Tic-tac-toe - help to understand a loop

I did my tic-tac-toe game, but I’m not very happy about the AI.

1/ I didn’t separate the DOM and logic.
2/ My logic isn’t very smart since I look at others like here:

But some points are obscure:
the page Game.js

I don’t understand, how he can get all the variable attached with this in len?

var len = old.board.length;

in the instance it don’t call any argument, what’s mean the argument old?

this.currentState = new State();