Retrieve DOM of two different HTML files with one JS file - javascript

I'm just trying to retrieve the dom of the two html page on the same javascript file.

When i try to retrive the the dom of an element of the first html page, it works, but the elements of the second page return me undefined. How can i try to retrieve the element of the second page ?

firstpage.html

<p id="first">test first</p>

secondpage.html

<p id="second">test second</p>

file.js

var first = document.getElementById("first").innerHTML;
console.log(first); // will show me "test first"
var second = document.getElementById("second").innerHTML;
console.log(second); // will show me "undefined" .. :(

Thanks for the help !