jQuery - Why .att doesn't replace the attribute value?

Based on the manual as follows:

http://api.jquery.com/attr/ 
attr( attributeName, value ) 

I should be able to replace the attribute value of a selected element.

             <div id="keynote1Fld" class="row ">
                <label for="keynote1" class="labeloptional">Topic 1</label>
                <div class="collection">
                 <input type="text" class="largetextfield" maxlength="96" size="96" value="" id="keynote1" name="keynote1" />
          开发者_StackOverflow      </div>
             </div>

I need to replace the class value of label for keynote1 from labeloptional to label.

Here is what have done:

$("label[for=keynote1]").att('class', 'label');

However, the class value for label of keynote1 still is 'labeloptional' after the execution of the above statement.

why?

Thank you

You need to use attr not att.

However, if you're changing classes, you shouldn't be using attr().

What you should do is use removeClass() and addClass().

$('label').removeClass('oldClass').addClass('newClass');

That way, you won't be overwriting any other classes that might be assigned to a tag.