Computer behavior for Ticktacktoe game

Hello i try to find a way how to make a simple Tic Tac Toe game between Player and Computer.

I want to ask you for a little help, i don't know how to create the behavior of the computer. 

This is my code : 

Javascript Code

I create a function for computerMove , i think I have to use random function and array, but I have no idea where to start ...