is function wrong with :first, :last, :last-child

[tigerf]

I have some code that result is not match expectation.

html:

 <span>1</span><span>2</span><span>3</span>

script:

 $('span:eq(0)').is('span:first') // yes, it is true
 $('span:eq(2)').is('span:first') // expect false but true

 $('span:eq(2)').is('span:last') // yes, it is true
 $('span:eq(0)').is('span:last') // expect false but true

 $('span:eq(0)').is('span:first-child') // yes, it is true
 $('span:eq(2)').is('span:first-child') // yes, it is false

 $('span:eq(2)').is('span:last-child') // expect true but false
 $('span:eq(0)').is('span:last-child') // yes, it is false

BTW: I don't know what is different between :first and :first-child.

[dmethvin]

These are good things to explore with other users in a forum. Please don't file a bug report until you are sure it's a bug.

The .is() method takes its context from the collection of nodes it is given, similar to .filter(). (The docs note this relationship.) If the filter expression yields at least one element, .is() returns true.

Your examples are all selecting a single span, so :first and :last will be true. You aren't showing the complete markup, so I don't know whether the :*-child should return true or false.