In FacetWP 3.9, we’ve removed some deprecated javascript methods. The old methods (and their replacements) are listed below:

FWP.build_post_data()       => FWP.buildPostData()
FWP.build_query_string()    => FWP.buildQueryString()
FWP.fetch_data              => FWP.fetchData()
FWP.load_from_hash()        => FWP.loadFromHash()
FWP.parse_facets()          => FWP.parseFacets()
FWP.set_hash()              => FWP.setHash()

If your site relies on any of the old methods, add the following hook to resolve issues:

add_filter( 'facetwp_load_deprecated', '__return_true' );