JavaScript loop running on console but not on code

I’m trying to make a a JS forEach loop that is supposed to run after the DOM is fully loaded, but for some reason it won’t run on code but when I put the same loop on console it runs fine.

window.onload = (event) => {

    var checkboxes = document.getElementsByName("plates")
    var labelPlate = document.getElementsByName("labelPlate")

    checkboxes.forEach((i, index) => {
        if (i.checked) {

            labelPlate[index].classList.add("active")
        } else {
            labelPlate[index].classList.remove("active")
        }
    })
};

Loop is supposed to run.