[code-infra] Support flat build output behind a flag #1064

brijeshb42 · 2026-01-22T13:52:26Z

New cli arg --flat for the new logic to create flat builds
~~Flat builds create mjs (and d.mts for types) files for esm and cjs (and d.cts for types) files for cjs builds for simplicity. There are no js files now.~~. I scrapped this due to an issue with tsc compilation where it needs the presence of d.ts files which was not present with this (only .d.cts and .d.mts files were present). This can perhaps be achieved when we move over to use a bundler but not currently.

Flat builds use the type value of package.json to determine the output file extensions.
For module -
1. .js files are es modules and .cjs files are commonjs.
2. main field points to index.cjs (if export['.'] is present and module field points to index.js
For commonjs or when no type is specified -
1. .js files are commonjs and .mjs files are es modules.
2. main field points to index.js (if export['.'] is present and module field points to index.mjs
Added validation for bin field in package.json. It should point to the source files instead of built files now.
Some refactoring/tests around exports and bin generation for output package.json.

To fix:

tsc build with project references throws error
Fix issues on windows.

In an effort to fix issues like -

Can't get MUI 7 working with module federation (MUI 6 worked fine) material-ui#47561
Codesandbox
Parcel vanilla mode

Downstream changes -

[code-infra] Setup flat build for packages base-ui#3830 (has bundler analyses as well)
[code-infra] Setup flat build for packages mui-x#21029
[code-infra] Setup flat build for packages material-ui#47670

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mui-bot · 2026-01-22T13:53:51Z

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui-components/react	0B^(0.00%)	0B^(0.00%)
@mui/x-charts-pro	0B^(0.00%)	0B^(0.00%)

Details of bundle changes

Check out the code infra dashboard for more information about this PR.

Janpot · 2026-01-22T14:40:06Z

packages/code-infra/src/cli/packageJson.d.ts

 		The executable files that should be installed into the `PATH`.
 		*/
-    bin?: string | Partial<Record<string, string>>;
+    bin?: string | Record<string, string>;


This would cause pkg.bin['any string here'] to be of type string. We want it to be string | undefined.

Suggested change

bin?: string | Record<string, string>;

bin?: string | Partial<Record<string, string>>;

But isn't that the case ? If pkg.bin has a key, it should have a value. It can't be undefined. Or maybe I am missing some use-case.

The way this is declared has a reason. It's to maximize type safety. Consider the following example under your changes:

function checkBin(pkg: PackageJson) { const foo: string = pkg.bin['some nonsense'] console.log(typeof foo); // prints undefined, even though we declared it as string }

Compiles without a problem and the type of foo is actually not corresponding to the actual value at runtime. this is a common source of bugs. If we do Partial, then it will complain about assigning something that can potentially be undefined to a string.

Moreover, it requires you to be defensive, typescript forces you to add a check whether the property exists before blindly using it:

function checkBin(pkg: PackageJson) { if (pkg.bin['some-script']) { const path: string = pkg.bin['some-script'] } }

Do you mean that there might be a case that someone has not specified a value for a key in package.json['bin'] ? Like this -

"bin": { "code-infra": null // since there cannot be undefined }

I don't see this happening but I have reverted any ways since it required 1-2 if checks.

no, the oppositie, that someone tries to access a key that is not in that object.

Janpot · 2026-01-22T14:47:37Z

Nice work. I think you may want to check

mui-public/packages/code-infra/src/utils/babel.mjs

Lines 148 to 151 in adc9a37

    
           // cjs for reexporting from commons only modules. 
        
           // If we need to rely more on this we can think about setting up a separate commonjs => commonjs build for .cjs files to .cjs 
        
           // `--extensions-.cjs --out-file-extension .cjs` 
        
           await cjsCopy({ from: sourceDir, to: outDir });

It's to handle an edge case with Next.js used here: https://github.com/mui/material-ui/blob/b09d704dcdc0de3ea0d54b10e1c0803e8256b415/packages/mui-material-nextjs/src/v13-appRouter/appRouterV13.tsx#L6

I suspect that:

it will now copy the file twice into the same location
it may conflict with other .cjs files we emit

Not sure though

csvan · 2026-02-06T22:27:51Z

Any news on this? It's a critical fix to make MUI work with Module Federation currently

brijeshb42 · 2026-02-10T06:14:53Z

Although this has been implemented, we need to test the build output with all the bundlers to make sure that it won't introduce any bugs.

brijeshb42 · 2026-02-10T10:40:33Z

@Janpot The PR is now ready. I've posted a detailed bundler comparison in Base UI PR.

Janpot · 2026-02-10T10:43:15Z

Does this work with codesandbox now? Is it portable to Core/X?

brijeshb42 · 2026-02-10T10:45:01Z

No. No codesandbox still. I was just trying to make sure anything that already works is not breaking. Codesandbox doesn't work with current structure as well as the new one.
I am checking out CSB as a followup but that should not block this.

