One; one one; two one; one two, one one; ...

one-one.js

// [1]
// [1,1]
// [2,1]
// [1,2,1,1]
// [1,1,1,2,2,1]
// [3,1,2,2,1,1]
// [1,3,1,1,2,2,2,1]
// [1,1,1,3,2,1,3,2,1,1]
// [3,1,1,3,1,2,1,1,1,3,1,2,2,1]
// [1,3,2,1,1,3,1,1,1,2,3,1,1,3,1,1,2,2,1,1]

const max = 10;
let largest = 0;

function append(arr, num) {
  if (num > largest) {
    largest = num;
  }
  arr.push(num);
}

let line = [1];
console.log(JSON.stringify(line));
for (let i = 1; i < max; i++) {
  const newLine = [];
  let current = line[0];
  let repeat = 0;
  for (let j = 1; j < line.length; j++) {
    if (line[j] === current) {
      repeat++;
    }
    else {
      append(newLine, repeat + 1);
      append(newLine, current);
      current = line[j];
      repeat = 0;
    }
  }
  append(newLine, repeat + 1);
  append(newLine, current);
  console.log(JSON.stringify(newLine));
  line = newLine;
}

console.log(`\nlargest number: ${largest}\n`);