Basic JS Leap Year Number Range Number of Days in the Month

**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

Determine the range of numbers

Determine the number of days in the current month.
Pay attention to the character data obtained from the input box of the page, and choose whether to convert it into a numeric value according to the actual situation.

Use case penetration to simplify repetitive code

<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 +"''";
        }
    }