advent12.js

const generateNextIteration = (arr, patterns) => {
  return arr.map((val, idx) => {
    const currentPattern = arr[idx - 2] + arr[idx - 1] + arr[idx] + arr[idx + 1] + arr[idx + 2];
    return patterns.hasOwnProperty(currentPattern) ? patterns[currentPattern] : '.';
  });
};

const pad50 = () => new Array(50).fill('.');

const addPadding = (arr) => [...pad50(), ...arr, ...pad50()];

const pottedPlants = (initialState, patterns, iterations = 20) => {
  const initialArr = initialState.split('');
  let arr = addPadding(initialArr);
  let i = 0;
  while (i < iterations) {
    arr = generateNextIteration(arr, patterns);
    i++;
  }

  // subtracting 50 because I added 50 dots to the left of the original first index
  return arr.reduce((acc, cv, idx) => cv === '#' ? acc + idx - 50 : acc, 0);
};

const realPatterns = {
  '.###.': '#',
  '###.#': '#',
  '#....': '.',
  '..#..': '.',
  '##.#.': '.',
  '...#.': '.',
  '.#...': '#',
  '.##..': '.',
  '..#.#': '.',
  '#..#.': '.',
  '....#': '.',
  '##..#': '#',
  '..##.': '#',
  '.##.#': '#',
  '.#.#.': '.',
  '.....': '.',
  '#####': '.',
  '.####': '#',
  '###..': '.',
  '.#..#': '#',
  '#.#.#': '#',
  '#..##': '#',
  '#...#': '#',
  '.#.##': '#',
  '##.##': '.',
  '..###': '.',
  '#.###': '.',
  '####.': '#',
  '#.##.': '#',
  '##...': '#',
  '#.#..': '.',
  '...##': '#',
};

const realInitialState = '##...#...###.#.#..#...##.###..###....#.#.###.#..#....#..#......##..###.##..#.##..##..#..#.##.####.##';

pottedPlants(realInitialState, realPatterns); // 3,405

Answer for Part 2: 3,350,000,000,000

Expanded the padding to 5,000 in code above, did 1,000 iterations, recognized the pattern and extrapolated in Excel