How do I get the reference for the map?

I wanted to put an infowindow on markers when I click them, but I can't seem to "open" the infowindow because I don't have the reference for the map initialized on the page.
Do you have a documentation for this package? I can't seem to understand much of how to use this, but somehow I have made it work now. Please observe the code I put on pastebin:

https://pastebin.com/VYMCvS9m

I don't know if I got the reference of map right, but everytime I run `infowindow.open(map, marker)`, I always get b.get is not a function. Please help!