script.js

// alert("hello world");
var imageNumber = 1;
var imageName = "images/classProject-";
var mainImage = document.getElementById("main_image");

var nextButton = document.getElementById("next");
var prevButton = document.getElementById("prev");

console.log(mainImage.src, nextButton, prevButton);
//on click
function goNext() {
  if( imageNumber < 5 ) {
    imageNumber = imageNumber + 1;
    var newSrc = imageName + imageNumber + ".jpg";
    console.log(newSrc);
    mainImage.src = newSrc;
  }
}

function goBack() {
  if( imageNumber > 1 ) {
    imageNumber = imageNumber - 1;
    var newSrc = imageName + imageNumber + ".jpg";
    console.log(newSrc);
    mainImage.src = newSrc;
  }
}

nextButton.onclick = goNext;
prevButton.onclick = goBack;