very interesting!

fishies.js

const days = 256;
const cycle = 6;

// Build a new 9 Length Array.
// Each array element will represent all fish
// with that remaining timer
const fishes = Array(cycle + 3).fill(0);

// load inputs into the index representing their timer
inputs.forEach(_ => fishes[_]++);

for (let i = 0; i < days; i++) {
    // How many fish have a timer of 0
    const newFish = fishes[0];
    
    // Move the front element to the back
    // think of these like the babies. because
    // they are new they must have a timer of 8
    // this is represented by placing them at index 8.
    fishes.push(fishes.shift());
    
    // think of this as reseting the timer for the fish that
    // reproduced. 6 days from now they reproduced they will
    // reproduce again. this is represented by them being added to
    // the amount of fish reproducing in 6 days.
    fishes[cycle] += newFish;
}

// Add all the fish together
let sum = 0
fishes.forEach(fishWithTimer => sum += fishWithTimer)
console.log(sum)