$ (& #39; # checkbox & #39;). attr (& #39; checked & #39;); Return checked or undefined Solution

$ ('# Checkbox'). attr ('checked'); The returned value is checked or undefined, not true or false. The solution to this problem is as follows:

In versions earlier than JQ1.6, We will write our Code as follows:

<Input type = 'checkbox' id = 'cb '/> <script> // obtain whether to select var isChecked = $ (' # cb '). attr ('checked'); // you can specify $ ('# cb '). attr ('checked', true); </script>

This is done before JQ1.6, but when we upgrade JQ1.6 to a later version, the problem arises. At this time, we will find:
$ ('# Cb'). attr ('checked'); The returned value is checked or undefined, not true or false. In addition, the checked attribute has been initialized during page initialization and will not change as the status changes. Therefore, if the checkbox is selected at the beginning, the returned value is checked. If it is not selected at the beginning, the returned value is undefined.

<Input type = 'checkbox' id = 'cb '/> <script> // obtain whether to select var isChecked = $ (' # cb '). prop ('checked'); // or var isChecked = $ ('# cb '). is (": checked"); // sets the selected $ ('# cb '). prop ('checked', true); </script>

The reason is analyzed. You can understand it as follows:
It distinguishes "attributes" from "Features". attributes refer to "name, id", and so on, and features refer to "selectedIndex, tagName, nodeName.
After JQ1.6, attributes can be obtained through the attr method and features can be obtained through the prop method.

$("#cb").attr("tagName"); //undefined $("#cb").prop("tagName"); //INPUT