Is it portable to Core/X?

Yes. Same structure for all our packages. I analyzed just Base UI since it was already setup with Michal's bundler project.

Janpot · 2026-02-10T10:50:23Z

packages/code-infra/src/utils/build.test.mjs

+      await createFile(
+        path.join(
+          outputDir,
+          `index${getOutExtension('esm', { isFlat: true, packageType: 'module' })}`,


I would avoid using getOutExtension in fixtures. If there's a bug in getOutExtension, it may cancel each other out in the implementation details. Instead, make the fixture more static and explicit:

`index.mjs`,

Tests must be as simple as possible, we don't want to have to create tests for tests.

Can these files be created in parallel?

Are these tests written by AI?

Yes. It has followed what was in the AGENTS file, ie, to make the tests closer to the real world. That's why its actually creating the files instead of simulating a filesystem.

1. New cli arg `--flat` for the new logic to create flat builds 2. Flat builds create `mjs` (and `d.mts` for types) files for esm and `cjs` (and `d.cts` for types) files for cjs builds for simplicity. There are no `js` files now. 3. Added validation for `bin` field in `package.json`. It should point to the source files instead of built files now.

packages/code-infra/src/utils/build.test.mjs

brijeshb42 · 2026-02-11T08:50:00Z

@csvan Could you verify if module federation related issues are resolved if you use @mui/material version https://pkg.pr.new/mui/material-ui/@mui/material@eca9727 ? All the other CI published packages are listed here (if required) - https://github.com/mui/material-ui/pull/47670/checks?check_run_id=63203520528

this will remove matching exports from the final package.json

The package might be trying to block an import

brijeshb42 added scope: code-infra Involves the code-infra product (https://www.notion.so/mui-org/5562c14178aa42af97bc1fa5114000cd). breaking change Introduces changes that are not backward compatible. labels Jan 22, 2026

brijeshb42 force-pushed the flat-build branch from 859d9e3 to 46312cb Compare January 22, 2026 13:58

brijeshb42 requested a review from a team January 22, 2026 14:04

Janpot reviewed Jan 22, 2026

View reviewed changes

brijeshb42 force-pushed the flat-build branch 3 times, most recently from 56c0ac3 to 414402a Compare January 22, 2026 18:43

oliviertassinari deployed to flat-build - code-infra-dashboard PR #1064 January 22, 2026 19:31 — with Render View deployment

oliviertassinari deployed to flat-build - mui-tools-public PR #1064 January 22, 2026 19:31 — with Render View deployment

brijeshb42 force-pushed the flat-build branch 3 times, most recently from 9687adf to da754b7 Compare January 23, 2026 04:58

ZeeshanTamboli mentioned this pull request Jan 23, 2026

Can't get MUI 7 working with module federation (MUI 6 worked fine) mui/material-ui#47561

Closed

brijeshb42 force-pushed the flat-build branch from d8d3202 to 744aff6 Compare January 26, 2026 06:59

oliviertassinari temporarily deployed to flat-build - code-infra-dashboard PR #1064 January 29, 2026 11:57 — with Render Destroyed

oliviertassinari temporarily deployed to flat-build - mui-tools-public PR #1064 January 29, 2026 11:57 — with Render Destroyed

brijeshb42 force-pushed the flat-build branch from 75e8dac to 0ef8561 Compare February 10, 2026 06:45

brijeshb42 requested review from a team and Janpot February 10, 2026 10:35

Janpot reviewed Feb 10, 2026

View reviewed changes

brijeshb42 added 4 commits February 10, 2026 21:24

single cjsCopy

532658b

refactor js/cjs/mjs file output extension

45e5eee

Add tests for ts generation logic

a92b4a0

Point default condition to mjs/esm modules

2040596

brijeshb42 force-pushed the flat-build branch from 0ef8561 to 06c1655 Compare February 10, 2026 15:54

Janpot reviewed Feb 10, 2026

View reviewed changes

packages/code-infra/src/utils/build.test.mjs Outdated Show resolved Hide resolved

Expand all globs in package.json

4286afa

brijeshb42 force-pushed the flat-build branch from 06c1655 to 4286afa Compare February 11, 2026 04:41

brijeshb42 requested a review from Janpot February 11, 2026 08:48

Add support for negation export

e35e0f9

this will remove matching exports from the final package.json

brijeshb42 force-pushed the flat-build branch from e7d71e4 to e35e0f9 Compare February 11, 2026 12:10

Preserve null paths that don't match

069dccf

The package might be trying to block an import

	bin?: string \| Record<string, string>;
	bin?: string \| Partial<Record<string, string>>;

[code-infra] Support flat build output behind a flag #1064

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