Grabbed dndbeyond's source code

```
~/go/bin/sourcemapper -output ddb -jsurl https://media.dndbeyond.com/character-app/static/js/main.90aa78c5.js
```

ddb_main/node_modules/axios/lib/helpers/combineURLs.js

@@ -0,0 +1,14 @@
+'use strict';
+
+/**
+ * Creates a new URL by combining the specified URLs
+ *
+ * @param {string} baseURL The base URL
+ * @param {string} relativeURL The relative URL
+ * @returns {string} The combined URL
+ */
+module.exports = function combineURLs(baseURL, relativeURL) {
+  return relativeURL
+    ? baseURL.replace(/\/+$/, '') + '/' + relativeURL.replace(/^\/+/, '')
+    : baseURL;
+};