6.js

function part1(input, days) {
  let gens = [0,0,0,0,0,0,0,0,0]
  input.split(',').forEach((x) => gens[x]++)
  let zeroes = 0;
  for(let d=0;d<days;d++) {
    [zeroes, ...gens] = gens;
    gens.push(zeroes)
    gens[6] += zeroes
  }
  console.log(gens.reduce((a,b) => a+b))
}