wierd problems in javascript program

I was trying to make a javascript program that makes us play tic-tac-toe. But wherever i click,it shows "Invalid Move".

Code:

Game

var droid = new Android(); var turn =1; function convert(event) { var et = event.target.src; if(et == "imgb.png") { if(turn == 1) { et = "imgx.png"; turn = 2; } else { et = "imgo.png"; turn = 1; } } else alert(" Invalid Move"); }