Sticky Zoom for geoJson - zooming

Question

Dynamically loading geoJson for Chloropleth map. When I zoom in and pan all is well but if I choose another Cell Carrier which rerenders the Chloropleth it always jumps back out to the original default Zoom/Center.
How can I implement a 'sticky zoom' to preserve the session bounds?
I've been playing with storing it in an hidden field but if always seems to be overwritten by the change in geoJson features.
var map = new mapboxgl.Map({
container: 'map',
style: 'mapbox://styles/mapbox/light-v10',
center: [-100, 38],
zoom: 3
});
if (document.getElementById('hfldBounds').value != 'null') {
map.fitBounds(document.getElementById('hfldBounds').value);
};
map.on('zoom', function (e) {
document.getElementById('hfldBounds').value = map.getBounds();
});
Ideally the user selects a carrier that renders the map. They zoom in and pan and then they want to see what another carrier owns in the same bounds so they change carrier, get the new geoJson data but maintain the previous map extents.

Answer 1

I finally worked this out myself so I'll post the solution here:
<script>
var map = new mapboxgl.Map({
container: 'map',
style: 'mapbox://styles/mapbox/light-v10',
center: [-100, 38],
zoom: 3
});
map.on('zoom', function (e) {
sessionStorage.setItem('bounds', map.getBounds());
});
map.on('drag', function (e) {
sessionStorage.setItem('bounds', map.getBounds());
});
if (sessionStorage.getItem('bounds') != null) {
let bounds = sessionStorage.getItem('bounds');
var araBounds = bounds.toString().split(',');
var swX = parseFloat(araBounds[0].replace('LngLatBounds(LngLat(',''));
var swY = parseFloat(araBounds[1].replace(')',''));
var neX = parseFloat(araBounds[2].replace('LngLat(',''));
var neY = parseFloat(araBounds[3].replace('))', ''));
var ne = new mapboxgl.LngLat(neX, neY);
var sw = new mapboxgl.LngLat(swX, swY);
var box = new mapboxgl.LngLatBounds(sw, ne);
map.fitBounds(box);
};
</script>
load map
save bounds to session variable on move or pan
check for bounds and construct a new bounds object
use bounds object with map.fitBounds() to zoom back to your previous extents.