Moment getting the value of the previous month, even after adding one month

Question

I’m trying to work with the Moment js.

var data = moment("2019-08-17", "YYYY-M-D").add('months', 1);

alert(data);

dia = data.date();
mes = data.month();
ano = data.year();

The alert(data) gives me the correct date but when I split mes = data.month(), return the current month and not the calculated one!

Where am I going wrong?

Answer 1

Let’s go in pieces:

---

Order of parameters in the method add

I don’t know what version of Moment js. you are using, but the documentation of the method add says the following:

Before version 2.8.0, the moment#add(String, Number) syntax was also supported. It has been deprecated in favor of moment#add(Number, String).

In free translation:

Before the version 2.8.0, the syntax moment#add(String, Number) was also borne, but this was depreciated in favour of moment#add(Number, String).

That is to say, add('months', 1) will also work, but when using the most current version of Moment.js, I got a message stating what the documentation indicates:

Deprecation Warning: Moment(). add(period, number) is deprecated. Please use Moment(). add(number, period). See http://momentjs.com/guides/#/warnings/add-Inverted-param/ for more info.

Note: I used the version 2.24.0. To see which one you’re using, just do console.log(moment.version).

Anyway, add('months', 1) works. But by being deprecated, suggest switching to new syntax (add(1, 'months')), if you are using a version >= 2.8.0.

---

Numerical value of the month

If we look at the documentation of method month, we will see that the value returned is between 0 and 11, because the months are indexed at zero (January is zero, February is 1, etc). Probably was done so to maintain compatibility with the Date javascript native, that has this same behavior.

That is, simply add 1 to get the correct numerical value (January equal to 1, February equal to 2, etc):

// usar a sintaxe nova: add(numero, string)
let data = moment("2019-08-17", "YYYY-M-D").add(1, 'months');

console.log(data);

let mes = data.month() + 1; // somar 1 pois janeiro é zero, fevereiro é 1, etc
console.log(mes); // 9

<script src="https://momentjs.com/downloads/moment.min.js"></script>

Only one point of attention: although the months are indexed in zero, the constructor that takes two strings (one with the date and one with the format) considers the correct values of the month. That’s why moment("2019-08-17", "YYYY-M-D") creates a date in August. But by obtaining the numeric value in the method month, then the January rule is used to be zero. Confusing, but that’s how it was done.

---

Another option to get the month value is to use format, who doesn’t use the "January is zero rule":

// usar a sintaxe nova: add(numero, string)
let data = moment("2019-08-17", "YYYY-M-D").add(1, 'months');

console.log(data);

console.log(data.format('M')); // 9

// com o zero antes, caso o mês seja menor que 10
console.log(data.format('MM')); // 09

<script src="https://momentjs.com/downloads/moment.min.js"></script>

The difference is that month() returns a number, while format returns a string. Depending on what you will do with this value, it can make a difference.

---

As to the your comment, about putting the value in a input date, to documentation says that, to set a date to this input, the value must be in the format ISO 8601 ("yyyy-mm-dd"). This can be obtained using format. Ex:

document.addEventListener('DOMContentLoaded', (event) => {
    let data = moment("2019-08-17", "YYYY-M-D").add(1, 'months');
    document.querySelector('#mesQueVem').value = data.format('YYYY-MM-DD');
});

<script src="https://momentjs.com/downloads/moment.min.js"></script>
<form>
  <div>
    <input type="date" id="mesQueVem" name="mesQueVem">
  </div>
</form>