file.js

// more lines, ever-so-slightly faster search, though.
function checkValidMove (xo, pos, state) {
    // invalid move.
    if (state[pos[0]][pos[1]] !== '') return false;

    state[pos[0]][pos[1]] = xo;

    // check row
    for (var i = 0, col = 0; i < 3; i ++) if (state[pos[0]][i] === xo) col ++;
    if (col === 3) return "a winner is you";

    // check col
    for (var i = 0, row = 0; i < 3; i ++) if (state[i][pos[1]] === xo) row ++;
    if (row === 3) return "a winner is you";

    // check \
    if (pos[0] === pos[1]) for (var i = 0, diag = 0; i < 3; i ++) if (state[i][i] === xo) diag ++;
    if (diag === 3) return "a winner is you";

    // check /
    if (pos[0] === 2-pos[1]) for (var i = 0, diag = 0; i < 3; i ++) if (state[i][2-i] === xo) diag ++;
    if (diag === 3) return "a winner is you";

    return "no winner for you!";
}