JS take the date of the previous month 【 pit filling 】

When working on writing code , There is a place to be based on years As a query condition

• First of all new Date() get date object
• Reuse his set Method , Will be set go in

var date = new Date();
date.setFullYear(2019);
date.setMonth(10); --  Here month The scope is （0~11）, Explain that I am now set to 11 month 

Then I think it should be 2019-11 The object of this date however ！！！
There is a ghost .. The return is 2019-12-01（ The second line ）

……

• Toss and turn for a long time , Still failed , Suddenly I see that today is 2019-12-31

I was thinking , It is the new year , I am still in vain , I didn't learn anything , It's hard. It's hard ... But ,,, Why ---- incorrect ！
If today is 31 Number , This is me. date Is it difficult to set it to 2019-11-31 了 ？（ Because I only set the year and month , Day is not set , So the day should be the initialization time ）
When you find a problem , I started searching for information again

https://www.cnblogs.com/jc2182/p/11273954.html

Unfortunately, there is no way to set the day , This method should not work

• Continue Baidu's other methods online , Finally, I found a brother's way （ Direct use of construction methods ）

/** *  Get the last month end date  */
function getLastMonthAndDay(){
    
    var nowDate = new Date();
    var year = nowDate.getFullYear();
    var month = nowDate.getMonth(); if(month == 0){
    
        month = 12;
        year = year - 1;
    }
    var lastDay = new Date(year,month,0);
    var yyyyMMdd = year+" year "+month+" month "+lastDay.getDate()+" Japan ";
    console.log(yyyyMMdd);
}

var lastDay = new Date(year,month,0); You can directly set the month and year you want