Legacy Code Modernization

Modernize this legacy code to use current best practices:

```
[PASTE_LEGACY_CODE]
```

**Current:** [WHAT_IT_USES — e.g., "class components, jQuery, callbacks, var"]
**Target:** [MODERN_PATTERNS — e.g., "React hooks, native fetch, async/await, const/let"]

Modernize step by step:
1. **Syntax upgrades** — var → const/let, callbacks → async/await, etc.
2. **Pattern updates** — class components → hooks, imperative → declarative
3. **API modernization** — jQuery → native DOM/React, XMLHttpRequest → fetch
4. **Type safety** — add TypeScript types where applicable
5. **Remove polyfills** — identify code that's no longer needed in modern browsers

For each change:
- Show the before and after
- Explain why the modern approach is better (not just "it's newer")
- Flag any behavioral differences to watch for

Provide the complete modernized file, followed by a migration checklist to verify nothing broke.