:nth-child(index/even/odd/equation)
匹配其父元素下的第N个子或奇偶元素
‘:eq(index)’ 只匹配一个元素，而这个将为每一个父元素匹配子元素。:nth-child从1开始的，而:eq()是从0算起的！
可以使用:
nth-child(even)
:nth-child(odd)
:nth-child(3n)
:nth-child(2)
:nth-child(3n+1)
:nth-child(3n+2)
Matches the nth-child of its parent.
While ‘:eq(index)’ matches only a single element, this matches more then one: One for each parent. The specified index is one-indexed, in contrast to :eq() which starst at zero.
返回值
Array<Element>
参数
index (Number) : 要匹配元素的序号，从1开始
示例
在每个 ul 查找第 2 个li
HTML 代码:
<ul>
<li>John</li>
<li>Karl</li>
<li>Brandon</li>
</ul>
<ul>
<li>Glen</li>
<li>Tane</li>
<li>Ralph</li>
</ul>
jQuery 代码:
$(“ul li:nth-child(2)”)
结果:
[ <li>Karl</li>, <li>Tane</li> ]
——————————————————————————–
:first-child
匹配第一个子元素
‘:first’ 只匹配一个元素，而此选择符将为每个父元素匹配一个子元素
Matches the first child of its parent.
While ‘:first’ matches only a single element, this matches more then one: One for each parent.
返回值
Array<Element>
示例
在每个 ul 中查找第一个 li
HTML 代码:
<ul>
<li>John</li>
<li>Karl</li>
<li>Brandon</li>
</ul>
<ul>
<li>Glen</li>
<li>Tane</li>
<li>Ralph</li>
</ul>
jQuery 代码:
$(“ul li:first-child”)
结果:
[ <li>John</li>, <li>Glen</li> ]
——————————————————————————–
:last-child
匹配最后一个子元素
‘:last’只匹配一个元素，而此选择符将为每个父元素匹配一个子元素
Matches the last child of its parent.
While ‘:last’ matches only a single element, this matches more then one: One for each parent.
