JUSTCodaborate

OXO

var currentPlayer = "O";
var won = false;
function place(box) {
  if(box.innerText != "" || won) return;
  box.innerText = currentPlayer;
  currentPlayer == "O" ? currentPlayer = "X" : currentPlayer = "O";
  checkGameBoard();
}

function checkGameBoard() {
  for(var i = 0; i <= 2; i++) {
   checkWinner(document.getElementById("0_"+i).innerText,
               document.getElementById("1_"+i).innerText,
               document.getElementById("2_"+i).innerText);
   checkWinner(document.getElementById(i+"_0").innerText,
               document.getElementById(i+"_1").innerText,
               document.getElementById(i+"_2").innerText);
  }
  checkWinner(document.getElementById("0_0").innerText,
              document.getElementById("1_1").innerText,
              document.getElementById("2_2").innerText);
  checkWinner(document.getElementById("0_2").innerText,
              document.getElementById("1_1").innerText,
              document.getElementById("2_0").innerText);
}

function checkWinner(first, second, third) {
  if(first != "" && first == second && first == third) {
    alert("Winner!");
    won = true;
  }
}