[feat] async transforms

[loynoir]

#469 (comment)

https://github.com/guybedford/es-module-shims/blob/2.6.1/src/core.js#L614

https://github.com/guybedford/es-module-shims/blob/2.6.1/src/core.js#L876

[guybedford]

I'd be curious to understand the use case for an async transform here further - are there really transformers that require it and split work up on a per-module level?

[loynoir]

Oh, I have forgotten why I opened feature request.

Maybe because all other popular toolchain transformer are async.

My current use case within userscript, should be without .tsTransform, but with .source.

source: async (url, fetchOptions, _parent, defaultSourceHook) => {

https://nodejs.org/api/module.html#caveat-in-the-asynchronous-load-hook

export async function load(url, context, nextLoad) { }

https://esbuild.github.io/plugins/#on-load

let exampleOnLoadPlugin = {
  name: 'example',
  setup(build) {
    // Load ".txt" files and return an array of words
    build.onLoad({ filter: /\.txt$/ }, async (args) => {
      let text = await fs.promises.readFile(args.path, 'utf8')
      return {
        contents: JSON.stringify(text.split(/\s+/)),
        loader: 'json',
      }
    })
  },
}

https://rollupjs.org/plugin-development/#transform

Type: (code: string, id: string) => TransformResult

Kind: async, sequential

[guybedford]

I've renamed this issue to "async transforms" as supporting this feature more generally and not just for TypeScript. I think we need some more discussion on it though around use cases before considering implementation as I'm not yet convinced it is the right approach for this project to try to be a general transform loader.