In the snippet below, I'm using the contains() method to check if a class that starts with banner__ exists in the document. If I define the class explicitly, then I can get a true response. But there are various classes that could exist in the document that start with banner__.
How do I check if a class that starts with banner__ exists?
const banner = document.querySelector('.banner');
const isBanner = banner.classList.contains('banner__');
if (isBanner) {
console.log('Exists')
} else {
console.log('Does not exist')
}<div class="banner banner__holiday">Banner</div>Convert the class list to an array, then use .some() to test if any of them begin with what you want.
const banner = document.querySelector('.banner');
const isBanner = Array.from(banner.classList).some(c => c.startsWith('banner__'));
if (isBanner) {
console.log('Exists')
} else {
console.log('Does not exist')
}<div class="banner banner__holiday">Banner</div>