jquery选择器问题：仅在隐藏所有元素时执行某些操作 [英] jquery selector question: do something only if all elements are hidden

问题描述

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。这似乎没有办法。有办法吗？

  if（$（'some_ul li：hidden'））{
 // do只有当给定ul中的所有'li'都被隐藏时才会出现问题
} 
  

解决方案

检查返回元素的长度属性。

  if（！$（'＃some_ul li：visible） '）。length）{
 //只有当给定ul中的所有'li'都被隐藏时才做某事
} 
  

编辑：隐藏更改为可见

---

编辑：澄清使用 .length 属性进行布尔测试。 请参阅此答案底部的进一步说明。

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

！为您提供其真/假值的负数。 / p>

所以如果 $（'＃some_ul li：visible'）。length 找到0个可见元素，那就是与返回 false 相同。

当您放置！在它后面，它的布尔值反转为 true 。因此，如果找不到可见元素， if（）中的代码将会运行。

与完成相同：

  if（$（'＃some_ul li： visible'）。length == 0）{
 //只有当给定ul中的所有'li'都被隐藏时才做某事
} 
  

...它取数值 $（'＃some_ul li：visible'）。length 然后转动它使用 == 运算符将其转换为布尔值。

---

To使用：隐藏您需要首先获得总长度，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）{
 //只有当给定ul中的所有'li'都被隐藏时才做某事
 } 
  

---

编辑：在回答您的评论时，为了澄清，在测试中有几个值等于true / false。

以下是一些等于false的值的示例：

  var someVariable; // someVariable未定义，等于false 
 
 var someVariable =''; // someVariable是一个空字符串，等于false 
 
 var someVariable = false; // someVariable是一个布尔值false，等于false 
 
 var someVariable = 0; // someVariable为数字零，等于false 
 
 var someVariable = Number（'a'）; // someVariable是NaN（非数字），等于false 
 
 var someVariable = null; // someVariable为null，等于false 
  

以下是一些等于true的示例：

  var someVariable =false; // someVariable是一个字符串，等于true 
 
 var someVariable = 123; // someVariable是一个大于0的数字，等于真
 
 var someVariable = -123; // someVariable是一个小于0的数字，等于true 
 
 var someVariable = true; // someVariable是一个布尔值true，等于true 
 
 var someVariable =！false; // someVariable是一个否定值，
 //否则为false，等于true 
  

如果您对任何值的有效布尔值感到好奇，请在它之前放置 !! 。

  alert（!! 123）; //警告为真
  

第一个！转换与有效布尔值相反的值。第二个！将其转换回有效的布尔值。

例如：

  var a; //未定义的变量
 
 alert（a）; //提醒undefined，逻辑上等于'false'
 
 alert（！a）; //提示未定义的布尔对面，即'true'
 
 alert（!! a）; //警告转换器将布尔对面的'true'反馈回'false'
  

I need to do something only if all the li's in a given ul are hidden.. this doesn't seem to do the trick. is there a way?

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

解决方案

Check the length property of the elements returned.

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

EDIT: changed hidden to visible

---

EDIT: Clarification on the use of the .length property for a boolean test. Please see further explanation at the bottom of this answer.

$('#some_ul li:visible').length returns the number of elements found. Numeric values equate to true or false.

The ! gives you the negative of its true/false value.

So if $('#some_ul li:visible').length find 0 visible elements, that is just the same as returning false.

When you place ! behind it, it's boolean value is reversed to true. So if no visible elements are found, the code in your if() will run.

It is exactly the same as doing:

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

...which takes the numeric value of $('#some_ul li:visible').length and turns it into a boolean value using the == operator.

---

To use :hidden you would need to first get the total length, then compare it to the hidden length.

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
}

---

EDIT: In response to your comment, for clarification, there are several values that will equate to true/false in a test.

Here are some examples of values that equate to false:

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

Here are some examples that equate to true:

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

If you're curious about the effective boolean value of any value, place !! before it.

alert( !!123 );   // Alerts true

The first ! converts the value to the opposite of its effective boolean value. The second ! converts the it back to its effective boolean value.

For example:

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'

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