How to calculate the age of a person with JS, from the date of birth?

Question

How can I calculate a person’s age in Javascript from the date of birth?

I tried something like:

function idade(dia, mes, ano) {
   return new Date().getFullYear() - ano;
}

idade(11, 12, 1980); //  33
idade(15, 2, 2011);  // 2
idade(5, 31, 1993);  // 20

Answer 1

— Peter: Hi, Maria. How old are you?

— Maria: Pedro, I was born on February 28, 1990.

— Peter: So I know how old you are!

— Maria: What is?

— Peter: Just take the current year and subtract by the year you were born. Soon, 2014 - 1990 = 24. You are 24 years old.

— Maria: No, actually I am 23 years old. I do 24 day 28 February.

— Maria: To calculate age, you subtract the current year by the year of birth. But if you have not yet passed the birthday date, you subtract 1.

— Peter: Ah, yeah. I forgot that detail!

---

function idade(ano_aniversario, mes_aniversario, dia_aniversario) {
    var d = new Date,
        ano_atual = d.getFullYear(),
        mes_atual = d.getMonth() + 1,
        dia_atual = d.getDate(),

        ano_aniversario = +ano_aniversario,
        mes_aniversario = +mes_aniversario,
        dia_aniversario = +dia_aniversario,

        quantos_anos = ano_atual - ano_aniversario;

    if (mes_atual < mes_aniversario || mes_atual == mes_aniversario && dia_atual < dia_aniversario) {
        quantos_anos--;
    }

    return quantos_anos < 0 ? 0 : quantos_anos;
}



console.log(idade(1980, 12, 11)); //  33

console.log(idade(2011, 2, 15));  // 2

console.log(idade(1993, 31, 5));  // 20

Answer 2

I suggest comparing the month and the day of the month: if today’s date is greater than or equal to that of birth (i.e. the birthday has passed), then it is enough to make the difference between the years. Otherwise, make a difference and subtract 1:

function idade(nascimento, hoje) {
    var diferencaAnos = hoje.getFullYear() - nascimento.getFullYear();
    if ( new Date(hoje.getFullYear(), hoje.getMonth(), hoje.getDate()) < 
         new Date(hoje.getFullYear(), nascimento.getMonth(), nascimento.getDate()) )
        diferencaAnos--;
    return diferencaAnos;
}

Example in jsFiddle.

Answer 3

A one-line solution, calculating from the number of days:

// calcula a idade considerando os parâmetros 
// 'nascimento' e 'hoje' como objetos Date
function calculaIdade(nascimento, hoje){
    return Math.floor(Math.ceil(Math.abs(nascimento.getTime() - hoje.getTime()) / (1000 * 3600 * 24)) / 365.25);
}

The logic used is as follows::

• Math.abs(nascimento.getTime() - hoje.getTime()) - returns the amount of milliseconds passed since nascimento until hoje. The function Math.abs returns the module of subtraction, that is, transforms a negative number into positive and maintains the signal of a positive.
• / (1000 * 3600 * 24) - calculates the number of days from the amount of milliseconds returned in the previous expression. Dividing by 1000, we have the number of seconds; the number of seconds dividing by 3600 we have the number of hours (because in 1 hour it fits 3600 seconds); and finally we divide the amount of hours by 24, we will have the corresponding amount of days.
• Math.ceil - round up the decimal value of the previous operation, as it is considered that a day has passed even if the amount of hours does not give 24 hours. As for example a baby born last night, we consider that by this morning already has 1 day of life.
• / 365.25 - finally, we calculate the year by dividing the total of days by the total of days that fit into a year. The number 365.25 is because a year has approximately 365 days and 6 hours, which is equal to 365.25 days. The bixesto year comes from this difference, because every 4 years, the 6 hours disregarded in the calendar become 24 hours, ie another day.
• Math.floor - round down the amount of years. Yeah, no matter if the person is birthday tomorrow and is 25,999 years old, she is 25 years old.

Answer 4

How about calculating in a much more minimalist way? The calculation with ms is highly superior considered performance. Ex: I was born on 17/12/1995. So:

function calcAge(dateString) {
  var birthday = +new Date(dateString);
  return ~~((Date.now() - birthday) / (31557600000));
}

console.log(calcAge("Thu Dec 17 1995 00:51:54 GMT-0300 (BRT)"));

If the code is executed before the day 17/12/2015. I will still be 19 years :P

The technique used is in: http://jsperf.com/birthday-calculation And I thank Caio Ribeiro for showing me

Answer 5

Hello, I adapted the code of @Gabriel S. to accept date in the Brazilian standard.

function calcIdade(data) {
    var d = new Date,
        ano_atual = d.getFullYear(),
        mes_atual = d.getMonth() + 1,
        dia_atual = d.getDate(),
        split = data.split('/'),
        novadata = split[1] + "/" +split[0]+"/"+split[2],
        data_americana = new Date(novadata),
        vAno = data_americana.getFullYear(),
        vMes = data_americana.getMonth() + 1,
        vDia = data_americana.getDate(),
        ano_aniversario = +vAno,
        mes_aniversario = +vMes,
        dia_aniversario = +vDia,
        quantos_anos = ano_atual - ano_aniversario;
    if (mes_atual < mes_aniversario || mes_atual == mes_aniversario && dia_atual < dia_aniversario) {
        quantos_anos--;
    }
    return quantos_anos < 0 ? 0 : quantos_anos;
}
// calcIdade('02/01/1978') => 39
// calcIdade('03/09/1980') => 36