Analyzing JavaScript Date object properties and methods through instances

In daily life, various forms of time characters are everywhere. The emergence of the concept of time, the invention of time units and timing tools, has brought about changes to mankind. Let’s talk about dates today. Let's take a look at the date object Date in JavaScript.

Get the number of days in the month

// Get the number of days in the monthfunction getMonthDayCount(year, month) {
 return new Date(year, month, 0).getDate();
}
(getMonthDayCount(2017, 10)); // 31

The essence of the third parameter of Date is the same as setDate.

Because the date is 0 automatically retreats to the last day of the previous month, the month here does not need to be reduced, which is just right.

Get the days in all months

function getAllMonthDayCount(year) {
 var days = [31, new Date(year, 2, 0).getDate(), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
 return days;
}
(getAllMonthDayCount(2016));// [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

This is an extension of the above and will not explain much.

Is it a leap year?

function isLeapYear(year) {
 return (year % 4 == 0) && (year % 100 != 0 || year % 400 == 0);
}

This is the relevant code in the online web front-end development, and I believe most people use it. But actually, do you really understand or can you remember it? HTML and CSS alone are enough to remember

function isLeapYear(year) {
 return new Date(year, 2, 0).getDate() === 29;
}
([
 isLeapYear(2000),
 isLeapYear(2016),
 isLeapYear(2017),
 isLeapYear(2018)
]); // [ true, true, false, false ]

Looking at it this way, isn’t it very simple and easy to understand?

And it doesn’t need to be remembered. Can’t you forget it even if you want to?

Add or subtract the number of days

I have seen someone using relative seconds to calculate the situation of crossing the month and New Year's Eve in a few days or a few days later.

In fact, just setDate directly, automatically deal with the situation of cross-month and New Year's Eve.

// What month will be 10 days later
var dt = new Date('2016-12-25');
(() + 10);
(()); // 2017/1/4

// What month was it 10 days ago
var dt = new Date('2017-01-04');
(() - 10);
(()); // 2016/12/25

Below I will summarize the objects and methods of JavaScript Date objects into a table for reference, and you can also follow the JavaScript reference manual.

Date object properties

Date object method

The above is all the content of this article. I hope it will be helpful to everyone's study and I hope everyone will support me more.