function calcAge(){
  var dateTime = new Date();
  var birthday = document.getElementById('birthday');
  var age = Math.abs(dateTime - birthday)
  document.getElementById("age").innerHTML = `Your age is `+age;
}

document.getElementById('btn').addEventListener("click", calcAge())

我正在习惯DOM，并认为下面的项目html代码将是一个很好的实践

function calcAge(){
  var dateTime = new Date();
  var birthday = document.getElementById('birthday');
  var age = Math.abs(dateTime - birthday)
  document.getElementById("age").innerHTML = `Your age is `+age;
}

document.getElementById('btn').addEventListener("click", calcAge())

顺晟科技：