Update filter.js

src/public/javascript/gym/filter.js

-var filter = document.getElementById("filter-gym");
-var filterButton = document.getElementById("button-filter");
-var searchFilter = document.getElementById("button-search-filter");
-var cancelFilter = document.getElementById("button-cancel-filter");
-var searchButton = document.getElementById("button-search");
-var inputSearch = document.getElementById("input-search");
-var inputPriceMin = document.getElementById("filter-min-price");
-var inputPriceMax = document.getElementById("filter-max-price");
-var toggleAlphAsc = document.getElementById("filter-alph-asc");
-var toggleAlphDesc = document.getElementById("filter-alph-desc");
-var togglePriceAsc = document.getElementById("filter-price-asc");
-var togglePriceDesc = document.getElementById("filter-price-desc");
+const filter = document.getElementById("filter-gym");
+const filterButton = document.getElementById("button-filter");
+const searchFilter = document.getElementById("button-search-filter");
+const cancelFilter = document.getElementById("button-cancel-filter");
+const searchButton = document.getElementById("button-search");
+const inputSearch = document.getElementById("input-search");
+const inputPriceMin = document.getElementById("filter-min-price");
+const inputPriceMax = document.getElementById("filter-max-price");
+const toggleAlphAsc = document.getElementById("filter-alph-asc");
+const toggleAlphDesc = document.getElementById("filter-alph-desc");
+const togglePriceAsc = document.getElementById("filter-price-asc");
+const togglePriceDesc = document.getElementById("filter-price-desc");
 filter.style.display = "none";
 filter.addEventListener("click", function (event) {
   // stop propagation but if clicks outside of the filter, it will close the filter