Get month in mm format in javascript

An alternative way:

var currentMonth=('0'+(currentDate.getMonth()+1)).slice(-2)

One line solution:

var currentMonth = (currentDate.getMonth() < 10 ? '0' : '') + currentDate.getMonth();

for the date:

("0" + this.getDate()).slice(-2)

and similar for the month:

("0" + (this.getMonth() + 1)).slice(-2)

ES6 version, inpired by Gert Grenander

let date = new Date();
let month = date.getMonth()+1;
month = `0${month}`.slice(-2);

If you do this

var currentDate = new Date();
var currentMonth = currentDate.getMonth() + 1;

then currentMonth is a number, which you can format as you want, see this question that will help you with formatting: How can I format an integer to a specific length in javascript?

In order for the accepted answer to return a string consistently, it should be:

if(currentMonth < 10) {
    currentMonth = '0' + currentMonth;
} else {
    currentMonth = '' + currentMonth;
}

Or:

currentMonth = (currentMonth < 10 ? '0' : '') + currentMonth;

Just for funsies, here's a version without a conditional:

currentMonth = ('0' + currentMonth).slice(-2);

Edit: switched to slice, per Gert G's answer, credit where credit is due; substr works too, I didn't realize it accepts a negative start argument

var CurrentDate = new Date();
    CurrentDate.setMonth(CurrentDate.getMonth());

    var day = CurrentDate.getDate();
    var monthIndex = CurrentDate.getMonth()+1;
    if(monthIndex<10){
        monthIndex=('0'+monthIndex);
    }
    var year = CurrentDate.getFullYear();

    alert(monthIndex);

An alternative with ES6 template strings

A solution for mm/yyyy. Not quite the question, but I guess remove the second part.

const MonthYear = `${dateObj.getMonth() < 10 ? '0' : '' }${dateObj.getMonth()+1}/${dateObj.getFullYear()}`

const Month = `${dateObj.getMonth() < 10 ? '0' : '' }${dateObj.getMonth()+1}`

An answer here referenced one on number formatting but without a code example. This is a general string padding problem that has been simplified since ES2017. You can use String.prototype.padStart and String.prototype.padEnd to make sure that a string is of a fixed minimum length.

The return of Date.prototype.getMonth is a number from 0 (January) to 11 (December). To be represented as strings from '01' to '12' you might do:

let date = new Date()
let month = date.getMonth() + 1
let display = month.toString().padStart(2, '0')
console.log(display) // one of "01", "02", ..., "09", "10", "11", "12"

This similarly applies to the return of Date.prototype.getDate, a number from 1 to 31:

console.log(
  new Date().getDate().toString().padStart(2, '0')
) // one of "01", "02", ..., "31"

Or simply any value that you want to be a string with a minimum length:

let greeting = 'world'.padStart(17, 'hello ')
console.log(greeting) // "hello hello world"