js实现当月首末日期

<script type="text/javascript">
function show(){
var today = new Date();
var year = today.getFullYear();
var month = today.getMonth() + 1;
var lastDay = new Date(year, month, 0); //次月第一天的前一天
        alert(
            "当前月第一天：" + year + "-" + month + "-01" + "\n" + 
            "当前月最后一天：" + year + "-" + month + "-" + lastDay.getDate()
        );
    }
</script>
<input type="button" onclick="show()" value="Show"/>