Detailed introduction to the usage of data objects in js (with code)

This article brings you a detailed introduction to the usage of data objects in js (with code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Introduction

Handling dates in JavaScript can be complex and often a pain for developers regardless of their skills.

JavaScript provides us with date processing functions through a powerful Date object.

DATE Object

Date object instance represents a single point in time.

Although named Date, it is also used Processing time.

Initialize Date object

We initialize a Date object through the following code:

new Date()

The above code creates a date object that represents the current moment.

Internally, the date represents the number of milliseconds since January 1, 1970 (UTC). This time is important because as far as the computer is concerned, this is when it started.

You may be familiar with UNIX timestamps: this represents the number of seconds that have elapsed since that famous date.

Note that UNIX timestamps are in seconds and JavaScript dates are in milliseconds

If we have a UNIX timestamp, we can initialize a JavaScript date object through the following method:

const timestamp = 1530826365
new Date(timestamp * 1000)

If we pass in 0, we will get the date representing Jan 1st 1970 (UTC).

new Date(0)

If we pass in a string instead of a value, the Date object will use the parse method to determine which date you pass in, such as:

new Date('2018-07-22')
new Date('2018-07') //July 1st 2018, 00:00:00
new Date('2018') //Jan 1st 2018, 00:00:00
new Date('07/22/2018')
new Date('2018/07/22')
new Date('2018/7/22')
new Date('July 22, 2018')
new Date('July 22, 2018 07:22:13')
new Date('2018-07-22 07:22:13')
new Date('2018-07-22T07:22:13')
new Date('25 March 2018')
new Date('25 Mar 2018')
new Date('25 March, 2018')
new Date('March 25, 2018')
new Date('March 25 2018')
new Date('March 2018') //Mar 1st 2018, 00:00:00
new Date('2018 March') //Mar 1st 2018, 00:00:00
new Date('2018 MARCH') //Mar 1st 2018, 00:00:00
new Date('2018 march') //Mar 1st 2018, 00:00:00

It’s very flexible here. You can add or omit leading zeros within months or days.

You need to pay attention to the position of month/day, otherwise the month may be parsed as a date.

Use Date.parse You can also process strings:

Date.parse('2018-07-22')
Date.parse('2018-07') //July 1st 2018, 00:00:00
Date.parse('2018') //Jan 1st 2018, 00:00:00
Date.parse('07/22/2018')
Date.parse('2018/07/22')
Date.parse('2018/7/22')
Date.parse('July 22, 2018')
Date.parse('July 22, 2018 07:22:13')
Date.parse('2018-07-22 07:22:13')
Date.parse('2018-07-22T07:22:13')

Date.parse will return a timestamp expressed in milliseconds instead of a Date object

You can also pass in values ​​in order to represent each part of the date. The parameter order is as follows: year, month (starting from 0), date, hour, minute, second, millisecond

new Date(2018, 6, 22, 7, 22, 13, 0)
new Date(2018, 6, 22)

At least three parameters need to be passed in, but most JavaScript engines can also parse less than three parameters

new Date(2018, 6) //Sun Jul 01 2018 00:00:00 GMT+0200 (Central European Summer Time)
new Date(2018) //Thu Jan 01 1970 01:00:02 GMT+0100 (Central European Standard Time)

The final result of the above code is a relative value that depends on your computer's time zone. This means that passing the same parameters may produce different results on different computers.

JavaScript, without any information about the time zone, treats the date as UTC and the result is automatically converted for the current computer time zone.

To summarize, there are four methods that allow you to create a new Date object:

• If no parameters are passed, a Date object will be created based on the current time;

• Pass in a value representing the number of milliseconds in the past since 1 Jan 1970 00:00 GMT;

• Pass in a string representing the date;

• Pass in a series of parameters that represent each item;

Time zone

When initializing the date, you can also pass in the time zone, At this point the date is not assumed to be UTC and is then converted to the local time zone.

The time zone can be passed in HPURS format or by adding a time zone name.

new Date('July 22, 2018 07:22:13 +0700')
new Date('July 22, 2018 07:22:13 (CET)')

If an incorrect time zone name is passed in during parsing, JavaScript will use UTC by default and will not report an error.

If a value in the wrong format is passed in, JavaScript will report an Invaild Date error.

