16 内置对象

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内置对象(系统自带)

Math、Date、Array、Object......

Math 数学内置对象

Math.PI 圆周率

console.log(Math.PI); //π

2.

Math.max( ) 最大值

Math.min( ) 最小值

console.log(Math.max(10,20,3,6,9)); //20
console.log(Math.min(10,20,3,6,9)); //3

3.

Math.abs( ) 绝对值

console.log(Math.abs('-1')); //1
console.log(Math.abs('string')); //NaN
console.log(Math.abs()); //

4.

Math.random( ) [0,1)之间的随机数

console.log(Math.random());  [0,1)之间的随机数
//获取0-5之间的随机数
console.log(Math.random()*5);
//获取0-5之间的整数
console.log(parseInt(Math.random()*5));
//获取1-5之间的数字
 console.log(parseInt(Math.random()*5 + 1)); //1-5

5.

Math.ceil( ) 向上取整

console.log(Math.ceil(12.3)); //13

6.

Math.floor( ) 向下取整

console.log(Math.floor(12.3)); //12

7.

Math.round( ) 四舍五入

console.log(Math.round(12.3)); //12

8.

Math.pow( ) 求指数次幂

console.log(Math.pow(2,4)); //16 2的4次方

9.

Math.sqrt( ) 求平方根

console.log(Math.sqrt(16)); //4

Date 时间戳

var dt = new Date(); //获取时间  年月日时分秒  系统自带
console.log(dt);
var year = dt.getFullYear(); //获取年
console.log(year); // 2022
var month = dt.getMonth() + 1; //获取月份要+1
console.log(month); //2
var day = dt.getDate(); //获取天
console.log(day); //24
var hours = dt.getHours(); //获取小时
console.log(hours); //21
var minute = dt.getMinutes(); //获取分钟
console.log(minute); 
var second = dt.getSeconds(); //获取秒
console.log(second);
var week = dt.getDay(); //获取周几
console.log(week); 

需求：定义一个函数，获取到年月日星期

function dateTime(){
    var dt = new Date(); //获取到全部时间
    var y = dt.getFullYear(); //获取年
    var m = dt.getMonth(); //获取月
    var d = dt.getDate(); //获取日
    var week = dt.getDay(); //获取星期
    switch(week){ //用switch来判断
        case 1 :
            week = "星期一";
        break;
        case 2 :
            week = "星期二";
        break;
        case 3 :
            week = "星期三";
        break;
        case 4 :
            week = "星期四";
        break;
        case 5 :
            week = "星期五";
        break;
        case 6 :
            week = "星期六";
        break;
        case 0 :
            week = "星期日";
        break;
    }
    return y + "年" + m + "月" + d + "天" + week; // 返回拼接
}
var result = dateTime();
console.log(result);

需求：随机点名

var arr = ["鸣人","佐助","春野樱","宁次","李洛克","天天","鹿丸","丁次","我爱罗","勘九郎","带土","卡卡西","凯","水门"];
var suiji = parseInt(Math.random() * arr.length);
alert(arr[suiji]);

需求：封装函数实现系统max最大值方法

function myMax(){
    var max = arguments[0]; //假设max为第一个
    for(var i = 0; i < arguments.length; i++){
       //判断
       if(max < arguments[i]){
         max = arguments[i];
       }
    }
    return max;
}
var result = myMax(10,20,30,100,50);
console.log(result); //100

需求：随机生成一个颜色(数字0-9，字母a-f)

function getColor(){
    var str = "#"; //颜色是以#开头的
    //定义一个数组
    var arr = ["1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"];
    for(var i = 0; i < 6; i++){ //颜色代码6位数
       var num = parseInt(Math.random() * arr.length); //取整
       str += arr[num]; 
    }
    return str;
}
var result = getColor();
console.log(result);