URL: /rules/perf/legacy-js

---
title: "Legacy JavaScript"
description: "Detects ES5 polyfills and legacy JavaScript code"
---

Detects ES5 polyfills and legacy JavaScript code

| | |
|---|---|
| **Rule ID** | `perf/legacy-js` |
| **Category** | [Performance](/rules/perf) |
| **Scope** | Per-page |
| **Severity** | info |
| **Weight** | 3/10 |

## Solution

Consider removing legacy polyfills if you don't need to support old browsers. Use differential serving (module/nomodule) to send modern code to modern browsers. Update Babel/TypeScript target to ES2020+ if your audience uses modern browsers. Check browserslist configuration.

## Enable / Disable

### Disable this rule

```toml squirrel.toml
[rules]
disable = ["perf/legacy-js"]
```

### Disable all Performance rules

```toml squirrel.toml
[rules]
disable = ["perf/*"]
```

### Enable only this rule

```toml squirrel.toml
[rules]
enable = ["perf/legacy-js"]
disable = ["*"]
```