pseudo-mocha.js

const registeredTests = [];

function it (testFn) {
  registeredTests.push(testFn);
}

function runTheNextTest () {
  var testFunction = registeredTests.pop();
  var isAsync = testFunction.length == 1;

  if (isAsync) {
    testFunction(function done (err) {
      if (err) /* report the failure */
      runTheNextTest();
    });
  }
  else {
    var maybeAPromise = testFunction();
    if (isAPromise(maybeAPromise)) {
      maybeAPromise
        .then(runTheNextTest)
        .catch(runTheNextTest);
    }
    else {
      runTheNextTest();
    }
  }
}



runTheNextTest();

I'd point out that then and catch have different behavior – then will pass the test, catch fail the test.