how to block someone from placing in the same spot TIC TAC TOE

I'm trying to learn how to block someone from placing an X or an O on the spots where has already been played. how do I make so when picture X is displayed, picture O cannot be placed in the same spot as the X? What i've done so far is the Nombreup and Nombre are the keybinds that you can use to place either X or O. I just do not know how to prevent it to place an O or X on a spot where an O or X has been placed (prevent cheating during a tic tac toe game). Here is my code :

document.onkeydown = Nombre;
Nombreup1 = 49; Nombreup2 = 50; Nombreup3 = 51;
Nombreup4   = 52; Nombreup5 = 53; Nombreup6 = 54;
Nombreup7   = 55; Nombreup8 = 56; Nombreup9 = 57;
Nombre7 = 103; Nombre8 = 104; Nombre9 = 105;
Nombre4 = 100; Nombre5 = 101; Nombre6 = 102;
Nombre3 = 99; Nombre2 = 98; Nombre1 = 97;

function Nombre(e){
  toucheCourante = e.keyCode;

  if (toucheCourante == Nombre7){
  lettreO7();

  } else if (toucheCourante == Nombre8){
    lettreO8();

  } else if (toucheCourante == Nombre9){
    lettreO9();


  } else if (toucheCourante == Nombre6){
    lettreO6();


  } else if (toucheCourante == Nombre5){
    lettreO5();


  } else if (toucheCourante == Nombre4){
    lettreO4();


  } else if (toucheCourante == Nombre3){
    lettreO3();


  } else if (toucheCourante == Nombre2){
    lettreO2();


  } else if (toucheCourante == Nombre1){
    lettreO1();
    }
/////////////////////////////////////////

    if (toucheCourante == Nombreup7){
    lettreX7();


  } else if (toucheCourante == Nombreup8){
      lettreX8();


  } else if (toucheCourante == Nombreup9){
      lettreX9();


  } else if (toucheCourante == Nombreup6){
      lettreX6();


  } else if (toucheCourante == Nombreup5){
      lettreX5();


  } else if (toucheCourante == Nombreup4){
      lettreX4();


  } else if (toucheCourante == Nombreup3){
      lettreX3();


  } else if (toucheCourante == Nombreup2){
      lettreX2();


  } else if (toucheCourante == Nombreup1){
      lettreX1();
      }

}

function casearret(){
 if (c1 < cup1) {

 }
}





function lettreO1()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 40, 265,95,93);

  }
}

function lettreX1()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 40, 265,95,93);

  }
}

function lettreO2()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 145, 265,110,93);

  }
}

function lettreX2()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 145, 265,110,93);

  }
}

function lettreO3()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 265, 265,95,93);
  }
}

function lettreX3()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 265, 265,95,93);
  }
}

function lettreO4()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 40,149,95,107);

  }
}

function lettreX4()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 40,149,95,107);

  }
}

function lettreO5()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 145,149,110,107);

  }
}

function lettreX5()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 145,149,110,107);

  }
}

function lettreO6()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 265,149,95,107);
  }
}

function lettreX6()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 265,149,95,107);

  }
}

function lettreO7()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 40,40,95,101);
  }
}

function lettreX7()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 40,40,95,101);

  }
}

function lettreO8()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 145,40,110,101);

  }
}
function lettreX8()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
context.drawImage(lettreX, 145,40,110,101);

  }
}
function lettreO9()
{
  lettreO = new Image();
  lettreO.src = 'O.jpg';
  lettreO.onload = function(){
  context.drawImage(lettreO, 265,40,95,101);
  }
}

function lettreX9()
{
  lettreX = new Image();
  lettreX.src = 'X.PNG';
  lettreX.onload = function(){
  context.drawImage(lettreX, 265,40,95,101);
  }
}