Workshop › Tower Defense › Step 19 of 42

Pick a target

What we are doing

A tower keeps asking: which enemy do I shoot? The classic answer: among those in range, the one that has walked the furthest, because it is the closest to getting through. With d, "furthest" is just the biggest d.

```js
// The enemy in range that has walked the furthest: it is the closest to getting through.
function targetFor(tower) {
  const range = TOWERS[tower.kind].range
  let target = null
  for (const e of enemies) {
    const p = pointAt(e.d)
    const inRange = Math.hypot(p.x - tower.col, p.y - tower.row) <= range
    if (inRange && (!target || e.d > target.d)) target = e
  }
  return target
}
```

What it means

Math.hypot(dx, dy) is the straight distance between two points, in tiles here.
target starts as null; an enemy in range replaces it if there is none yet or if it has walked further.
With nobody in range, the answer is null.

Your turn

Above function update() { write the comment and targetFor.

Skills: Collision detection

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