Workshop › Missile Command-style Defense › Step 15 of 35

An explosion that grows and shrinks

What we are doing

Interceptors will explode where they arrive. An explosion is a position and an age; its radius is worked out from the age: it grows for the first half of its life and shrinks in the second.

```js
const BLAST = 32 // the biggest radius of an explosion
const BLAST_FRAMES = 50 // how long an explosion lasts, growing then shrinking

// How big an explosion is at its age: it grows for the first half and shrinks in the second.
function radius(b) {
  const t = b.age / BLAST_FRAMES
  return BLAST * (t < 0.5 ? t * 2 : (1 - t) * 2)
}
```

What it means

t goes from 0 to 1 over the explosion's life.
In the first half t * 2 goes 0 → 1; in the second (1 - t) * 2 goes 1 → 0. Times BLAST that is 0 → 32 → 0.
Computing the size from the age, instead of storing it, means it can never get out of step.

Your turn

Under const SHOT_SPEED = 7 write the two constants.
Above function update() {, write radius with its comment, followed by an empty line.

Skills: Velocity and gravity

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