chronometer.js

class Chronometer {
  constructor() {
    this.currentTime = 0
    this.intervalId = null
    // ... your code goes here
  }

  start(callback) {
    this.intervalId = setInterval (() => {
      this.currentTime++
      if (callback) callback()
    }, 1000)
    // ... your code goes here
  }

  getMinutes() {
    return Math.floor(this.currentTime / 60)
    // ... your code goes here
  }

  getSeconds() {
    return this.currentTime % 60
    // ... your code goes here
  }

  computeTwoDigitNumber(value) {
    return value < 10 ? `0${value}` : `${value}`;
    // ... your code goes here
  }

  stop() {
     clearInterval(this.intervalId)
    // ... your code goes here
  }

  reset() {
    this.currentTime = 0
    // ... your code goes here
  }

  split() {
    const minutes = this.computeTwoDigitNumber(this.getMinutes())
    const seconds = this.computeTwoDigitNumber(this.getSeconds())
    return `${this.computeTwoDigitNumber(this.getMinutes())}:${this.computeTwoDigitNumber(this.getSeconds())}`;
    // ... your code goes here
  }
}

// The following is required to make unit tests work.
/* Environment setup. Do not modify the below code. */
if (typeof module !== 'undefined') {
  module.exports = Chronometer;
}