refactoredCode.js

const timeCodeRegEx = /\1(\d\d):\2(\d\d),\3(\d+)\4([sm])/

const isTimeCode = (str) => timeCodeRegEx.test(str) 

const convertTime = (str) => {
  let arr = timeCodeRegEx.exec(str)

  return { 
    min: Number(arr[1]),
    sec: Number(arr[2]),
    duration: Number(arr[3]),
    time: arr[4]
  }
}

const convert = (line) => {
  if(isTimeCode(line)) {
    const { min, sec, duration, time } = convertTime(line)

    let addMin = sec + duration > 59 ? 1 : 0 
    let subSec = addMin === 1 ? 60 : 0
  
    return {
      startTime: {
        min,
        sec
      },
      endTime: {
        min: min + (time === 'm' ? duration : 0) + addMin,
        sec: sec + (time === 's' ? duration : 0) - subSec
      }
    }
  } else {
    return ''
}