[ts] Insert `export {}` when necessary to imply ESM #13314

wbinnssmith · 2021-05-14T23:22:14Z

This diff is best viewed with whitespace diff disabled: https://github.com/babel/babel/pull/13314/files?w=1

This makes the TypeScript transformer insert an empty export {} declaration into files that would otherwise start as modules but become ambiguous after transformation, specifically when there are no imports of exports in the transformed result. See #12568 for context.

Since type imports and exports are removed, this change tracks when imports and exports are left behind as value imports/exports. If there are any left behind, the export {} expression does not get added.

Q	A
Fixed Issues?	Fixes #12568
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	Yes
Tests Added + Pass?	Adjusted existing, which cover use
Documentation PR Link	None
Any Dependency Changes?	None
License	MIT

codesandbox-ci · 2021-05-14T23:25:39Z

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wbinnssmith · 2021-05-14T23:22:50Z

packages/babel-plugin-transform-typescript/src/index.ts

@@ -63,6 +63,8 @@ export default declare((api, opts) => {
    var { allowDeclareFields = false } = opts;
  }

+  let hasValueExports = false;


I believe this could also be attached to state instead of closing over it

We can either use a WeakMap from the program node to this variable, or attach it to state. I prefer the WeakMap approach since it's similar to what we use in many other plugins.

Closing over causes problems, because Babel doesn't reinstantiate the plugin for each file but only when the options change.

Btw, do we need both hasValueExports/hasValueImports, or can it be a single boolean?

I prefer the WeakMap approach since it's similar to what we use in many other plugins.

Great idea 🙂

Btw, do we need both hasValueExports/hasValueImports, or can it be a single boolean?

Nope, only one is necessary. I'll combine them.

JLHwung

LGTM with some nits.

For reviewers: please ignore whitespace changes when reviewing.

JLHwung · 2021-05-18T02:14:30Z

packages/babel-plugin-transform-typescript/src/index.ts

        }
+
+        IMPLICITLY_ESM.add(state.file.ast.program);


nit: Alternatively we can add this to the ExportDeclaration visitor, it covers ExportExportAllDeclaration, ExportDefaultDeclaration and ExportNamedDeclaration.

We would have to be careful about ordering, since we only want to add the node to the WeakSet if it's not removed.

JLHwung · 2021-05-18T02:14:55Z

packages/babel-plugin-transform-typescript/src/index.ts

        }
+
+        IMPLICITLY_ESM.add(state.file.ast.program);


Q: Why do we need this for ExportSpecifier?

Yeah good point — looks like this is already handled above.

nicolo-ribaudo

https://github.com/babel/babel/pull/13314/files?diff=unified&w=1#r633982388 could probably be removed, but except for that lgtm 👍

nicolo-ribaudo · 2021-05-18T15:43:01Z

Oh it looks like GH actions are down

Regarding the failing test, I checked how TS handles var a: string: it doesn't inject export {} (https://www.typescriptlang.org/play?module=6#code/G4QwTgBCBcEM4BcwEsB2BzA3EA). However, with this PR Babel transforms it to var a; export {};. I think it would be better to align our behavior and only inject export {} if we actually removed import/export statements.

babel-bot · 2021-05-18T21:24:24Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/46307/

nicolo-ribaudo · 2021-05-19T14:14:21Z

Could you address #13314 (comment)? 🙏

wbinnssmith · 2021-05-20T02:08:58Z

Could you address #13314 (comment)? 🙏

Ah, missed what you meant here. Just implemented this and checked a few of Babel's test cases agains the typescript playground and they behave the same now 😄

packages/babel-plugin-transform-typescript/src/index.ts

nicolo-ribaudo · 2021-05-20T22:54:50Z

Thanks!

wbinnssmith changed the title ~~Export an empty named export declaration to imply ESM~~ TypeScript transform: export an empty named export declaration to imply ESM May 14, 2021

wbinnssmith changed the title ~~TypeScript transform: export an empty named export declaration to imply ESM~~ TypeScript transform: insert an empty named export declaration when necessary to imply ESM May 14, 2021

wbinnssmith commented May 14, 2021

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wbinnssmith force-pushed the wbinnssmith/empty-export-named-decl branch from 23a2480 to 8589908 Compare May 18, 2021 00:18

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Export an empty named export declaration to imply ESM

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nicolo-ribaudo changed the title ~~TypeScript transform: insert an empty named export declaration when necessary to imply ESM~~ [ts] Insert export {} when necessary to imply ESM May 20, 2021

nicolo-ribaudo merged commit b4c798e into babel:main May 20, 2021

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Babel plugin transform typescript 7.14.4 Shopify/web-configs#256

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Fix ExportNamedDeclaration with babel 7.14.4 ui5-community/babel-plugin-transform-modules-ui5#53

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[ts] Insert `export {}` when necessary to imply ESM #13314

[ts] Insert `export {}` when necessary to imply ESM #13314

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[ts] Insert export {} when necessary to imply ESM #13314

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[ts] Insert `export {}` when necessary to imply ESM #13314

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