2. Keep the trip in native code

trip.lucent.ts

src/trip.lucent.ts

  const seconds = (b.time - a.time) / 1000;
  return seconds > 0 ? distance(a, b) / seconds : 0;
}

export type TripSummary = { fixes: number; meters: number; seconds: number; topSpeed: number };

/** A trip keeps its fixes in native code; JavaScript holds the Trip. */
export class Trip {
  private fixes: Fix[] = [];

  add(fix: Fix): void {
    this.fixes.push(fix);
  }

  get count(): number {
    return this.fixes.length;
  }

  summary(): TripSummary {
    let meters = 0;
    let topSpeed = 0;
    for (let i = 1; i < this.fixes.length; i++) {
      const a = this.fixes[i - 1]!;
      const b = this.fixes[i]!;
      meters += distance(a, b);
      topSpeed = Math.max(topSpeed, speed(a, b));
    }
    const first = this.fixes[0];
    const last = this.fixes[this.fixes.length - 1];
    const seconds = first && last ? (last.time - first.time) / 1000 : 0;
    return { fixes: this.fixes.length, meters, seconds, topSpeed };
  }
}

App.tsx

App.tsx

import { Text } from "react-native";
import { distance, speed } from "./src/trip.lucent";
import { Trip } from "./src/trip.lucent";

// The Eiffel Tower, then the Louvre 40 minutes later.
const start = { latitude: 48.8584, longitude: 2.2945, time: 0 };
const end = { latitude: 48.8606, longitude: 2.3376, time: 40 * 60 * 1000 };
// A walk along the Seine, one fix every ten minutes.
const walk = [
  { latitude: 48.8584, longitude: 2.2945, time: 0 },
  { latitude: 48.8637, longitude: 2.3023, time: 10 * 60 * 1000 },
  { latitude: 48.8661, longitude: 2.3125, time: 20 * 60 * 1000 },
  { latitude: 48.8638, longitude: 2.3252, time: 30 * 60 * 1000 },
  { latitude: 48.8606, longitude: 2.3376, time: 40 * 60 * 1000 },
];

const trip = new Trip();
for (const fix of walk) trip.add(fix);

export default function App() {
  const summary = trip.summary();
  return (
    <Text>
      {(distance(start, end) / 1000).toFixed(2)} km at {speed(start, end).toFixed(1)} m/s
      {summary.fixes} fixes, {(summary.meters / 1000).toFixed(2)} km, top speed{" "}
      {summary.topSpeed.toFixed(1)} m/s
    </Text>
  );
}

Trip is a class, so JavaScript gets a handle on one native object rather than a copy. Its fixes stay in native code: JavaScript sends each fix once, with add.

What's copied, and what isn't

• trip.add(fix) copies the fix. Changing fix in JavaScript afterwards doesn't change the trip.
• trip.summary() returns a new object on each call: a copy of a TripSummary struct.
• trip itself crosses by reference. Passing it back to Lucent later gives the same object, with its fixes.
• trip.count is a getter: reading it calls into native code.

This is the shape Design the boundary first recommends: the data stays native, and small values cross.

Run it

The screen shows 5 fixes, 3.55 km, top speed 1.6 m/s.