If statement not executing correct code | Javascript

So I am trying to develop a Tic Tac Toe game for practice with javascript but I've hit a roadblock. I have an if statement that should be returning true but it isn't, here is a sample.

var game = true;
var x = 'X';
var o = 'O';
var blank = '';

var turn = x;
var board = [blank, blank, blank,
             blank, blank, blank,
             blank, blank, blank];

function write() {

    $('td').click(function() {
        //Making sure that the block that was clicked can only be clicked once
        var id = $(event.target).attr('id');
        var digit = parseInt(id.slice(-1));
        //check to see of the block has been clicked on
        if (board[digit] = blank) {
            board[digit] = turn;
            $(board[digit]).html(turn.toUpperCase());
            if (turn = x) {
                turn = o;
            } else if (turn = o) {
                turn = x;
            }

        } else {
            alert("That box has already been clicked on!")
        }
    }); 

}