browser-car-speed

Tiny, typed browser GPS speed and movement detection.

npm install browser-car-speed

browser-car-speed reads the standard Geolocation API, prefers the browser's reported coords.speed, and uses a guarded rolling-coordinate estimate when that value is missing or stuck at zero. It has no runtime dependencies and does nothing until you call start().

Live example: Speedo · Simulated demo without location

Quick start

import {
  start,
  stop,
  subscribe,
  getCarSpeed,
  isCarMoving,
} from "browser-car-speed";

const unsubscribe = subscribe((snapshot) => {
  console.log(snapshot.status);                 // "locating" | "live" | ...
  console.log(snapshot.reading?.kilometersPerHour);
});

// Call from a click/tap because the browser may show a location prompt.
startButton.addEventListener("click", start);

getCarSpeed();        // km/h: number | null
getCarSpeed("mph");  // mph:  number | null
isCarMoving();        // boolean | null

unsubscribe();
stop();

null always means unknown or unavailable. It is never silently converted to zero. This distinction matters when GPS accuracy is too weak to prove that the device is stationary.

Independent trackers

Use createCarSpeed() when an app needs custom thresholds, an injected adapter, or more than one tracker.

import { createCarSpeed } from "browser-car-speed";

const car = createCarSpeed({
  movingThresholdKmh: 4,
  stationaryThresholdKmh: 2,
  staleAfterMs: 8_000,
});

const unsubscribe = car.subscribe(({ status, reading, moving }) => {
  if (status !== "live") return;
  console.log(reading.kilometersPerHour, moving);
});

car.start();

// Later:
unsubscribe();
car.stop();

API

Convenience singleton

• start() / startCarSpeed() — begin high-accuracy location tracking.
• stop() / stopCarSpeed() — clear the watcher, timer, and retained state.
• getCarSpeed(unit?) — current full-precision speed in "km/h" (default), "mph", or "m/s"; null when unavailable or stale.
• isCarMoving() — hysteresis-based true or false; null when unknown.
• getSnapshot() / getCarSpeedSnapshot() — current state and metadata.
• subscribe(listener, options?) / subscribeCarSpeed(...) — state updates; returns an idempotent unsubscribe function.
• carSpeed — the shared tracker used by these functions.

Tracker

createCarSpeed(options?) returns:

interface CarSpeedTracker {
  start(): void;
  stop(): void;
  getCarSpeed(unit?: "km/h" | "mph" | "m/s"): number | null;
  isCarMoving(): boolean | null;
  getSnapshot(): CarSpeedSnapshot;
  subscribe(listener: CarSpeedListener, options?: { emitCurrent?: boolean }): () => void;
}

Snapshot

interface CarSpeedSnapshot {
  status: "idle" | "locating" | "live" | "stale" | "error" | "unsupported";
  reading: SpeedReading | null;      // fresh reading only
  lastReading: SpeedReading | null;  // retained while stale
  moving: boolean | null;
  error: CarSpeedError | null;
  running: boolean;
  supported: boolean;
  accuracyMeters: number | null;
  lastPositionTimestamp: number | null;
}

interface SpeedReading {
  metersPerSecond: number;
  kilometersPerHour: number;
  milesPerHour: number;
  source: "sensor" | "calculated";
  accuracyMeters: number | null;
  timestamp: number;
}

Snapshots and readings are immutable.

Options

Option	Default	Purpose
movingThresholdKmh	3	Enter the moving state at or above this speed.
stationaryThresholdKmh	1.5	Leave the moving state at or below this speed.
staleAfterMs	10000	Stop exposing a reading after this age.
maximumSpeedKmh	260	Reject implausibly high values.
maximumFallbackAccuracyMeters	15	Accuracy ceiling for coordinate-derived speed.
coordinateFallback	true	Estimate speed when coords.speed is unavailable.
minimumFallbackIntervalMs	500	Shortest coordinate baseline.
fallbackWindowMs	20000	Longest rolling coordinate baseline.
fallbackMinimumDisplacementMeters	3	Minimum absolute displacement evidence.
smoothingWindow	3	Accepted readings in the median filter (1–9).
maximumAccelerationKmhPerSecond	60	Maximum filtered change per second.
speedJumpToleranceKmh	15	Free jump tolerance before limiting changes.
freshnessIntervalMs	500	Stale-state notification interval; 0 disables it.
positionOptions	high accuracy, no cache, 15 s timeout	Native watch options.
geolocation	browser API	Structural adapter for testing or hybrid runtimes.
now	Date.now	Epoch-millisecond clock, injectable for tests.

How fallback avoids common GPS mistakes

• Displacement inside the reported accuracy radius remains unknown, not stationary.
• A rolling window preserves evidence of slow movement instead of repeatedly resetting the baseline.
• Two consistent, progressing coordinate estimates are required before fallback speed is accepted.
• A fixed GPS jump is rejected because its displacement does not continue.
• Old smoothing samples are reset after a stale gap.
• A browser-provided zero can be overridden by sustained coordinate evidence.
• Separate enter/exit thresholds keep movement state from flickering near zero.

Browser and vehicle limitations

This package estimates whether the browser/device is moving. It does not read Tesla CAN data, vehicle telemetry, wheel speed, or a calibrated dashboard sensor. Results are indicative and depend on the browser's Geolocation implementation, GPS conditions, hardware, permissions, and software version.

• Use HTTPS (or localhost); browsers normally restrict Geolocation on insecure origins.
• Ask for location from a user gesture where possible.
• Do not use this package as a safety system or a replacement for the vehicle's speedometer.
• Tesla fullscreen/Theater workarounds belong in the application layer and are deliberately not included here.

Utilities

import { convertSpeed, haversineMeters } from "browser-car-speed";

convertSpeed(100, "km/h", "mph");
haversineMeters(41.0082, 28.9784, 41.0151, 28.9795);

Development

npm install
npm run verify

The verification suite type-checks and builds the package, runs behavior tests, checks package exports/types, inspects the tarball allow-list, and installs the packed tarball into a clean temporary project.

License

MIT