browser compatibility broken

[oliver-sanders]

We used to use Babel to patch contemporary ECMA into something with more reasonable browser support.

Along the line we switched implementation, but it looks like things have got a bit broken.

See #2370 (comment)

[MetRonnie]

Copying the comment for reference:

I would have expected Babel to handle this

(We have not been using Babel since the move to Vite; it uses esbuild^[1] instead)

Note Vite does transform some newer ECMAScript syntax (stuff like operators, e.g. &&= was introduced relatively recently and might be transformed into a more compatible syntax if you set an old enough build target). But it can't transform APIs (e.g. ResizeObserver or indeed RegExp.escape)

Ah, after typing that all out I realise it's stated here lol: https://esbuild.github.io/api/#target

In #1341 I introduced a check for our build to ensure we don't have too-bleeding-edge APIs in our built code. But unfortunately it seems this plugin is falling out of maintenance:

• Use of RegExp.escape() not flagged with browserlist-config-wikimedia's ~last-3-years config amilajack/eslint-plugin-compat#668
• Several fucntions not flagged when using "last 7 years" amilajack/eslint-plugin-compat#656
• Compatibility info appears to be missing, eslint 9 config, 6.0 amilajack/eslint-plugin-compat#636

Footnotes

1. Although the next major version of Vite will be moving away from esbuild to their own (feature-equivalent) tool: https://vite.dev/blog/announcing-vite8-beta ↩

N.B. Babel provides back-compat for new APIs by using the core-js polyfill library - think of that XKCD comic about the dependency stack but replace "random person in Nebraska" with random person in Russia.

If we wanted to introduce polyfills (via core-js), it seems vite-plugin-legacy might be the way to go. However this would increase the build size.

If you don't have hard reliance on bleeding edge runtime features, it is not that hard to avoid having to use polyfills in the modern build altogether.

That was the aim with using eslint-plugin-compat. Perhaps we could find a similar plugin to replace it.