This project is made with the help of Claude (1M context).

CollideJS

IntersectionObserver for any two elements.

Proximity, collision, and time-to-collision detection for the DOM. Tiny, shape-agnostic, TypeScript-first.

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Why CollideJS?

The browser gives you IntersectionObserver — but only against the viewport (or one ancestor). There's nothing built-in for "is this element near, or hitting, that other element?"

CollideJS fills that gap. It's ~2KB, has zero deps, and uses a single shared RAF loop to batch all observer reads.

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Quick Start

<script src="https://cdn.jsdelivr.net/npm/@buildwithdarsh/collidejs"></script>
<script>
  const pair = Collide.watch(cardA, cardB, { near: 80 });
  pair.on('near',     (m) => console.log('near:', m.distance, 'px'));
  pair.on('collide',  (m) => console.log('hit:', m.overlap));
  pair.on('separate', ()  => console.log('clear'));
</script>

Or with a bundler:

npm install @buildwithdarsh/collidejs

import Collide from '@buildwithdarsh/collidejs';

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API

Collide.watch(a, b, opts?) → Pair

Watch two elements. Emits near, collide, separate, leave, tick.

const pair = Collide.watch(el1, el2, {
  near: 60,           // px threshold for 'near' event (default 60)
  shape: 'aabb',      // 'aabb' | 'circle' (default aabb)
  trackVelocity: true,// compute velocity + TTC (default true)
});
pair.on('near',    (m) => ...);
pair.on('collide', (m) => ...);
pair.metrics();      // one-off snapshot
pair.destroy();

Metrics shape:

{
  colliding: boolean;
  distance: number;       // min distance between shapes
  overlap: { x, y };      // overlap dimensions when colliding
  centerA, centerB: Vec2;
  direction: Vec2;        // unit vector A → B
  angle: number;          // radians
  velocity: Vec2;         // B relative to A, px/sec
  closingSpeed: number;   // positive = approaching, px/sec
  ttc: number | null;     // time-to-collision in seconds
}

Collide.group(source, targets, opts?) → Group

Watch one element against many targets (drag-to-drop zones).

const g = Collide.group(drag, dropZones, { near: 80 });
g.on('near',    (zone, m) => zone.classList.add('highlight'));
g.on('collide', (zone, m) => commitDrop(zone));
g.closest();  // { target, metrics }

Collide.all(elements, opts?) → All

N×N observer — every pair tested every frame. Great for crowd avoidance, packing.

const a = Collide.all(nodes);
a.on('collide', (a, b, m) => separate(a, b, m.direction));

Collide.check(a, b, opts?) → CollisionMetrics

One-shot check. No observer, no RAF — use for discrete hit-tests.

if (Collide.check(bullet, target).colliding) score();

Collide.shapes(a, b) → CollisionMetrics

Low-level geometry: collide two Shape objects directly. Framework-agnostic.

Collide.shapes(
  { kind: 'aabb', x: 0, y: 0, w: 10, h: 10 },
  { kind: 'circle', cx: 5, cy: 20, r: 4 },
);

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Performance

• Single shared RAF — every observer registers into the same loop. Runtime cost scales with the number of observed pairs, not the number of observer instances.
• Batched getBoundingClientRect — one read per element per frame, never during event handlers.
• Auto-stops when idle — the loop pauses when the last observer is destroyed.

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License

MIT © Darsh Gupta