Workshop › Snake › Step 19 of 28

No U-turns

What we are doing

A snake cannot turn straight back into its own neck. We ignore the opposite direction, and keep the chosen turn in nextDir until the snake actually moves.

```js
let nextDir = dir

  // Ignore a turn straight back into the snake's own neck.
  if (turn.x === -dir.x && turn.y === -dir.y) return
  nextDir = turn

function update() {
  dir = nextDir
```

What it means

• The opposite of { x: 1, y: 0 } is { x: -1, y: 0 }: both numbers negated. If the new turn is exactly that, ignore it.
• Keys only set nextDir; update copies it into dir right before moving, once per move.

Your turn

1. Under let dir = ... write let nextDir = dir.
2. In the listener, add the comment and the reverse check, and change dir = turn to nextDir = turn.
3. Make dir = nextDir the first line of update.

Skills: Keyboard and mouse input, Game states

← A table of turns Crash into the walls →

All steps

1. Find the canvas
2. Get the pen
3. Paint the board
4. Think in cells
5. Keep the position in a variable
6. A function that draws
7. A function that moves
8. The game loop
9. Slow it down
10. A direction to move in
11. Steer with the arrow keys
12. A body made of a list
13. Move the whole body
14. Pick a spot for the food
15. Draw the food
16. Eat and grow
17. Never put food on the snake
18. A table of turns
19. No U-turns
20. Crash into the walls
21. Crash into yourself
22. The game over message
23. Show the score
24. One function that starts a game
25. Play again with Space
26. Remember the best score
27. Speed up as you grow
28. Build it yourself: through the walls