Get Number of Day In Year

dayNumber.js

//Const from date. 
const pokebithday = document.getElementById("date-pick");
//Function to convert date into day Number.
const getUserId = () => {
  //Get the Value from the input
  let userBirthday = pokebithday.value
  // use Date.parse() to convert a string representation of a date, and returns the number of milliseconds.
  let parseDate = Date.parse(userBirthday)
  //Slice the year from the input and set the fist day of Year. 
  let startOfYear = userBirthday.slice(0, 4) + '-01-01'
  //Parse the Fisrt day of Year with Date.parse()
  let parseStart = Date.parse(startOfYear)
  //Get the number of day convert the Milliseconds into Days using this formula: 1000 * 60 * 60 * 24
  //I use 86400000 which are the seconds in 24 hours
  let days = 86400000
  //Get the difference between the first day of the year and the select date.
  let diff = parseDate - parseStart
  //Round up the number obtained from the division and add one
  let numberOfDate = Math.ceil(diff / days) + 1
  //Set the UserId as the result
  userIdPokemon = numberOfDate
}