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**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

利用了case的穿透简化的重复代码

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 +"''";        }    }