I found that there are many frameworks will check the duplicate class name before adding the new class name on the element that i think will slow down the performance.
There is nothing "wrong" with having a duplicate class name, it's just redundant. There's probably a small performance impact, but that'll only really make much difference if you have a lot of duplication.
Also, preventing duplicates just helps to keep things tidy.
ES6 provides a convenient API for adding and removing and testing class-names of DOM-elements: classList.add(name) and classList.remove(name) and classList.contains(name).
In the ES6 context the question could perhaps be rephrased as:
"If I use .classList.add() to add the same className more than once, and then want to remove that class with classList.remove(), do I need to call classList.remove() multiple times?
Luckily the answer seems to be a single call to .classList.remove() is enough to remove a given class no matter how many times you have added it.
I couldn't find an easy answer to this by Google so I wrote the following test to tell me that is how it behaves. Calling the below function with DOM-element as argument does NOT throw an error:
function testClassListRemove (dem)
{
dem.classList.add ("hello");
dem.classList.add ("hello");
dem.classList.add ("hello");
ok (dem.classList.contains ("hello") );
dem.classList.remove ("hello");
ok (! dem.classList.contains ("hello") );
function ok (b)
{ if (! b)
{ throw new Error ('not ok ');
}
}
}
Semantic UI heavily uses attribute selectors
like here
.ui.grid [class*="left floated"].column {
margin-right: auto;
}
.ui.grid [class*="right floated"].column {
margin-left: auto;
}
If you want margin-left
and margin-right
to have the auto value
, you must have a duplicate of the classname segment floated
(ex. <div class="left floated right floated">Lorem</div>
)