Refactored Code: 

// function demo(food, hay, cover, weight) {
function demo(input) {
    // exam input comes as array
    let [food, hay, cover, weight] = input;

    let foodGr = food * 1000;
    let hayGr = hay * 1000;
    let coverGr = cover * 1000;
    let weightGr = weight * 1000;
    let days = 1;

    while (days <= 30) {
        foodGr -= 300;
        if (days % 2 === 0) {
            hayGr -= (foodGr * 5) / 100;
        }
        if (days % 3 === 0) {
            coverGr -= weightGr * 0.3333;
        }
        // days++ used at the end of the cycle
        days++;
    }
    let foodKg = foodGr / 1000;
    let hayKg = hayGr / 1000;
    let coverKg = coverGr / 1000;

    // foodKg, hayKg, coverKg in validations
    if (foodKg >= 0 && hayKg >= 0 && coverKg >= 0) {
        console.log(`Everything is fine! Puppy is happy! Food: ${foodKg.toFixed(2)}, Hay: ${hayKg.toFixed(2)}, Cover: ${coverKg.toFixed(2)}.`)
        // || must be used for other case
    } else if (foodKg < 0 || hayKg < 0 || coverKg < 0) {
        console.log('Merry must go to the pet store!') // '!' must be used in message
    }

}

;-)