Use of document.getElementById in JavaScript

Question

Can someone explain what the document.getElementById(\"demo\") line does in the example below?

I understand getElementById gets the id of demo but the id is

Answer 1

Consider

 var x = document.getElementById("age");

Here x is the element with id="age".

Now look at the following line

var age = document.getElementById("age").value;

this means you are getting the value of the element which has id="age"