Look and say sequence - JavaScript version

look-and-say.js

/*
look-and-say-sequence by Jaeho Lee,
public domain

1 => [[1, 1]]
11 => [[1, 2]]
21  => [[2, 1], [1, 1]]
1211
111221
*/

function getLookAndSaySequence(len) {
  const seq = ['1']

  function getNextSeq(input) {
    const split = input.split('')
    const result = []

    for (
      let i = 0, count = 0, current = split[i];
      i < split.length;
      i++
    ) {
      count += 1

      const next = split[i + 1]
      if (
        next == null // reaches end
        ||
        next !== current // next
      ) {
        result.push([String(count), current])
        current = next
        count = 0
      }
    }
    return result
  }

  function flatNumber(seq) {
    return seq.reduce((prev, next) => prev.concat(next), []).join('')
  }

  const f = a => flatNumber(getNextSeq(a))

  let i = 0
  while (i < len - 1) {
    seq[i + 1] = f(seq[i])
    i++
  }

  return seq
}


const result = getLookAndSaySequence(8);
["1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211"].forEach((e, i) => {
  console.assert(e === result[i])  
})