Complete mastery of JavaScript's Date object

This article brings you relevant knowledge about javascript, which mainly introduces related issues about date objects. The Date object is a constructor, so we must go through object instantiation, that is It can only be used after passing new. Let’s take a look at it. I hope it will be helpful to everyone.

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Next, we will explain the second type of common built-in object in JS - Date object

#Date object is different from Math object. Math object is not a constructor and can be used directly. The Date object is a constructor, so we must instantiate the object, that is, before it can be used after passing new. The Date object is mostly used to deal with time and date issues in development.

Date object instantiation:

var date=new Date();

Date object instantiation can have parameters or no parameters. The output without parameters is the current system The standard time at that time, if there are parameters, we can output the time we want to display

1: Instantiation without parameters

It will be displayed after instantiation without parameters The time and date of the current system

var date=new Date();  //没有参数

console.log(date);  //会输出当前时间

2: Instantiation with parameters

Instantization with parameters is divided into two The two types are numeric type and string type . The following are examples of the two types respectively

1. Instantiation of numeric parameters:

var date=new Date(2021,1,18);  //数字型参数

console.log(date);

You can see that the parameter we input is January, but the output result is Feb (February). The numerical output will be smaller than the month we input. One month older.

2. Instantiation of string parameters:

var date=new Date('2021-1-18 12:56:00');  //字符串型参数

console.log(date);

The parameter is January, and the output result is also January. Therefore, string parameters are used more often than numeric parameters.

3: Formatting year, month and day

We already know that the Math object has many properties and methods that can be used directly, and the same is true for the Date object, which can also be used after instantiation There are many methods, and there are three commonly used methods to format the year, month and day

getFullYear() Output the current year

getMonth() Output the current month (It should be noted that the output month is 1 less than the actual month, and the output real month should be increased by 1)

getDate() Output the current day

##getDay() Output the current day of the week (Corresponding numbers from Monday to Sunday: 1 2 3 4 5 6 0)

var Date=new Date();

 console.log(Date.getFullYear());  //输出当前年份

 console.log(Date.getMonth() + 1);  //输出结果为当前月份的前一月，要手动加1才能返回当前月份

 console.log(Date.getDate());  //输出当前几号

 console.log(Date.getDay());  //输出当前周几

Tuesday, January 18, 2021, you can do the following

var arr=['星期天','星期一','星期二','星期三','星期四','星期五','星期六'];

var Date=new Date();

var year=Date.getFullYear(); 

var month=Date.getMonth() + 1; 

var date=Date.getDate();

var day=Date.getDay();  

 console.log(year + '年' + month + '月' + date + '日' + arr[day]);

Four: Formatting hours, minutes and seconds

getHours() Output the current hour

getMinutes() Output the current minutes

getSeconds()    输出当前秒

var Date=new Date();  

console.log(Date.getHours());  //输出当前小时

console.log(Date.getMinutes());  //输出当前分钟

console.log(Date.getSeconds());  //输出当前秒

 输出连续格式时分秒：

将其封装在了函数内，并利用三元运算符将不足10的数字补0，符合平常看时间的习惯

function time()

{

var time=new Date();

var h=time.getHours();

h = h<10 ? '0'+h : h;

var m=time.getMinutes();

m = m<10 ? '0'+m : m;

var s=time.getSeconds();

s = s<10 ? '0'+s : s;

return h+'时'+m+'分'+s+'秒';

}

console.log(time());

五：获取当前时间总毫秒数（时间戳）

这里所说的总毫秒数是指当前时间距离1970年1月1日的总毫秒数，共有四种方法可以表示

valueOf()

getTime()

var date=new Date();

console.log(date.valueOf());   

console.log(date.getTime());

 或者使用另外一种简便写法 var date=+new Date();返回的就是总毫秒数

var date=+new Date();

console.log(date);

 以及H5新增加的一种方法，这个方法不需要实例化对象即可获得，更为简便

console.log(Date.now());

六：倒计时案例（重点）

在日常开发中很多地方都会用的到倒计时，例如淘宝，京东的双十一秒杀倒计时等，我们如何也写出一个倒计时效果呢，我们首先会想到刚才学到的获取当前时间，再减去我们设置好的时间即可，但是我们获取到的标准时间很可能会出现减去之后是负数的情况（例如02-12）这怎么办呢？于是我们的时间戳便有利用价值了，时间戳即刚才讲到过的总毫秒数，这个时间是永远不会重复的，对此我们可以使用设置好的总毫秒数减去当前的总毫秒数，在进行一系列单位换算，就可以得到一个简单的倒计时案例了，首先我们需要熟练记清楚单位换算之间的关系：

1秒=1000毫秒

天数=秒数/60/60/24

小时数=秒数/60/60%24

分钟数=秒数/60%60

秒数=秒数%60

对于无法整除的秒数，我们利用 parseInt() 方法取整即可，有了这样一个换算关系，我们就可以轻松地完成这个倒计时案例了

function count(time)

{

    var nowtime=+new Date(); //得到当前时间

    var aimtime=+new Date(time);  //得到目标时间（结束时间）

    var times=(aimtime-nowtime)/1000;  //得到倒计时时间差（毫秒） 除1000得到秒

    var d=parseInt(times/60/60/24)  //得到倒计时天数

    d=d<10?'0'+d:d;  //将不足10的时间补0

    var h=parseInt(times/60/60%24)  //得到倒计时小时数

    h=h<10?'0'+h:h;  //将不足10的时间补0

    var m=parseInt(times/60%60)  //得到倒计时分钟数

    m=m<10?'0'+m:m;  //将不足10的时间补0

    var s=parseInt(times%60)  //得到倒计时秒数

    s=s<10?'0'+s:s;  //将不足10的时间补0

    return d + '天' + h + '时' + m + '分' + s +'秒';  //返回倒计时

}

alert('倒计时还剩下' + count('2022-1-18 16:30:00'));  //调用并输入目标的结束时间

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