jquery和js如何判断checkbox是否选中

<h3>请选择你爱吃的水果：</h3>
<form id="checkboxform" method="post" action="">
<input type="checkbox" name="bjjb" value="1">西瓜<br>
<input type="checkbox" name="bjjb" value="2">草莓<br>
<input type="checkbox" name="bjjb" value="3">葡萄<br>
<input type="checkbox" name="bjjb" value="4">火龙果<br>
<input type="checkbox" name="bjjb" value="5">苹果<br>
<a href="javascript:void(0);" onclick="selectAll()">全选</a>
<a href="javascript:void(0);" onclick="selectNone()">全不选</a>
<a href="javascript:void(0);" onclick="selectInvert()">反选</a>
<a href="javascript:void(0);" onclick="selectOne()">必须选择一个</a>
</form><script>
//全选
function selectAll() {
var names = document.getElementsByName("bjjb");
for (var i = 0; i < names.length; i++) {
names[i].checked = true;
}
}

//全不选
function selectNone() {
var names = document.getElementsByName("bjjb");
for (var i = 0; i < names.length; i++) {
names[i].checked = false;
}
}

//反选
function selectInvert() {
var names = document.getElementsByName("bjjb");
for (var i = 0; i < names.length; i++) {
if (names[i].checked) {
names[i].checked = false;
} else {
names[i].checked = true;
}
}
}

//必须选择一个
function selectOne() {
var names = document.getElementsByName("bjjb");
var flag = false; //标记判断是否选中一个
for (var i = 0; i < names.length; i++) {
if (names[i].checked) {
flag = true;
break;
}
}
if (!flag) {
alert("请最少选择一项！");
return false;
}
}</script>