Support TypeScript const enums

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #8741 (which is the issue with most 👍s), fixes #6476
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2550
Any Dependency Changes?
License	MIT

When using --isolatedModules, TypeScript compiles const enum declarations as if they were normal enums: https://www.typescriptlang.org/play?#code/MYewdgzgLgBApmArgWxgQRgbwFAxgD2wF9ts58AHEAJ1lElgRRgCEtcYBPGAXhgCZipNADp8AbmyjO4oA (click on "TS Config" and enable isolatedModules). We can do the same thing by default, rather than throwing.

Additionally, this PR introduces a new optimizeConstEnums option to the TS plugin and preset:

• For const enums only used in the same file where they are declared, we can safely inline the enum values. This matched the TS behavior without --isolatedModules, and produces a much smaller output.
• When the const enum is exported, we can generate an object mapping from the enum keys to its values. This is way smaller than the default enum output, because it doesn't need to support reverse mappings (i.e. E[E.x]).

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[JLHwung]

🎉

[JLHwung]

Can we default it to true in next minor? I know we have onlyRemoveTypeImports default to true in Babel 8, but supporting new language features should be opt-in by default, like we did for other ES features.

[nicolo-ribaudo]

I'm not sure we should.

In TypeScript with --isolatedModules, code like this works and logs x:

const enum A {
  x
}

// @ts-expect-error
console.log(A[A.x]);

export {};

However, with optimizeConstEnums it stops working because we don't ignore const anymore.

For the default behavior, I'd prefer to match --isolatedModules as closely as possible where it's easy to do so.

[JLHwung]

Interesting, I think it should be a TS issue, they could have optimized the const enums here.

[dosentmatter]

TypeScript docs mentions why ambient const enums (ie. declare const enum) don't work with isolatedModules.

When isolatedModules: false, it seems const enum and declare const enum function the same.

When isolatedModules: true, it seems const enum transpiles as if it was just enum since you get the reverse-mapped object. declare const enum transpiles into the usual empty output, because it's expected to be inlined. However, the isolatedModules: true will produce error:

Cannot access ambient const enums when the '--isolatedModules' flag is provided.(2748)

No idea why they only document declare const enum and not const enum. But I assume their reasoning of converting const enum to just enum is to have it work across modules. And I guess they treat declare const enum as an error since you are explicitly saying it is ambient.

---

Anyway, @nicolo-ribaudo. I'm not familiar with babel code, but did you make sure you also support declare const enum when optimizeConstEnums: true? When isolatedModules: false, declare const enum seems to function the same as const enum, so you can just strip the declare.

Also, if we want to copy TypeScript functionality with isolatedModules: true and optimizeConstEnums: false, should we transpile const enum by treating it as enum ie. transpile it to the reverse-mapped object instead of throwing an error?

[nicolo-ribaudo]

The default Babel behavior is to match --isolatedModules (as mentioned in our docs at the 4th caveat).

Thus, this PR makes it so that by default const enums are compiled to "normal" enums, which also matches the default behavior of your babel-plugin-const-enum plugin. It's implemented in 933ba59 (#13324).

With optimizeConstEnums: true the plugin seems to correctly work: https://github.com/babel/babel/pull/13324/files#diff-d0d0efefe2a94935f80a8f399bbca77e2a0fbbc8e7a3ba2a1a2f07eff7bcb0de. When optimizeConstEnums: false, it produces some broken output:

declare const enum Animal {
  Dog,
  Cat
}
let x: Animal = Animal.Dog;

// becomes

let x = Animal.Dog; 

however, I'm not too much concerned about it because:

• It's exactly the same output that TS generates with --isolatedMoudles
• TypeScript will report a type-checking error saying that declare const enum are not supported.