How to add property to existing class

Question

I have got a CSS class like so:

.simpleClass {
    width: 25px;
}

And I have a matching element:

Answer 1

The property will be added to div if it already not exist in css class. Hence, it's always better to switch css class with new properties. Eg.

$( ".referenceClass" ).switchClass( "oldClass", "newClass");