Getting document is not defined , from document.getElementById

Question

I am learning JavaScript and I am using Atom (Text Editor). On my HTML file I got only this:

Answer 1

You have hit some menu option or key combination which is trying to execute the JS file using Node.js.

Your code, however, is designed to run, embedded in a web page, using the APIs supplied by web browsers.

Web browsers, under those circumstances, will provide a document object. Node.js will not.

You need to open the HTML document in a web browser. The open in browser extension might be useful.

You can see any error reports using the Developer Tools that every major browser supplies.

(NB: The first error you will then encounter is explained by this question and answer).