Get the total length or duration of a Youtube playlist

youtube-playlist-length.js

(() => {
  const formatTime = (seconds) => {
    const units = {
      days: Math.floor(seconds / 60 / 60 / 24),
      hours: Math.floor(seconds / 60 / 60 % 24),
      minutes: Math.floor(seconds / 60 % 60),
      seconds: seconds % 60
    }

    // remove empty units, singularize if necessary, and return a string
    return Object.entries(units).filter(([unit, value]) => value !== 0)
      .map(([unit, value]) => `${value} ${value === 1 ? unit.slice(0, -1) : unit}`)
      .reduce((totalTime, x) => totalTime + ', ' + x)
  }

  const timestamps = [...document.querySelectorAll('span.ytd-thumbnail-overlay-time-status-renderer')]
    .map(x => x.innerText.trim())
    .map(x => {
      const [min, sec] = x.split(':')
      return min * 60 + sec * 1
    })

  const totalTime = formatTime(
    timestamps.reduce((totalSeconds, x) => totalSeconds + x)
  )

  console.log('%c Playlist total: %s', 'font-size: 2.5em; color: cyan', totalTime)
})()

Navigate to a youtube playlist and paste into the Javascript console (⌥ ⌘ J on macOS)
You don't need to be logged in, so if the console makes you nervous, just open it in an incognito window.

Honestly, I probably wouldn't iterate an object in this way in a production application but for this case I thought it was interesting to be able to slice off the 's' to make the unit singular.
Also I will admit it doesn't look as hacky in python, which is where I took the code for the formatTime function