jquery选择器问题:仅在隐藏所有元素时执行某些操作

Question

只有当给定ul中的所有li都被隐藏时,我才需要做一些事情.这似乎并没有成功.有办法吗？

if ($('#some_ul li:hidden')) {
  // do something only if all 'li's in a given ul are hidden
}

Answer 1

检查返回的元素的length属性.

if ( !$('#some_ul li:visible').length ) {
  // do something only if all 'li's in a given ul are hidden
}

编辑:将隐藏更改为可见

编辑:澄清使用.length属性进行布尔测试.请参阅本答复底部的进一步说明.

$('#some_ul li:visible').length返回找到的元素数.数字值等于true或false.

它!给你的真/假值否定.

因此,如果$('#some_ul li:visible').length找到0个可见元素,那就与返回相同false.

当你放在!它后面时,它的布尔值反转为true.因此,如果找不到任何visible元素,您的代码if()就会运行.

它与完成相同:

if ( $('#some_ul li:visible').length == 0 ) {
  // do something only if all 'li's in a given ul are hidden
}

...取数字值$('#some_ul li:visible').length并使用==运算符将其转换为布尔值.

要使用,:hidden您需要先获取总长度,然后将其与隐藏长度进行比较.

var total = $('#some_ul li');

if ( $('#some_ul li:hidden').length == total ) {
  // do something only if all 'li's in a given ul are hidden
}

编辑:为了回应您的评论,为了澄清,有几个值将等同于测试中的真/假.

以下是一些等于false的值的示例:

var someVariable;  // someVariable is undefined, equates to false

var someVariable = '';   // someVariable is an empty string, equates to false

var someVariable = false; // someVariable is a boolean false, equates to false

var someVariable = 0;  // someVariable is number zero, equates to false

var someVariable = Number('a');  // someVariable is NaN (Not a Number), equates to false 

var someVariable = null;   // someVariable is null, equates to false

以下是一些等同于true的示例:

var someVariable = "false";   // someVariable is a string, equates to true

var someVariable = 123;   // someVariable is a number greater than 0, equates to true

var someVariable = -123;   // someVariable is a number less than 0, equates to true

var someVariable = true;   // someVariable is a boolean true, equates to true

var someVariable = !false;   // someVariable is a negated value that would
                             //    otherwise be false, equates to true

如果您对任何值的有效布尔值感到好奇,请放在!!它之前.

alert( !!123 );   // Alerts true

第一个!将值转换为其有效布尔值的相反值.第二个!将它转换回有效的布尔值.

例如:

var a;  // an undefined variable

alert( a );   // alerts undefined, logically equates to 'false'

alert( !a );   // alerts the boolean opposite of undefined, which is 'true'

alert( !!a );   // alerts the converts the boolean opposite 'true' back to 'false'