How to iterate over the entire database and find the desired value?

Question

JavaScript

(Pre-story)
I'm learning AngularFire2 and ran into a problem, at this point (Fourth of Five)
https://github.com/angular/angularfire2/blob/maste...
If we scroll down we can understand what this application does,
When you click on the button (For example, small), the script enters items and checks item1, item2, item3 there, if it finds small in any of the item(1,2,3), then it takes its text and displays it.
(What is the problem)
And how to enumerate the entire database without giving exact values ​​check item1/2/3
That is, enumerate this:

And not this:

Answer 1

"Waterfall" descent along the hash "tree".