window.document
is a BOM object that refers to the currently loaded document (page). Its methods and properties fall into the DOM category of objects.
var doc = window.document
<!DOCTYPE html> <html> <head> <title>Welcome to Rookie Nerd!</title> </head> <body> <script type="text/javascript"> var doc = window.document; console.log(doc.title); // Welcome to Rookie Nerd! </script> </body> </html>