基础JS 闰年 数字范围 当月天数

**判断是否是闰年**
能被4整除不能被100整除
二选一
能被400整除

判断数字的范围

判断当月的天数
注意从页面的输入框获取到的内容时字符数据，根据实际情况选择是否转换成数值

利用了case的穿透简化的重复代码

<input type="text" id="txt"><br>
    <input type="button" id="btn1" value="判断平闰年">
    <input type="text" id="res1" readonly><br>


    <input type="button" id="btn2" value="判断数字的范围">
    <input type="text" id="res2" readonly><br>


    <input type="button" id="btn3" value="判断当月份天数">
    <input type="text" id="res3" readonly><br><br>

 var txt = document.getElementById("txt");
    var btn1 =document.getElementById("btn1");
    var res1 = document.getElementById("res1")
    

    btn1.onclick = function(){
    
    
        var y = txt.value;
        if(y%4===0 && y/100!==0 || y%400===0){
    
    
            res1.value = y+ "是闰年";
        }
        else{
    
    
            res1.value = y + "是平年";
        }
    }

var btn2 = document.getElementById("btn2");
    var res2 = document.getElementById("res2");

    btn2.onclick = function(){
    
    
        var n = Number(txt.value);
        if( n > 0){
    
    
            res2.value = n + "大于0";
        }
        else if(n === 0){
    
    
            res2.value = n + "等于0";
        }
        else if(n < 0){
    
    
            res2.value = n + "小于0";
        }
    }

 var btn3 = document.getElementById("btn3");
    var res3 = document.getElementById("res3");

    btn3.onclick = function(){
    
    
        var m = Number(txt.value)
        switch(m){
    
    
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                    res3.value = m + "月是31天";
                    break;
        case 2:
            res3.value = m + "月是28天";
            break;
        
        case 4:
        case 6:
        case 9:
        case 11:
            res3.value = m + "月是30天";
            break;

            default:
            res3.value = "输入错误'" + m +"''";
        }
    }