$('tr:even').addClass('alt');

why does $('tr:even').addClass('a开发者_Go百科lt'); select from the 1st row and $('tr:nth-child(even)').addClass('alt'); selects from the 2nd row ?

It's because :even gets even elements using 0-based indexing and :nth-child() uses 1-based indexing.

maybe the selector looks at the index and nth-child(even) looks at the length? Seems like an oversight.

got it! :odd and :even selectors use Javascript native zero based numbering. so the first row counts as 0(even)