Grabbed dndbeyond's source code

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

ddb_main/packages/rules-engine/es/reducers/utils.js

@@ -0,0 +1,15 @@
+/**
+ *
+ * @param spellSlots
+ * @param spellLevel
+ * @param slotsUsed
+ */
+export function updateSpellSlots(spellSlots, spellLevel, slotsUsed) {
+    let newSpellSlots = [...spellSlots];
+    const levelIdx = newSpellSlots.findIndex((entity) => entity.level === spellLevel);
+    if (levelIdx > -1 && slotsUsed !== null) {
+        const newSlot = Object.assign(Object.assign({}, newSpellSlots[levelIdx]), { used: slotsUsed });
+        newSpellSlots = [...newSpellSlots.slice(0, levelIdx), newSlot, ...newSpellSlots.slice(levelIdx + 1)];
+    }
+    return newSpellSlots;
+}