How to zoom in/out in react-native-map?

This is what I did and it worked out for me very well:

function getRegion(origin, destination, zoom) {
  const oLat = Math.abs(origin.latitude);
  const oLng = Math.abs(origin.longitude);
  const dLat = Math.abs(destination.latitude);
  const dLng = Math.abs(destination.longitude);

  return {
      latitude: (origin.latitude + destination.latitude) / 2,
      longitude: (origin.longitude + destination.longitude) / 2,
      latitudeDelta: Math.abs(oLat - dLat) + zoom,
      longitudeDelta: Math.abs(oLng - dLng) + zoom,
  };                                                                                  
}

New React Native Maps API gives you option to call animateCamera method with zoom parameter.

const MapComponent= (props: any) => {

const map: LegacyRef<MapView> = useRef(null);

const onZoomInPress = () => {
    map?.current?.getCamera().then((cam: Camera) => {
        cam.zoom += 1;
        map?.current?.animateCamera(cam);
    });
};

return (
        <View>
            <MapView
                ref={map}
                provider={PROVIDER_GOOGLE}
                region={region}>
            </MapView>
            <ButtonComponent
                style={{position: 'absolute', bottom: 400, left: 0}}
                onPress={onZoomInPress}>
                Zoom In
            </MainButtonBlue>
        </View>
    );
}

I was able to make this work using Dimensions.get('window');

            const window = Dimensions.get('window');
            const { width, height }  = window
            LONGITUDE_DELTA = LATITUD_DELTA + (width / height)

and by default set LATITUD_DELTA = 0.0922. Then just update this values with the prop onRegionChangeComplete in the <MapView>

You should use the animateToRegion method (see here)

It takes a region object which has latitudeDelta and longitudeDelta. Use these to set the zoom level.

Updated:

in a Region object the latitude and longitude specify the center location and latitudeDelta and longitudeDelta specify the span of the viewable map area.

This image from this blog post illustrates it well (LatΔ and LngΔ).