Getting the previous month's first date from current date in JavaScript

Straightforward enough, with the date methods:

  var x = new Date();
  x.setDate(1);
  x.setMonth(x.getMonth()-1);

Simplest way would be:

var x = new Date();
x.setDate(0); // 0 will result in the last day of the previous month
x.setDate(1); // 1 will result in the first day of the month

Important Note: Some of the answers using setMonth() here are wrong:

One liners for use in 2019 (using ES6 syntax; supported by all major browsers and Node):

    const date = new Date().toISOString(); // "2019-09-18T13:49:12.775Z"
    const [yyyy, mm, dd, h, i, s] = date.split(/T|:|-/);


    // previous month's last day
    const prev = new Date(new Date().setDate(0)).toISOString();
    const [pyyyy, pmm] = prev.split(/T|:|-/);

Note that Array destructuring allows you to skip parts:

    const date = new Date().toISOString();
    const [, , dd, , i] = date.split(/T|:|-/);

Explanation: The code above gets the ISO date 2019-09-18T13:49:12.775Z and splits it on : or - or T which returns an array [2019, 09, 18, 13, 49, 12] which then gets destructured.

Using setMonth() is wrong:

date = new Date("Dec 31, 2019")
date.setMonth(date.getMonth() - 1);
date; // Dec 1, 2019!

Check this link:

http://blog.dansnetwork.com/2008/09/18/javascript-date-object-adding-and-subtracting-months/

EDIT: I have drummed up an example:

Date.prototype.SubtractMonth = function(numberOfMonths) {
var d = this;
d.setMonth(d.getMonth() - numberOfMonths);
d.setDate(1);
return d;
}

$(document).ready(function() {
    var d = new Date();
    alert(d.SubtractMonth(1));
});

This worked for me

var curDateMonth = new Date();
var prvDateMonth = new Date(curDateMonth.getFullYear(),curDateMonth.getMonth()-1,curDateMonth.getMonth());
console.log(curDateMonth.toLocaleString('en-US', { month: 'long' }) +' vs '+ prvDateMonth.toLocaleString('en-US', { month: 'long' }));

To get 00:00:00 am of previous month use this:

let d = new Date();
d.setDate(1);
d.setMonth(d.getMonth() - 1);
d.setHours(0,0,0,0);
const lastMonthStart = new Date(d);

Hope this helps!!

//  Previous Month Detail
let prevStartDate = new Date(this.date.getFullYear(), this.date.getMonth() - 1, 1);
console.log(prevStartDate);
let preEndDate = new Date(this.date.getFullYear(), this.date.getMonth() - 1 + 1, 0);
console.log(preEndDate);
//Current Month Detail
let cStartDate = new Date(this.date.getFullYear(), this.date.getMonth(), 1);
console.log(cStartDate);
let cEndDate = new Date(this.date.getFullYear(), this.date.getMonth(), 1, 0);
console.log(cEndDate);
//Next Month Detail
let nStartDate = new Date(this.date.getFullYear(), this.date.getMonth() + 1, 1);
console.log(nStartDate);
let nendDate = new Date(this.date.getFullYear(), this.date.getMonth() + 1 + 1, 0);
console.log(nendDate);

//just try this will work fine

let date = new Date();
let month = new Date().getMonth();
let prevMonth = date.setMonth(month - 1)
let formatPrevMonth = new Date(date.setMonth(month - 1));

console.log(formatPrevMonth)

Here, first we assign getMonth() to a variable and incremented it by 1:

var currentMonth = date.getMonth()+1;
var current_date = date.getFullYear()+"/"+currentMonth+"/"+date.getDate();
document.getElementById("p3").innerHTML = "Today,s date:"+current_date;