Now that you understand how the event handler is working, the application is almost ready. There are two functions that need to be written, the one that checks for a winner, and the one that checks for ties. Checking for a winning row is done here by just going row by row and verifying that all three squares have the same player written on it:

function rowComplete() {
  var lastPlayedBy = grid[1][1];
  if ((lastPlayedBy != "") &&
      (grid[2][1] == lastPlayedBy) &&
      (grid[3][1] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[1][2];
  if ((lastPlayedBy != "") && 
      (grid[2][2] == lastPlayedBy) &&
      (grid[3][2] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[1][3];
  if ((lastPlayedBy != "") && 
      (grid[2][3] == lastPlayedBy) &&
      (grid[3][3] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[1][1];
  if ((lastPlayedBy != "") && 
      (grid[1][2] == lastPlayedBy) &&
      (grid[1][3] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[2][1];
  if ((lastPlayedBy != "") && 
      (grid[2][2] == lastPlayedBy) &&
      (grid[2][3] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[3][1];
  if ((lastPlayedBy != "") && 
      (grid[3][2] == lastPlayedBy) &&
      (grid[3][3] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[1][1];
  if ((lastPlayedBy != "") && 
      (grid[2][2] == lastPlayedBy) &&
      (grid[3][3] == lastPlayedBy)) return(true);
  lastPlayedBy = grid[1][3];
  if ((lastPlayedBy != "") && 
      (grid[2][2] == lastPlayedBy) &&
      (grid[3][1] == lastPlayedBy)) return(true);
}

The function that checks for a tie is much shorter. We scan the grid and return if there is still empty clickable boxes:

function itsAtie() {
  for( var i = 1; i <= 3; i++)
    for( var j = 1; j <= 3; j++)
      if (grid[i][j] == "") return(false);
  return(true);
}