delete countries index.js

index.js

/* New import at the top*/
import { fromLonLat } from 'ol/proj'

// Wait for source to render
countriesSource.once('addfeature', () => {
    // For each visited place
    visitedPlaces.forEach(place => {
        // Obtain map coordinates from longitude and latitude
        const coordinate = fromLonLat(place)

        // For each feature at coordinate, remove it from the source
        // Because OpenLayers observes for changes, this will visually delete the countries from the map.
        countriesSource.getFeaturesAtCoordinate(coordinate).forEach(f => countriesSource.removeFeature(f))
    })
})