JS: Remove all references to global URL constants

Instead they are passed in wherever needed. Cleaner code,
and path towards everything being more testable

binderhub/static/js/index.js

@@ -18,10 +18,32 @@ import "bootstrap/dist/css/bootstrap-theme.min.css";
 import "../index.css";
 import { setUpLog } from "./src/log";
 import { updateUrls } from "./src/urls";
-import { BASE_URL, BADGE_BASE_URL } from "./src/constants";
 import { getBuildFormValues } from "./src/form";
 import { updateRepoText } from "./src/repo";
 
+/**
+ * @type {URL}
+ * Base URL of this binderhub installation.
+ *
+ * Guaranteed to have a leading & trailing slash by the binderhub python configuration.
+ */
+const BASE_URL = new URL(
+ document.getElementById("base-url").dataset.url,
+ document.location.origin,
+);
+
+const badge_base_url = document.getElementById("badge-base-url").dataset.url;
+/**
+ * @type {URL}
+ * Base URL to use for both badge images as well as launch links.
+ *
+ * If not explicitly set, will default to BASE_URL. Primarily set up different than BASE_URL
+ * when used as part of a federation
+ */
+const BADGE_BASE_URL = badge_base_url
+ ? new URL(badge_base_url, document.location.origin)
+ : BASE_URL;
+
 async function build(providerSpec, log, fitAddon, path, pathType) {
  updateFavicon(new URL("favicon_building.ico", BASE_URL));
  // split provider prefix off of providerSpec

binderhub/static/js/src/repo.js

@@ -1,5 +1,3 @@
-import { BASE_URL } from "./constants";
-
 /**
  * Dict holding cached values of API request to _config endpoint
  */
@@ -21,10 +19,12 @@ function setLabels() {
 
 /**
  * Update labels for various inputboxes based on user selection of repo provider
+ *
+ * @param {URL} baseUrl Base URL to use for constructing path to _config endpoint
  */
-export function updateRepoText() {
+export function updateRepoText(baseUrl) {
  if (Object.keys(configDict).length === 0) {
- const configUrl = new URL("_config", BASE_URL);
+ const configUrl = new URL("_config", baseUrl);
  fetch(configUrl).then((resp) => {
  resp.json().then((data) => {
  configDict = data;