advent.js

// 1.1
data.split(/\n/g).map(e => parseInt(e, 10)).reduce((m, v) => m+v, 0)

// 1.2
Array.from({
  [Symbol.iterator]() {
    const list = data.split(/\n/g).map(e => parseInt(e, 10));
    let index = 0;
    let sum = 0;
    const seen = { 0: 0 };
    let done = false;

    return {
      next() {
        const current = list[index];
        sum += current;

        Object.assign(seen, { [sum]: (seen[sum] || 0) + 1 })

        try {
          return {
            value: { sum, count: seen[sum] },
            done,
          }
          
        } finally {
          index = ((index+1) % list.length)
          done = seen[sum] > 1;
        }
      }
    }
  }
}).slice(-1)[0].sum

// 2.1
Object.values(
  data
    .split(/\n/g)
    .map((e) => Array.from(e).sort().join('').match(/((\w)\2+)/g))
    .reduce((memo, t) => (
      Object.assign(memo,
        Object.keys(
          t.map(t => t.length).reduce(
            (m, i) => Object.assign(m, { [i]: 1 }), {}
          )
        ).reduce(
          (m3, v) => Object.assign(m3, { [v]: (m3[v] || 0) + 1 }), memo
        )
    )), {})
).reduce((m, v) => m*v, 1)

// 2.2
data
  .split(/\n/g)
  .map((t1, index, list) => (
    [
      list.find(
        (t2) => (
          1 === Array.from(t2).reduce(
            (count, c, index) => (
              count += c === t1[index] ? 0 : 1
            ),
            0
          )
        )
      ),
      t1
    ]
  ))
  .filter(([a, b]) => Boolean(a))
  .map(([a, b]) => {
    const ary = Array.from(a);
    ary.splice(Array.from(a).findIndex((c, index) => c !== b[index]), 1);
    return ary.join('');
  })
;