Support for custom loaders is missing in tsdown (and not documented in migration guide)

[mhdcodes]

The current version of tsdown does not support [custom loaders], whereas tsup does https://tsup.egoist.dev/#custom-loader.

This isn’t mentioned anywhere in the migration guide, so users relying on this functionality will hit a wall trying to migrate, in fact we rely on this for an open source astro project codiume/orbit#998, I only discovered this when I tried to migrate from tsup to tsdown. we use custom loaders to skip parsing .astro files

Questions / Roadmap:

• Are there plans to add loader support in a future tsdown release?

• If not, could the migration guide be updated to explicitly note this limitation and suggest any recommended patterns or plugins?

[Gugustinette]

To add a bit more context, tsup leverage esbuild's loader feature and exposes it as a custom loader option.

Rolldown exposes a similar -experimental- option called module types.

Then having "custom loader" should already be possible by overriding Rolldown's config like so :

// tsdown.config.ts
import { defineConfig } from 'tsdown'

export default defineConfig({
  entry: ['./src/index.ts'],
  // Override Rolldown's input options here
  inputOptions: {
    moduleTypes: {
      '.custom': 'json',
    },
  }
})

However, in your case @mhdcodes , an asset loader was discussed here when a file loader (similar to the copy loader) was proposed.
I must say I'm not sure about what it is exactly as I couldn't find any reference to it, it seems the Rolldown loader issue was closed so I guess something similar should exist ?

[Gugustinette]

Alright so the Rolldown engine (in Rust) actually has the asset loader in the ModuleType enum.

But the TypeScript package's ModuleTypes enum does not exposes it for some reason.

Using the asset loader might actually work if you ignore the type error I guess ?

// tsdown.config.ts
import { defineConfig } from 'tsdown'

export default defineConfig({
  entry: ['./src/index.ts'],
  inputOptions: {
    moduleTypes: {
      // @ts-expect-error
      '.astro': 'asset',
    },
  }
})

But it seems like an issue in Rolldown's TypeScript package to me.

[mhdcodes]

I will try this an let you know @Gugustinette thanks for the valuable feedback.

[mhdcodes]

apparently this doesn't work:

For the "moduleTypes..astro". Invalid type: Expected ("base64" | "binary" | "css" | "dataurl" | "empty" | "js" | "json" | "jsx" | "text" | "ts" | "tsx") but received "asset". 

[Gugustinette]

Oh yeah it seems they have a ModuleTypes validator so it's not just some type error.

I'll open the issue on Rolldown's repo.

[ryoppippi]

can we use some rollup plugins for assets like @rollup/plugin-wasm ?

[Gugustinette]

It seems the issue was resolved on Rolldown's side, now we are waiting for a release.

[MatthijsKok]

Should pnpx tsdown migrate create a mapping from tsup loaders to tsdown moduleTypes?
e.g. running the migration on a simple example

import { defineConfig } from 'tsup'

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm'],
  loader: {
    '.jpg': 'copy',
  },
})

yields an almost identical config (which has an error for unknown loader field):

import { defineConfig } from 'tsdown'

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm'],
  loader: {
    '.jpg': 'copy',
  },
})

I would expect the tsdown migrate command to atleast update the loader object location into inputOptions.moduleTypes, and ideally also the copy => asset module type itself.
e.g.

import { defineConfig } from 'tsdown'

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm'],
  inputOptions: {
    moduleTypes: {
      '.jpg': 'asset',
    },
  },
})

[MatthijsKok]

It seems the issue was resolved on Rolldown's side, now we are waiting for a release.

Latest canary includes the fix.
This PR updates tsdown to that version: #220

[sxzz]

Added via 7132a58