Real-time tracking demos usually assume a GPS device already exists. Early on, it doesn’t — but the map still needs to move convincingly.
The core idea
Position becomes a pure function of elapsed time rather than a stream of device updates:
const progress = (now - createdAt) / TRIP_DURATION;
const position = interpolate(route, progress);
No background job, no drift, no state to keep in sync — refreshing the page always reconstructs the same position for the same wall-clock time.
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