Utility for getting information about a playlist. Type `window[Symbol.for('userscript::PlaylistInfo')]` in your console to use the functions.

youtube-playlist-info.meta.js

// ==UserScript==
// @name         YouTube - Playlist Info
// @namespace    larryc5
// @version      1.0
// @description  Utility for getting information about a playlist. Type `window[Symbol.for('userscript::PlaylistInfo')]` in your console to use the functions.
// @author       Larry Costigan
// @match        http://www.youtube.com/*
// @match        https://www.youtube.com/*
// @grant        none
// @downloadURL  https://gist.githubusercontent.com/larryc5/585b4bd176385894900a81d53a7dec7f/raw/youtube-playlist-info.user.js
// @updateURL    https://gist.githubusercontent.com/larryc5/585b4bd176385894900a81d53a7dec7f/raw/youtube-playlist-info.meta.js
// ==/UserScript==

youtube-playlist-info.user.js

// ==UserScript==
// @name         YouTube - Playlist Info
// @namespace    larryc5
// @version      1.0
// @description  Utility for getting information about a playlist. Type `window[Symbol.for('userscript::PlaylistInfo')]` in your console to use the functions.
// @author       Larry Costigan
// @match        http://www.youtube.com/*
// @match        https://www.youtube.com/*
// @grant        none
// @downloadURL  https://gist.githubusercontent.com/larryc5/585b4bd176385894900a81d53a7dec7f/raw/youtube-playlist-info.user.js
// @updateURL    https://gist.githubusercontent.com/larryc5/585b4bd176385894900a81d53a7dec7f/raw/youtube-playlist-info.meta.js
// ==/UserScript==

(function() {
  'use strict';
  const exports = window[Symbol.for('userscript::PlaylistInfo')] = {
    get items() {
      return Array.from(document.querySelectorAll('span.ytd-thumbnail-overlay-time-status-renderer'));
    },
    time(min, max) {
      return (exports.items
        .slice(min, max)
        .map(e => e.textContent
             .trim()
             .split(':')
             .filter(Boolean)
             .reduce((t, n, i, a)=> t + parseInt(n)*Math.pow(60, a.length - i - 1), 0))
        .filter(Boolean)
        .reduce((t, n)=> t + n, 0));
    },
    timeF(format, min, max) {
      if (typeof format !== 'string') {
        max = min;
        min = format;
        format = '+H:MM:SS';
      }
      const t = exports.time(min, max);
      return String(format).replace(/\+D|D+|\+H|H+|\+M|M+|\+S|S+/g, ($1) => {
        let n = 0;
        if ($1[0] === '+') {
          switch ($1[1]) {
            case 'D': n = Math.floor(t / 60 / 60 / 24); break;
            case 'H': n = Math.floor(t / 60 / 60); break;
            case 'M': n = Math.floor(t / 60); break;
            case 'S': n = Math.floor(t); break;
          }
          return String(n);
        } else {
          switch ($1[0]) {
            case 'D': n = Math.floor(t / 60 / 60 / 24); break;
            case 'H': n = Math.floor(t / 60 / 60) % 24; break;
            case 'M': n = Math.floor(t / 60) % 60; break;
            case 'S': n = Math.floor(t) % 60; break;
          }
          if (n > Math.pow(10, $1.length)) {
            return String(n);
          }
          return ($1.replace(/./g, '0') + n).slice(-$1.length);
        }
      });
    },
    whereIs(percent) {
      percent = Math.min(1, Math.max(0, percent));
      const times = (exports.items
        .map(e => e.textContent
             .trim()
             .split(':')
             .filter(Boolean)
             .reduce((t, n, i, a)=> t + parseInt(n)*Math.pow(60, a.length - i - 1), 0)));
      const total = times.reduce((t, n)=> t + n, 0);
      const position = total * percent;
      let totalBefore = 0;
      let totalAt = 0;
      const index = (p => times.findIndex(n => {
        if (p - n < 0) {
          totalAt = n;
          return true;
        }
        totalBefore += n;
        p -= n;
        return false;
      }))(position);
      return index + ((position - totalBefore) / totalAt);
    },
  };
}());