Upgrade To ESLint 9

[JamieB-gu]

ESLint 9 comes with breaking changes¹. We publish some Guardian-specific ESLint config from CSNX which has already been updated to support these changes², and typescript-eslint v8 also supports this new version³.

The upgrade will require changes to our configuration, as demonstrated by the failing build in #13496.

Footnotes

1. https://eslint.org/blog/2024/04/eslint-v9.0.0-released/ ↩

2. https://github.com/guardian/csnx/pull/1658 ↩

3. https://typescript-eslint.io/blog/announcing-typescript-eslint-v8 ↩