make element
const myElem = document.createElement('span');
Attributes like classes and the id can be set as well
myElem.classList.add('foo');
myElem.id = 'bar';
myElem.dataset.attr = 'baz';
Can attach to the body
document.body.appendChild(myElem);
After setting the tag will look like this:
<span class="foo" id="bar" data-attr="baz"></span>
changing existing elements
document.querySelector('.class'); // get first element with class of "class"
document.querySelector('#id'); // select the first element with the id of id
document.querySelector('[data-other]'); // select the first element with the data-other attribute
document.querySelectorAll('.multiple'); // return array of all element with the 'multiple' class