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# 1. js获取某年某月的天数

## 实现代码

```javascript
/**
 * 获取某年某月的天数
 * @param {Number} year  年
 * @param {Number} month 月份
 */
function getDays(year, month) {
    return (new Date(year, month, 0)).getDate();
}
console.log(getDays(2019, 9));//30
```

## 代码解释

```javascript
new Date(year, month, day);
```

Date 对象提供的构造函数之一,初始化对象为指定的年月日

**注:** 月是从0开始计算(即:0代表1月),日是(1-31)

```javascript
new Date(2019,9,0)
```

这里day传入的0是因为如果日的范围是(1-31),传入0 就像前寻一天即等价于

```javascript
new Date(2019,8,30)//2019-09-30
```

```javascript
(new Date()).getDate()
```

getDate()：返回实例对象对应每个月的几号

所以

```javascript
(new Date(2019, 9, 0)).getDate();//30
```


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