

  runner.stats = stats;

  runner.once(Runner.constants.EVENT_RUN_BEGIN, function () {
    stats.start = new Date();
  });
  runner.on(Runner.constants.EVENT_SUITE_BEGIN, function (suite) {
    suite.root || stats.suites++;
  });
  runner.on(Runner.constants.EVENT_TEST_PASS, function () {
    stats.passes++;
  });
  runner.on(Runner.constants.EVENT_TEST_FAIL, function () {
    stats.failures++;
  });
  runner.on(Runner.constants.EVENT_TEST_END, function () {
    stats.tests++;
  });
  runner.once(Runner.constants.EVENT_RUN_END, function () {
    stats.end = new Date();
    stats.duration = stats.end - stats.start;
  });
}