Workshop › Endless Runner › Step 18 of 19

Duck

What we are doing

Holding the down arrow on the ground makes the runner shorter, low enough to pass under a bird. While ducking it cannot jump.

```js
const DUCK_H = 26

  // Duck only on the ground; keep the feet in place when the height changes.
  const h = keys.ArrowDown && onGround() ? DUCK_H : STAND_H
  runner.y += runner.h - h
  runner.h = h

  if (onGround() && runner.h === STAND_H) runner.vy = JUMP
```

What it means

• h is the height the runner should have this frame: short while the down arrow is held on the ground.
• runner.y += runner.h - h moves the top down by as much as the box shrinks (or up as it grows), so the feet stay put.
• A ducking runner is not STAND_H tall, so jump does nothing.

Your turn

1. Under STAND_H, write DUCK_H.
2. In update, under the ground block, write the comment and the three duck lines.
3. In jump, add && runner.h === STAND_H.

Skills: Keyboard and mouse input, Collision detection

← Which keys are down? Build it yourself: jump buffering →

All steps

1. The ground
2. The runner
3. Draw again and again
4. Gravity
5. Jump
6. Short hops
7. Tap to jump
8. Things that come at you
9. Cacti keep coming
10. Wait for the start
11. Crash
12. Try again
13. Faster and faster
14. Remember the best run
15. The score counter
16. Birds
17. Which keys are down?
18. Duck
19. Build it yourself: jump buffering