JavaScript Function to Calculate Age from Date of Birth

This function written in JavaScript calculates the age of a person. The input data is the month, the day, and the full year this person was born. All this data must be written using numbers, the month being a number from 1 to 12.

This is the function:

<script type="text/javascript">

function calculateAge(birthMonth, birthDay, birthYear)
{
  todayDate = new Date();
  todayYear = todayDate.getFullYear();
  todayMonth = todayDate.getMonth();
  todayDay = todayDate.getDate();
  age = todayYear - birthYear;

  if (todayMonth < birthMonth - 1)
  {
    age--;
  }

  if (birthMonth - 1 == todayMonth && todayDay < birthDay)
  {
    age--;
  }
  return age;
}

</script>

Try it yourself »

Next, we will see an example of how to call this function. It calculates the age of a girl born on November the 2nd of 1992.

<script type="text/javascript">

document.write("She is " + calculateAge(11,2,1992)
+ " years old" );

</script>

Try it yourself »

Since this function adds a year of age only once the date of the birthday has arrived or passed, it works perfectly fine calculating the age of people born on February the 29th of leap years.

Enjoy!

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JavaScript Manual >> JavaScript Function to Calculate Age