JavaScript

const list = [
    "../images/img1.png",
    "../images/img2.png",
    "../images/img3.jpeg",
];

const img = document.getElementById('main');

const [leftArrow,rightArrow] = document.getElementsByTagName('span');

const div = document.getElementById('photo');

let imgN = 0;

document.addEventListener('DOMContentLoaded',function(){
    
    img.src = list[imgN];

    const p = document.createElement('p');
    p.classList.add('addClass');
    div.insertBefore(p,img.nextElementSibling);
    p.textContent = `${imgN + 1} / ${list.length}`;

    rightArrow.addEventListener('click',function(){
        imgN ++;
        
        
        //ここから追加
        if(imgN >= list.length){
            imgN = 0;
        }
        //ここまで追加
        
        
        img.src = list[imgN];
    
        p.textContent = `${imgN + 1} / ${list.length}`;
    });
    leftArrow.addEventListener('click',function(){
        imgN --;
        
        
        //ここから追加
        if(imgN < 0){
            imgN = list.length - 1;
        }
        //ここまで追加
        
        
        img.src = list[imgN];
    
        p.textContent = `${imgN + 1} / ${list.length}`;
    
    });
     
});