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Title: Respect package.json `"type"` and module-format-specific file extensions in more `module` modes by andrewbranch · Pull Request #57896 · microsoft/TypeScript · GitHub

Open Graph Title: Respect package.json `"type"` and module-format-specific file extensions in more `module` modes by andrewbranch · Pull Request #57896 · microsoft/TypeScript

X Title: Respect package.json `"type"` and module-format-specific file extensions in more `module` modes by andrewbranch · Pull Request #57896 · microsoft/TypeScript

Description: Fixes #54752 Fixes #50647 Closes #54788 Related #55221 Prerequisite for #54102 Suggest reviewing individual commits: 96bffce: source changes 09330b8: tests showing new behavior c89137d: noise due to always looking up package.json 11e5727: noise due to the same, plus impliedNodeFormat always existing in buildinfo f9f46fd: less noisy changes to existing test behavior, worth reviewing This PR allows us to use module-format-specific file extensions and package.json "type" fields in all module modes—not just in node16/nodenext as today—but the way we use them can vary by module/moduleResolution mode. This has two noticeable effects: Emit respects file extension / package.json "type" more often. When module is es2015–esnext, we will now treat .cts/.cjs files, as well as .ts/.js files when the nearest package.json has "type": "commonjs", as CommonJS and emit CommonJS outputs accordingly. Note that unlike in --module nodenext, a lack of package.json "type" does not cause .ts/.js files to be treated as CommonJS. We only override the default behavior of the module setting when "type" is explicitly set to "commonjs", or when the file extension is .cts/.cjs. When module is commonjs, we will now treat .mts/.mjs files, as well as .ts/.js files when the nearest package.json has "type": "module", as ESM and output ESM outputs accordingly. When module is preserve, ESM import/export declarations are now forbidden in .cts/.cjs files, as well as in .ts/.js files when the nearest package.json has "type": "commonjs". We want to ensure that --module preserve means the module syntax you write gets emitted without significant transformation, but we also want to prevent ESM syntax from being used in unambiguously CommonJS files, so this situation acts much like verbatimModuleSyntax is enabled. Unambiguously ESM-mode declaration files do not get synthesized default exports This is #54752. Now that we always collect information about the package.json "type" and file extension, we gain the ability to notice when a declaration file, likely in a node_modules dependency, must represent an ESM JavaScript file, which changes how a default import targeting that file works. Previously, under all module modes except node16/nodenext, this was allowed: // @Filename: node_modules/esm-dep/foo.d.mts export declare const x: any; // @Filename: index.ts import esm from "esm-dep/foo.mjs"; esm.x; Since foo.d.mts has an unambiguously-ESM file extension, we should not allow a default import from a module that doesn’t declare a default export. However, it was allowed because we believed that foo.d.mts might actually represent a CommonJS module with a module.exports.x property, in which case a default import should be allowed and would link to the module.exports object. With this PR, when a declaration file has an ESM file extension or exists in a "type": "module" context, we use that to prevent us from letting a default import happen where it shouldn’t. This work also unlocks a fix for #54102. Webpack and esbuild use module-format-specific file extensions and package.json "type" to vary their ESM/CJS interop rules, so if we want to provide correct types for projects using those bundlers, we will need to be able to leverage that module format info under --module esnext or --module preserve. There is one test failing that represents an pre-existing bug that was not exposed until now because the test did not previously look up package.json files, and a particular sequence of edits causes a failure in combination with that. It has been isolated to a failure that doesn’t rely on this PR’s changes in #57757 so it can be investigated separately.

Open Graph Description: Fixes #54752 Fixes #50647 Closes #54788 Related #55221 Prerequisite for #54102 Suggest reviewing individual commits: 96bffce: source changes 09330b8: tests showing new behavior c89137d: noise due ...

X Description: Fixes #54752 Fixes #50647 Closes #54788 Related #55221 Prerequisite for #54102 Suggest reviewing individual commits: 96bffce: source changes 09330b8: tests showing new behavior c89137d: noise due ...

Opengraph URL: https://github.com/microsoft/TypeScript/pull/57896

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