Testing a JavaScript framework on-the-cheap, with no test framework or test runner

front-end-testing-example.js

if (getTimeRemaining(new Date('2018-07-29T03:24:45'), new Date('2018-07-29T03:24:45')) != '00:00') {
  throw new Error('should return no time if end time is the same as the current time');
}

if (getTimeRemaining(new Date('2018-07-29T03:24:45'), new Date('2018-07-29T03:24:12')) !== '00:33') {
  throw new Error('should returning the time difference when subtracting the second arg from the first');
}

if (getTimeRemaining(new Date('2018-07-29T03:24:45'), new Date('2018-07-29T03:04:36')) !== '20:09') {
  throw new Error('should returning a time difference that includes minutes and seconds');
}

if (getTimeRemaining(new Date('2018-07-29T03:24:45'), new Date('2018-07-29T03:22:46')) !== '01:59') {
  throw new Error('should returning a time difference where minutes less than 10');
}

function getTimeRemaining(stopTime, currentTime) {
  const stopTimestamp = stopTime.getTime();
  const currentTimestamp = currentTime.getTime();
  const totalSecondsRemaining = ((stopTimestamp - currentTimestamp) / 1000);

  const minutesRemaining = Math.floor(totalSecondsRemaining / 60);
  const secondsRemaining = totalSecondsRemaining % 60;

  const formattedMinutes = (`0${minutesRemaining}`).slice(-2);
  const formattedSeconds = (`0${secondsRemaining}`).slice(-2);

  return `${formattedMinutes}:${formattedSeconds}`;
}