slack-transcripts.js

const arr = $$('.p-media_transcript__scrollable > p').map((line) => {
    return { 
      timestamp: line.querySelector('.p-media_transcript__timestamp').textContent, 
      text: line.querySelector('.p-media_transcript__text').textContent 
    }
  });

function hhmmssToMillis(hhmmss) {
    const time = hhmmss.split(':').reverse();
    switch(time.length) {
       case 1: 
         return parseInt(time[0]) * 1000;
       case 2: 
         return (parseInt(time[1]) * 60 + parseInt(time[0])) * 1000;
       case 3: 
         return (parseInt(time[2]) * 60 * 60 + parseInt(time[1]) * 60 + parseInt(time[0])) * 1000;
       default:
        break;
    }
    return 0; 
}

function msToTime(duration) {
    var milliseconds = parseInt((duration%1000)/100)
        , seconds = parseInt((duration/1000)%60)
        , minutes = parseInt((duration/(1000*60))%60)
        , hours = parseInt((duration/(1000*60*60))%24);

    hours = (hours < 10) ? "0" + hours : hours;
    minutes = (minutes < 10) ? "0" + minutes : minutes;
    seconds = (seconds < 10) ? "0" + seconds : seconds;

    return hours + ":" + minutes + ":" + seconds + "." + milliseconds;
}

function formatTime(hhmmss) {
    return msToTime(hhmmssToMillis(hhmmss))
}

const video = document.querySelector('video');
let str = `WEBVTT

`
arr.forEach((line, index) => { str += `
${formatTime(line.timestamp)} --> ${ arr[index + 1] ? formatTime(arr[index + 1].timestamp) : msToTime(video.duration.toFixed()*1000)}
${line.text}
`
})

console.log(str)