Leap year checker in javascript

4 thoughts on “Leap year checker in javascript”

1. const isLeap = (year) => (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
   
   const calcAge = (DOBString) => {
     const UTC_today = new Date();
     const UTC_DOB = new Date(DOBString);
     if (!UTC_DOB) return "Please, use a valid date";
     let age = UTC_today.getUTCFullYear() - UTC_DOB.getUTCFullYear();
     let m = UTC_today.getUTCMonth() - UTC_DOB.getUTCMonth();
     if (m < 0 || (m === 0 && UTC_today.getUTCDate() < UTC_DOB.getUTCDate())) age--;
     return {
       age: age,
       isLeap : isLeap(UTC_DOB.getUTCFullYear())
     }
   };
   
   const showAgeResult = () => {
     const ageObj = calcAge(EL_DOB.value);
     EL_result.textContent = `Age: ${ageObj.age}`;
     EL_leap.textContent = ageObj.isLeap ? "Born on a leap year! so cool" : "";
   };
   
   const EL_DOB = document.querySelector("#DOB");
   const EL_result = document.querySelector("#result");
   const EL_leap = document.querySelector("#leap");
   EL_DOB.addEventListener("input", showAgeResult);
   showAgeResult();

   <input id="DOB" type="date" value="2000-02-29"/>
   <span id="result"></span><br>
   <span id="leap"></span>

   Important:
   notice that if user DOB is "2000-02-29", only the "2001-03-01" will show 1y – such depends on

   Reply

2. I suggest you look at this version

   Calculate age given the birth date in the format YYYYMMDD

   To fix YOUR code we need to pass the year to the function and not try to use a non-existing year field

   If there was such a field, you could access its value with document.querySelector("[name=year]").value since document.getElementsByName is a collection and the value would be document.getElementsByName("year")[0].value

   So what did I do?

   • passed the DOB year to the leap function
   • calculated the age from the year of the date difference

   I tested with 2000-02-29 and 2001-02-28

   NOTE the script ALSO works with 02/29/2016 but not with 29/02/2016 (European format)

   var year;
   
   function ageCalculator() {
     var userinput = document.getElementById("DOB").value;
     var dob = new Date(userinput);
   
     if (isNaN(dob)) {
       document.getElementById("message").innerHTML = "**Choose a date please!";
       return false;
     } else {
   
       //extract year from date
       var year = dob.getFullYear();
       console.log(year)
       calYear(year);
   
       //calculate difference from current date in milliseconds
       var date_diff = Date.now() - dob.getTime();
   
       //convert the calculated difference in date format
       var age_dt = new Date(date_diff);
   
      //now calculate the age of the user
       var age = Math.abs(1970 - age_dt.getFullYear());
   
       //display the calculated age
       return document.getElementById("result").innerHTML =
         "Age is: " + age + " years. ";
     }
   }
   
   function calYear(yr) {
     const isLeap = (yr % 400 == 0 || (yr % 100 != 0 && yr % 4 == 0)) 
     alert(isLeap ? "This is a leap!" : "This is not a leap!");
   }

   <input type="text" id="DOB" />
   <button type="button" onclick="ageCalculator()">Calculate</button>
   <span id="message"></span><br/>
   <span id="result"></span>

   Reply

3. this is function to check if this year is leap year

   function isLeapYear(year){
    return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
   }
   

   isLeapYear(2000) //true
   isLeapYear(2001) // false
   

   EDIT:
   so the problem is in document.getElementsByName("year")
   i think you should have a HTML named year which is have the year value

   <input type="number" name="year" />
   

   Reply

4. Here’s another Method I can make sure it will works

   function ageCalculator() {
     var userinput = document.getElementById("DOB").value;
     var dob = new Date(userinput);
   
     if (userinput == null || userinput == '') {
       document.getElementById("message").innerHTML = "**Choose a date please!";
       return false;
     } else {
       //calculate month difference from current date in time
       var month_diff = Date.now() - dob.getTime();
   
       //convert the calculated difference in date format
       var age_dt = new Date(month_diff);
   
       //extract year from date
       var year = age_dt.getUTCFullYear();
       calYear(year);
       //now calculate the age of the user
       var age = Math.abs(year - 1970);
   
       //display the calculated age
       return document.getElementById("result").innerHTML =
         "Age is: " + age + " years. ";
     }
   }
   
   function calYear(yr) {
     if (yr % 400 == 0 || (yr % 100 != 0 && yr % 4 == 0)) {
       window.alert("This is not leap!");
     } else {
       window.alert("This is a leap!");
     }
   }
   

   Reply