Though you have proposed a JavaScript solution for your problem (displaying a textbox when a checkbox is checked), this problem could be solved just by css. With this approach, your form works for users who have disabled JavaScript.

Assuming that you have the following HTML:

<label for="show_textbox">Show Textbox</label>
<input id="show_textbox" type="checkbox" />
<input type="text" />

You can use the following CSS to achieve the desired functionality:

 #show_textbox:not(:checked) + input[type=text] {display:none;}

For other scenarios, you may think of appropriate CSS selectors.

Here is a Fiddle to demonstrate this approach.

1）如果您的HTML标记是：

<input type="checkbox"  />

attr used:

$(element).attr("checked"); // Will give you undefined as initial value of checkbox is not set

If prop is used:

$(element).prop("checked"); // Will give you false whether or not initial value is set

2) If your HTML markup is:

 <input type="checkbox"  checked="checked" />// May be like this also  checked="true"

attr used:

$(element).attr("checked") // Will return checked whether it is checked="true"

Prop used:

$(element).prop("checked") // Will return true whether checked="checked"

我的方法是：

if ( $("#checkbox:checked").length ) {       
    alert("checkbox is checked");
} else {
    alert("checkbox is not checked");
}

这为我工作：

$get("isAgeSelected ").checked == true

isAgeSelected控件的ID 在哪里。

另外，@ karim79的答案也很好。我不确定测试时错过了什么。

注意，这是使用Microsoft Ajax而不是jQuery的答案

从jQuery 1.6开始，的行为jQuery.attr()已更改，鼓励用户不要使用它来检索元素的选中状态。相反，您应该使用jQuery.prop()：

$("#txtAge").toggle(
    $("#isAgeSelected").prop("checked") // For checked attribute it returns true/false;
                                        // Return value changes with checkbox state
);

其他两种可能性是：

$("#txtAge").get(0).checked
$("#txtAge").is(":checked")

我正在使用此，这绝对正常：

$("#checkkBoxId").attr("checked") ? alert("Checked") : alert("Unchecked");

注意：如果选中此复选框，它将返回true，否则未定义，因此最好检查“ TRUE”值。