document.querySelector(`.class/#id/tag`) - HTML dagi istalgan elementni JavaScript da tanlaydi
let title = document.querySelector('#title');
let button = document.querySelector('.btn');
addEventListener() - tanlangan elementda biror hodisa ro'y berganda, biror ishni bajaradi
button.addEventListener('click', () = > { console.log('welcome'); });
element.classList.add('class') - elementga class qo'shadi
element.classList.remove('class') - elementdan class ni o'chiradi
element.classList.contains('class') - elementda ma'lum class borligini tekshiradi, bor bo'lsa true qaytaradi 
const btn = document.querySelector('.btn1');
const image = document.querySelector('.image');

btn.addEventListener('click', () => {
    if(image.classList.contains('changeimage')){
        image.classList.remove('changeimage');
    } else {
        image.classList.add('changeimage');
    }
});
Yuqoridagi kod knopka bosilganda rasm ning changeimage nomli class i bor yoki yo'qligini tekshiradi, agar chabgeimage class i bor bo'lsa changeimage class ini o'chiradi, yo'q bo'lsa qo'shadi

photo