Changement diaporama flèches du clavier solution 2

function diapoSuivante() {
affichage (diaporama ++);
}

function diapoPrecedente() {
affichage (diaporama --);
}
   document.addEventListener("keydown", function(touche){
    if(touche.keyCode === 37){
        return diapoPrecedente();
    }
    else if(touche.keyCode === 39){
        return diapoSuivante();
    }
    });



// Reste du script JS
var diaporama = 1;
affichage(diaporama);

function boutons(n) {
affichage(diaporama += n);
}
function affichage(n) {
var i;
var diapoImg = document.getElementsByClassName("diapo");
var diapo = document.getElementsByClassName("demo");
if (n > diapoImg.length) {diaporama = 1}
if (n < 1) {diaporama = diapoImg.length}
for (i = 0; i < diapoImg.length; i++) {

diapoImg[i].style.opacity = "0";
}
for (i = 0; i < diapo.length; i++) {
diapo[i].className = diapo[i].className.replace("numeros");
}
diapoImg[diaporama-1].style.opacity = "1";
diapo[diaporama-1].className += "numeros";
}