Untitled

const input = document.getElementById('input');
const button = document.getElementById('button');
const ans = document.getElementById('ans');

button.addEventListener('click', function(){
    const birthdate = new Date(input.value);
    if(birthdate){
        const today = new Date();
        var age = today.getFullYear() - birthdate.getFullYear();
        var month = today.getMonth() - birthdate.getMonth();
        var date = today.getDate() - birthdate.getDate();
        if(month < 0 || (month === 0 && date < 0 )){
            age--;
        } 
        ans.innerHTML = age;
    }
});