index.js

//글자 제한(day - 2 / month - 2 / year - 4)

const day = document.querySelector("#day");
const month = document.querySelector("#month");
const year = document.querySelector("#year");
const yearNow = new Date().getFullYear();
const monthNow = new Date().getMonth() + 1;
const dayNow = new Date().getDate();
const today = new Date();
const btn = document.querySelector(".btn");
const year_input = document.querySelector(".results__year");
const month_input = document.querySelector(".results__month");
const day_input = document.querySelector(".results__day");

function Max(unit, num, min) {
  numLength = `${num}`;
  if (unit.value.length) {
    if (unit.value >= num) {
      unit.value = num;
    } else if (unit.value.length == numLength.length && unit.value <= min) {
      unit.value = min;
    }
  }
}

day.setAttribute("oninput", "Max(day, 31, 1)");
month.setAttribute("oninput", "Max(month, 12, 1)");
year.setAttribute("oninput", "Max(year, yearNow, 1900)");

// 나이 계산
function errFocus(unit) {
  day.classList.remove("err");
  month.classList.remove("err");
  year.classList.remove("err");
  if (unit) {
    unit.focus();
    unit.classList.add("err");
  }
}
let dayResult = 0;
let monthResult = 0;
let yearResult = 0;

btn.addEventListener("click", () => {
  let dayValue = day.value;
  let monthValue = month.value;
  let yearValue = year.value;
  if (dayValue && monthValue && yearValue) {
    if (yearValue == yearNow && monthValue > monthNow) {
      monthValue = monthNow;
      month.value = monthValue;
      dayValue = dayNow;
      day.value = dayValue;
    } else if (
      yearValue == yearNow &&
      monthValue == monthNow &&
      dayValue > dayNow
    ) {
      dayValue = dayNow;
      day.value = dayValue;
    } else if (yearValue.length < 4) {
      errFocus(year);
    } else {
      errFocus();
      dayResult = dayValue;
      monthResult = monthValue - 1;
      yearResult = yearValue;
      theDay = new Date(yearResult, monthResult, dayResult);
      let interval = today.getTime() - theDay.getTime();
      let result = dayjs(interval).format("YYYY-MM-DD");
      let resultYear = result.split("-")[0] - 1970;
      let resultmonth = result.split("-")[1] - 1;
      let resultDay = result.split("-")[2] - 1;
      year_input.textContent = resultYear;
      month_input.textContent = resultmonth;
      day_input.textContent = resultDay;
    }
  } else if (!dayValue) {
    errFocus(day);
  } else if (!monthValue) {
    errFocus(month);
  } else if (!yearValue) {
    errFocus(year);
  }
});

// 나이 계산 후 출력