img_gallery.js

const displayImgEl = document.querySelector('main div img');

// Bind an "onImgClick" function to all image elements found
const els = document.querySelectorAll('.thumb-bar img');
els.forEach((el => el.addEventListener('click', onImgClick));

function onImgClick(event) {
    // Get the current element which was clicked.
    // Could also use event.target too
    const el = event.currentTarget;

    // Get the image "src" attribute value
    const src = el.getAttribute('src');

    // Set the display element "src" attribute,
    // to use the src value of the element selected
    displayImgEl.setAttribute('src', src);
}