add game over to javascript game - game-development

Question

let canvas = document.getElementById("snake");
let context = canvas.getContext("2d"); //....
let box = 42;
let snake = [];
snake[0] ={
x: 8 * box,
y: 8 * box
}
let direction = "rechts";
let food ={
x: Math.floor(Math.random() * 15 + 1) * box,
y: Math.floor(Math.random() * 15 + 1) * box
}
function maakachtergrond(){
context.fillStyle = "#FFEAF0";
context.fillRect(0, 0, 16*box, 16*box);
}
function maakslang (){
for(i = 0; i < snake.length; i++){
context.fillStyle = "#FFFFFF";
context.fillRect(snake[i].x, snake[i].y, box, box);
}
}
function heteten (){
context.fillStyle = "#010101";
context.fillRect(food.x, food.y, box, box);
}
document.addEventListener('keydown', update);
function update(event){
if(event.keyCode == 37 && direction != 'rechts') direction = 'left';
if(event.keyCode == 38 && direction != 'down') direction = 'up';
if(event.keyCode == 39 && direction != 'left') direction = 'rechts';
if(event.keyCode == 40 && direction != 'up') direction = 'down';
}
function starthetspel(){
if(snake[0].x > 15*box && direction == "rechts") snake[0].x = 0;
if(snake[0].x < 0 && direction == 'left') snake[0].x = 16 * box;
if(snake[0].y > 15*box && direction == "down") snake[0].y = 0;
if(snake[0].y < 0 && direction == 'up') snake[0].y = 16 * box;
for(i = 1; i < snake.length; i++){
if(snake[0].x == snake[i].x && snake[0].y == snake[i].y){
clearInterval(jogo);
}
}
maakachtergrond();
maakslang();
heteten();
let snakeX = snake[0].x;
let snakeY = snake[0].y;
if(direction == "rechts") snakeX += box;
if(direction == "left") snakeX -= box;
if (direction == "up") snakeY -= box;
if(direction == "down") snakeY += box;
if(snakeX != food.x || snakeY != food.y){
snake.pop();
}else{
food.x = Math.floor(Math.random() * 15 +1) * box;
food.y = Math.floor(Math.random() * 15 +1) * box;
}
let newHead ={
x: snakeX,
y: snakeY
}
snake.unshift(newHead);
}
let jogo = setInterval(starthetspel, 100);
how to add a game over if i touch the edge of the game. For now the snake just respawn on the other side of the game, but i would prefer it to make it a game over
Maybe something like
if fillStyle = "#FFEAF0"
window.alert(game over);
and allowing the snake to go outside his gamefield, but I don't know how. It's for a small asignment I got from my teacher and maybe someone can help me out.