Be Careful , Perhaps , you have a class and a subclass .
<div id='id' class='myclass mysubclass' >dfdfdfsdfds</div>
If you use previous solutions , you will have :
myclass mysubclass
So if you want to have the class selector, do the following :
var className = '.'+$('#id').attr('class').split(' ').join('.')
and you will have
.myclass.mysubclass
Now if you want to select all elements that have the same class such as div above :
var brothers=$('.'+$('#id').attr('class').split(' ').join('.'))
that means
var brothers=$('.myclass.mysubclass')
OR can be implemented with vanilla javascript in 2 lines:
const { classList } = document.querySelector('#id');
document.querySelectorAll(`.${Array.from(classList).join('.')}`);