Date conversion and formatting

For a given date object, there are many methods to produce a string based on the date

const date = new Date('July 22, 2018 07:22:13')

date.toString() // "Sun Jul 22 2018 07:22:13 GMT+0200 (Central European Summer Time)"
date.toTimeString() //"07:22:13 GMT+0200 (Central European Summer Time)"
date.toUTCString() //"Sun, 22 Jul 2018 05:22:13 GMT"
date.toDateString() //"Sun Jul 22 2018"
date.toISOString() //"2018-07-22T05:22:13.000Z" (ISO 8601 format)
date.toLocaleString() //"22/07/2018, 07:22:13"
date.toLocaleTimeString()    //"07:22:13"
date.getTime() //1532236933000
date.getTime() //1532236933000

GETTER method of Date object

The Date object provides several methods for checking its value. The results of these methods all depend on the current time zone of the computer

const date = new Date('July 22, 2018 07:22:13')

date.getDate() //22
date.getDay() //0 (0 means sunday, 1 means monday..)
date.getFullYear() //2018
date.getMonth() //6 (starts from 0)
date.getHours() //7
date.getMinutes() //22
date.getSeconds() //13
date.getMilliseconds() //0 (not specified)
date.getTime() //1532236933000
date.getTimezoneOffset() //-120 (will vary depending on where you are and when you check - this is CET during the summer). Returns the timezone difference expressed in minutes

The above methods have corresponding versions for obtaining UTC time:

date.getUTCDate() //22
date.getUTCDay() //0 (0 means sunday, 1 means monday..)
date.getUTCFullYear() //2018
date.getUTCMonth() //6 (starts from 0)
date.getUTCHours() //5 (not 7 like above)
date.getUTCMinutes() //22
date.getUTCSeconds() //13
date.getUTCMilliseconds() //0 (not specified)

Editing the Date object

The Date object provides Several editing date value methods

const date = new Date('July 22, 2018 07:22:13')

date.setDate(newValue)
date.setDay(newValue)
date.setFullYear(newValue) //note: avoid setYear(), it's deprecated
date.setMonth(newValue)
date.setHours(newValue)
date.setMinutes(newValue)
date.setSeconds(newValue)
date.setMilliseconds(newValue)
date.setTime(newValue)
date.setTimezoneOffset(newValue)

setDay and setMonth all start processing from the value 0, for example, March should be the value 2

Here is one Trivia: These methods "overlap", so for example if you use date.setHours (48), the result will affect the day.

There is another piece of trivia: you can pass in multiple parameters to the setHours() method to set minutes, seconds, and milliseconds, such as setHours(0, 0, 0, 0) Similar situations exist for , setMinutes and setSeconds.

Similar to many methods of getting dates, there are also methods of setting dates for the UTC version:

const date = new Date('July 22, 2018 07:22:13')

date.setUTCDate(newalue)
date.setUTCDay(newValue)
date.setUTCFullYear(newValue)
date.setUTCMonth(newValue)
date.setUTCHours(newValue)
date.setUTCMinutes(newValue)
date.setUTCSeconds(newValue)
date.setUTCMilliseconds(newValue)

Get the current timestamp

If you want to get the milliseconds The current timestamp in units, it is recommended to use the following method:

Date.now()

instead of

new Date().getTime()

JavaScript always try to get the most accurate results

As mentioned above, you The number of days passed in will affect the total date. This will not report an error and will directly update the month.

new Date(2018, 6, 40) //Thu Aug 09 2018 00:00:00 GMT+0200 (Central European Summer Time)

The above phenomenon also takes effect on dates, hours, minutes, seconds and milliseconds

依据本地情况格式化日期

Internationalization API 在现代浏览器中有很好的支持(除了 UC浏览器)，允许你转换日期。

本地化方法通过，通过 Int1 对象暴露，这个对象还可以用来帮助本地化数值，字符串以及货币。

这里我们用到的是 Intl.DateTimeFormat()

我们可以通过下述方法来依据电脑的本地情况格式化一个日期：

const date = new Date('July 22, 2018 07:22:13')
new Intl.DateTimeFormat().format(date) //"22/07/2018" in my locale

也可以依据不同的时区格式化日期：

new Intl.DateTimeFormat('en-US').format(date) //"7/22/2018"

Intl.DateTimeFormat 方法还接收一个可选的参数用以自定义输出格式，可以用来展示 小时，分钟和秒

const options = {
  year: 'numeric',
  month: 'numeric',
  day: 'numeric',
  hour: 'numeric',
  minute: 'numeric',
  second: 'numeric'
}

new Intl.DateTimeFormat('en-US', options).format(date) //"7/22/2018, 7:22:13 AM"
new Intl.DateTimeFormat('it-IT', options2).format(date) //"22/7/2018, 07:22:13"

点击这个链接可以查看所有可以用到的属性

两个日期的对比

可以通过 Date.getTime() 获取两个日期之间的差别

const date1 = new Date('July 10, 2018 07:22:13')
const date2 = new Date('July 22, 2018 07:22:13')
const diff = date2.getTime() - date1.getTime() //difference in milliseconds

同样也可以通过这个方法检测两个日期是否相同：

const date2 = new Date('July 10, 2018 07:22:13')
if (date2.getTime() === date1.getTime()) {
  //dates are equal
}

需要注意的是，getTime() 方法比较的是毫秒，所以 July 10, 2018 07:22:13 和 July 10, 2018 并不相等。不过你可以通过 setHours(0, 0, 0, 0) 来重置时间。

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