refactor: remove const enum

[yangmingshan]

const enum caused many trouble for both user (#1228) and Vue itself. So this PR is trying to remove all the const enum in Vue codebase, with non-breaking way of course.

For types I will use union type to replace const enum, for runtime I will replace const enum with literal value. It's not a breaking change because const enum is compatible with union type, so the code below can continue to work:

const enum TrackOpTypes {
  GET = 'get',
  HAS = 'has',
  ITERATE = 'iterate'
}

type TrackOpTypesUnion = 'get' | 'has' | 'iterate'

function track(target: object, type: TrackOpTypesUnion, key: unknown) {
  // ...
}

track(rawTarget, TrackOpTypes.GET, key) // NO ERROR!

Currently I only refactored the @vue/reactivity package as an example, and it works. I kept the const enum declaration and its exports for compatibility.

Do you think this is worth a try? If so, please let me know so I can continue. Or if you don't think this is the right path, I'll stop there.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	85.9 kB	32.6 kB	29.5 kB
vue.global.prod.js	132 kB	49.3 kB	44.4 kB

Usages

Name	Size	Gzip	Brotli
createApp	47.9 kB	18.8 kB	17.2 kB
createSSRApp	50.6 kB	19.9 kB	18.2 kB
defineCustomElement	50.3 kB	19.6 kB	17.9 kB
overall	61.2 kB	23.7 kB	21.6 kB

[sxzz]

Thanks for your contribution. After the team discussion, we think we can keep enums but change them from const enum to enum. For internal usage, we can have a rollup plugin to transform the enum reference to raw value. For example, we replace TriggerOpTypes.SET with 'set'. This is for reducing bundle size and taking into account DX.

[yangmingshan]

That is very clever, enum to users, const enum to Vue. Impressive!

Should I close this PR?

[sxzz]

You can do it in this PR or redo in a new one. It's up to you ❤️

[yangmingshan]

I'll open another PR, thank you.

[yangmingshan]

@sxzz If we use this approach, we will need contain enum in bundle, otherwise user cannot use these enum at runtime, but it will increase the bundle size. Is it ok?

[sxzz]

It’s ok. If it is not referenced, it will be removed by tree-shaking.

[yangmingshan]

Not all of them, some enum will be exported, tree-shaking won't help, bundle size will increase a little.

But you can see the results first and then decide.

[xiaoxiangmoe]

@yangmingshan hi, I create a plugin to inline enum, see https://github.com/vuejs/core/pull/9261/files#diff-52a2558e5292b080df0a8a6b519b0ed62af0605833abe1766af51524e1985a34.

Maybe you can refer to this to continue improving your PR.