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Migrate from rolldown to tsdown#4025

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nstepien merged 1 commit intomainfrom
tsdown
Apr 2, 2026
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Migrate from rolldown to tsdown#4025
nstepien merged 1 commit intomainfrom
tsdown

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@nstepien nstepien commented Apr 1, 2026 •
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  • rolldown dropped support for CSS, unclear when it'll be restored, if ever
  • vite's library mode does not support outputting declaration files
  • tsdown, built on top of rolldown, is the ideal lib bundling tool for rdg

https://tsdown.dev/
https://github.com/rolldown/tsdown

before

<DIR>/styles.css      asset │ size:   9.78 kB
<DIR>/styles.css.map  asset │ size:  12.75 kB
<DIR>/index.js.map    asset │ size: 216.46 kB
<DIR>/index.d.ts      chunk │ size:  23.08 kB
<DIR>/index.js        chunk │ size:  99.18 kB

✔ rolldown v1.0.0-rc.5 Finished in 481.46 ms

after

ℹ tsdown v0.21.7 powered by rolldown v1.0.0-rc.12
ℹ config file: D:\repos\react-data-grid\.claude\worktrees\vitelib\tsdown.config.ts 
ℹ entry: src\index.ts
ℹ tsconfig: tsconfig.json
ℹ Build start
ℹ Cleaning 5 files
ℹ lib\index.js       98.39 kB │ gzip: 21.95 kB
ℹ lib\index.js.map  217.24 kB │ gzip: 53.86 kB
ℹ lib\styles.css      8.22 kB │ gzip:  2.12 kB
ℹ lib\index.d.ts     23.10 kB │ gzip:  5.14 kB
ℹ 4 files, total: 346.95 kB
✔ Build complete in 187ms

build output changes

We've lost the sourcemaps for CSS files, not a major concern.

In the JS, besides empty lines being removed, some const values are inlined:

image image

In the declaration file, the name of namespace imports changed, that's it:

image

The CSS file is processed by lightningcss, so it has more changes, but nothing major, some highlights:

image image image image

@nstepien nstepien self-assigned this Apr 1, 2026
@nstepien nstepien marked this pull request as ready for review April 1, 2026 22:49
@nstepien nstepien requested a review from amanmahajan7 as a code owner April 1, 2026 22:49
amanmahajan7
amanmahajan7 reviewed Apr 2, 2026
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@amanmahajan7 amanmahajan7 left a comment

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This is great. What is the default build target? Does it use baseline-widely-available?

tsdown.config.ts
cleanDir: true
},
outDir: 'lib',
platform: 'neutral',
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Should this be platform: 'browser'?

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@nstepien nstepien Apr 2, 2026

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I don't think so, we want to support SSR as well, so the lib should be compatible with node.

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FYI there's no build output difference at the moment if we set platform: 'browser'

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nstepien commented Apr 2, 2026

What is the default build target? Does it use baseline-widely-available?

https://tsdown.dev/options/target

If you don't specify a target and your package.json doesn't have an engines.node field, tsdown will behave as if target: false was set, preserving all modern syntax.

No syntax downgrade unless we set target or engines.node.
Which is probably fine, our rolldown config wasn't transpiling code either.
We can revisit if it's ever needed.

@nstepien nstepien requested a review from amanmahajan7 April 2, 2026 14:15
@nstepien nstepien merged commit 37a841f into main Apr 2, 2026
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@nstepien nstepien deleted the tsdown branch April 2, 2026 14:28
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