Question

$(".wrap table tr:first").addClass("tr-top");

it works for the first table, but i have many tables under the div .wrap.
what should i do? thanks!

Answer 1

In your example, this line returns the first in the set of all tr elements found.

$(".wrap table tr:first").addClass("tr-top");  // First <tr> of all that are found

So if you have 3 table elements, it will only return the first tr from the first table, since that will be the first tr element matched.

If you want the first tr for each table, you need first-child:

$(".wrap table tr:first-child").addClass("tr-top");  // First <tr> for each <table>

…which will return each tr that is a first child of its parent.

http://api.jquery.com/first-selector/

http://api.jquery.com/first-child-selector/