jquery获得checkbox是否选中的状态

Concerning boolean attributes, consider a DOM element defined by the HTML markup

<input type="checkbox" checked="checked" />

, and assume it is in a JavaScript variable named

elem

:

elem.checked	true (Boolean) Will change with checkbox state
$(elem).prop("checked")	true (Boolean) Will change with checkbox state
elem.getAttribute("checked")	"checked" (String) Initial state of the checkbox; does not change
$(elem).attr("checked") (1.6)	"checked" (String) Initial state of the checkbox; does not change
$(elem).attr("checked") (1.6.1+)	"checked" (String) Will change with checkbox state
$(elem).attr("checked") (pre-1.6)	true (Boolean) Changed with checkbox state

注意：elem.getAttribute("checked")和$(elem).attr("checked");都是获得checkbox的初始的状态，并且不会因为你是否勾选而改变状态，所以判断checkbox是否勾选不能用这2个方法

According to the
W3C forms specification, the

checked

attribute is a boolean attribute, which means the corresponding property is
true if the attribute is present at all—even if, for example, the attribute has no value or is set to empty string value or even "false". This is true of all boolean attributes.

Nevertheless, the most important concept to remember about the

checked

attribute is that it does not correspond to the

checked

property.
The attribute actually corresponds to the

defaultChecked

property and should be used only to set the
initial value of the checkbox. The

checked

attribute value does not change with the state of the checkbox, while the

checked

property does. Therefore, the cross-browser-compatible way to determine if a checkbox is checked is to use the property:

最重要的概念是记住checked attribute不相当于checked property！checked attribute不会随着checkbox状态而改变，而checked property 会

if ( elem.checked )

if ( $(elem).prop("checked") )

if ( $(elem).is(":checked") )

The same is true for other dynamic attributes, such as

selected

and

value

参考：http://api.jquery.com/prop/