Can you help me understand this code used to calculate age?

Question

Everything naveen wrote in this post makes sense: Calculate age given the birth date in the format YYYYMMDD

...besides the last bit with the if (m < 0)-part. Could anyone explain it to me? So far I get that: if month is less than 0, then why take the age and reduce by 1?

function getAge(dateString) {
    var today = new Date();
    var birthDate = new Date(dateString);
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;
    }
    return age;
}

It means "*If you haven't had a birthday yet this year, then you're one year younger than I thought*" — p.s.w.g, May 09 '19 at 18:58
`m` is not the month, it's the difference between the two months. — Bergi, May 09 '19 at 19:02
I don't understand, could you rephrase that in a different way? — Luka Momcilovic, May 09 '19 at 19:02
@p.s.w.g I'm not as good as you in javascript, could you rephrase? — Luka Momcilovic, May 09 '19 at 19:13
`m` is the difference in months between the month it is now and the month of the birthday. It's trying to figure out if it's past the current birthday. If you were born in 2009-01-31, and today is 2019-01-30, then you are not 10 years old yet. You have to subtract one after you subtracted 2009 from 2019 . — Ruan Mendes, May 09 '19 at 19:17
Here's a simpler implementation, takes a Date instead :D `function getAge(dob) { return Math.trunc((new Date() - dob) / (1000 * 60 * 60 *24 * 365)); }` — Ruan Mendes, May 09 '19 at 19:19
@JuanMendes I don't want simpler, I want to understand this. Otherwise, I'm a failure of a human being. — Luka Momcilovic, May 09 '19 at 19:20
Luka you have a lot of explanations here, just think about it for a sec. — , May 09 '19 at 19:26
Why take age = age - 1; if m is less than 0, I don't comprehend. — Luka Momcilovic, May 09 '19 at 19:28
`m` is the difference between today's month and the birthday's month. If `m` is under 0 then we are certain that `age` should be reduced by 1 since the birthday definitely hasn't happened yet. — , May 09 '19 at 19:33

ludovico · Answer 1 · 2019-05-09T19:17:06.210

0

var age = today.getFullYear() - birthDate.getFullYear();

Make an initial calculation of your age (current year - the year when you were born)

var m = today.getMonth() - birthDate.getMonth();

m = number of the current month - number of the month where you where born (note: it can be a negative value)

if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {

Check if one of the following is true: - m is negative (current month is before the month when you were born), OR - m is equals to 0 AND current day of the month is before the day of the month when you were born.

In that case, you need to substruct 1 from the age (number of years) that was calculated earlier.

Consider this scenario:

Today: 20 of September 2018.
You were born on: 10 of September 2000.

age calculated in the beginning = 18 (2018 - 2000).
m == 0 AND current day of the month is before the day of the month when you were born.

Real age (after substracting one): 17

edited May 09 '19 at 19:17

answered May 09 '19 at 19:05

ludovico

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I guess the OP can translate the code to English as well, but can you explain *why* these things are done? – Bergi May 09 '19 at 19:09
I'm just a rookie in javascript, and everything surrounding the var = m is mumbo jumbo to me :| – Luka Momcilovic May 09 '19 at 19:12
@LukaMomcilovic There's nothing very specific to JavaScript in that code. – Ruan Mendes May 09 '19 at 19:18
@JuanMendes Okay...I still don't understand it though. – Luka Momcilovic May 09 '19 at 19:19
If you were born in year 2000 and it's currently 2020, it doesn't mean you're 20 years old. It means you are 19 or 20 years old. The result depends on the difference between the current date and the date you were born. – ludovico May 09 '19 at 19:21

Can you help me understand this code used to calculate age?

1 Answers1