nth 类型元素选择器

当我们要一组 class 同名，或者连续的一组元素的其中，或者某种规律的元素单独样式的时候，不妨看看这类的元素选择器。

nth-child(n)、 nth-last-child(n) 、nth-of-type(n) 都是用来匹配父元素内部子元素的。不过也有些区别：
nth-child 按照个数来算；
nth-of-type 按照类型来计算；
nth-last-child(n) 从最后子元素往前开始计算。

.item:nth-child(2n+1){

}
.item:nth-of-type(n){

}
.item:nth-last-child(2n){

}

n 从  开始计数的正整数。

选择 demo 内第 3 个子元素背景为红色。

.item{
    width: px;
    height: px;
    text-align: center;
    line-height: px;
    border: px solid #ccc;
    background: #f2f2f2;
}
.item:nth-child(3){
    background: red;
}

图：

<!DOCTYPE html>
<html lang="en">
<head>
    < charset="UTF-8">
    < name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .item{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }
        .item:nth-child(3){
            background: red;
        }
    </style>
</head>
<body>
    <div class="demo">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="item">4</div>
    </div>
</body>
</html>

.item{
    width: px;
    height: px;
    text-align: center;
    line-height: px;
    border: px solid #ccc;
    background: #f2f2f2;
}
.item:nth-last-child(2){
    background: red;
}

图

<!DOCTYPE html>
<html lang="en">
<head>
    < charset="UTF-8">
    < name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .item{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }
        .item:nth-last-child(2){
            background: red;
        }
    </style>
</head>
<body>
    <div class="demo">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="item">4</div>
    </div>
</body>
</html>

.item{
    width: px;
    height: px;
    text-align: center;
    line-height: px;
    border: px solid #ccc;
    background: #f2f2f2;
}
.item:nth-of-type(3){
    background: red;
}

图

<!DOCTYPE html>
<html lang="en">
<head>
    < charset="UTF-8">
    < name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .item{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }
        .item:nth-of-type(3){
            background: red;
        }
    </style>
</head>
<body>
    <div class="demo">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="item">4</div>
    </div>
</body>
</html>

<div class="demo">
    <p class="item">我是 p </p>
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
    <div class="item">4</div>
</div>
<div class="demo">
    <p class="item-2">我是 p </p>
    <div class="item-2">1</div>
    <div class="item-2">2</div>
    <div class="item-2">3</div>
    <div class="item-2">4</div>
</div>

   .demo{
           float: left;
       }
       .item,.item-2{
           width: px;
           height: px;
           text-align: center;
           line-height: px;
           border: px solid #ccc;
           background: #f2f2f2;
       }        
       .item:nth-of-type(3){
           background: red;
       }
       .item-2:nth-child(3){
           background: red;
       }

图

通过图我们就清楚的明白他们的差异了。
简述实例展现，通过实例分析他们两个的区别

<!DOCTYPE html>
<html lang="en">
<head>
    < charset="UTF-8">
    < name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .demo{
            float: left;
        }
        .item,.item-2{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }        
        .item:nth-of-type(3){
            background: red;
        }
        .item-2:nth-child(3){
            background: red;
        }
    </style>
</head>
<body>
    <div class="demo">
        <p class="item">我是 p </p>
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="item">4</div>
    </div>
    <div class="demo">
        <p class="item-2">我是 p </p>
        <div class="item-2">1</div>
        <div class="item-2">2</div>
        <div class="item-2">3</div>
        <div class="item-2">4</div>
    </div>
</body>
</html>

下面是让所有偶数的背景变红。

 .item{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }
        .item:nth-of-type(2n){
            background: red;
        }

图：

<!DOCTYPE html>
<html lang="en">
<head>
    < charset="UTF-8">
    < name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .item{
            width: px;
            height: px;
            text-align: center;
            line-height: px;
            border: px solid #ccc;
            background: #f2f2f2;
        }
        .item:nth-of-type(2n){
            background: red;
        }
    </style>
</head>
<body>
    <div class="demo">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
        <div class="item">4</div>
    </div>
</body>
</html>

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