Workshop › Angry Birds-style Slingshot › Step 10 of 47

Launch!

What we are doing

Space launches the bird: a body of 20 by 20 on the sling, with a velocity along the aim that grows with the pull.

```js
const LAUNCH = 0.2 // speed per pixel of pull

// The launch velocity: pulled back by `pull` pixels, the bird flies the opposite way.
const launchVelocity = () => ({ vx: Math.cos(aim.angle) * aim.pull * LAUNCH, vy: Math.sin(aim.angle) * aim.pull * LAUNCH })

function launch() {
  const { vx, vy } = launchVelocity()
  bird = body('bird', SLING.x - BIRD, SLING.y - BIRD, BIRD * 2, BIRD * 2)
  bird.vx = vx
  bird.vy = vy
  bodies.push(bird)
}

  else if (event.key === ' ') launch()
```

What it means

launchVelocity() returns { vx, vy }: the aim's direction (cos, sin) times the pull times LAUNCH.
const { vx, vy } = ... takes the two fields out into two names.
The bird's box starts centered on the sling (SLING.x - BIRD is its left edge), gets the velocity and joins bodies, so the loop in draw paints it.
' ' is the name of the Space key.

Your turn

Under MAX_PULL write the LAUNCH line.
Above the keydown listener write launchVelocity and launch.
In the listener, above else return, add the Space line.

Skills: Velocity and gravity, Keyboard and mouse input

← Bodies Move by the velocity →

Launch!

What we are doing

What it means

Your turn

All steps