I have a slider. When clicking next button image is loading.

var t=3
        function next_btn(){
            
            t+=3;
            var imgEl = document.querySelectorAll('.lazy');
            for (let a = 0; a < t; a++) {
                if(imgEl[a].getAttribute('data-original')) {
                imgEl[a].setAttribute('src',imgEl[a].getAttribute('data-original'));
                imgEl[a].removeAttribute('data-original'); 
            }
            }
            

        }

but when clicking previous button image is not loading.

function previous_btn(){
     
            var imgEl = document.querySelectorAll('.lazy');
            
            for (let a =imgEl.length; a >3; a--) {
                if(imgEl[a].getAttribute('data-original')) {
                imgEl[a].setAttribute('src',imgEl[a].getAttribute('data-original'));
                imgEl[a].removeAttribute('data-original'); 
            }
            }
            
           
        }

I tried other things but it didn't work. How can I fix?