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/@mui/utils/deepmerge/deepmerge.js

@@ -0,0 +1,41 @@
+import _extends from "@babel/runtime/helpers/esm/extends";
+// https://github.com/sindresorhus/is-plain-obj/blob/main/index.js
+export function isPlainObject(item) {
+  if (typeof item !== 'object' || item === null) {
+    return false;
+  }
+  const prototype = Object.getPrototypeOf(item);
+  return (prototype === null || prototype === Object.prototype || Object.getPrototypeOf(prototype) === null) && !(Symbol.toStringTag in item) && !(Symbol.iterator in item);
+}
+function deepClone(source) {
+  if (!isPlainObject(source)) {
+    return source;
+  }
+  const output = {};
+  Object.keys(source).forEach(key => {
+    output[key] = deepClone(source[key]);
+  });
+  return output;
+}
+export default function deepmerge(target, source, options = {
+  clone: true
+}) {
+  const output = options.clone ? _extends({}, target) : target;
+  if (isPlainObject(target) && isPlainObject(source)) {
+    Object.keys(source).forEach(key => {
+      // Avoid prototype pollution
+      if (key === '__proto__') {
+        return;
+      }
+      if (isPlainObject(source[key]) && key in target && isPlainObject(target[key])) {
+        // Since `output` is a clone of `target` and we have narrowed `target` in this block we can cast to the same type.
+        output[key] = deepmerge(target[key], source[key], options);
+      } else if (options.clone) {
+        output[key] = isPlainObject(source[key]) ? deepClone(source[key]) : source[key];
+      } else {
+        output[key] = source[key];
+      }
+    });
+  }
+  return output;
+}