Feature Request: Make "loaders" a first class citizen of the language

[donaldpipowitch]

This follows some thoughts I had with "The Maybe Future of Frontend Tooling" and picks up some use cases which aren't covered by the current (awesome!) language server plugin support as far as I know. I also created https://github.com/Microsoft/TypeScript/issues/15589 which refers to linter rules. This issue is about loaders - in the sense how they were introduced by webpack.

In general both issues - this one and https://github.com/Microsoft/TypeScript/issues/15589 - remind me of Cheng Lous talks about "Taming the Meta Language" and "What's in a language?". Linters and loader are not a part of the current language in a strict sense, but part of our meta language and it is good to formalize more of them so they become a language construct. (Part of Cheng Lous talks is about why reason for example doesn't have a separate linter.)

Current state

• Loaders like they are used by webpack allows us to require non JavaScript files (or non TypeScript files) and transform them into a JavaScript/TypeScript "file" or require JavaScript/TypeScript files and transform them so they'll have a different interface.

• TypeScript supports loaders in two ways:

  • Either write an interface for them directly (e.g. declare module 'foo-loader?param=bar!../test';).
  • Or use a wild card (e.g. declare module 'foo-loader?*).

• This is either cumbersome/error prone and/or limited, because they interfaces can't be typed in a very dynamic way.

• Two use cases which we currently have, which I'd like to point out:

Use case 1

We have translations in a file called en.properties (and other files for other languages) which could look like this:

say-something={gender, select, f {She} m {He}} is great.

I'd like to import this file as import { saySomething } from '../i18n/en.properties'; which would have an interface like this:

declare module '../i18n/en.properties' {
  /**
   * `en`: {gender, select, f {She} m {He}} is great.
   * `de`: {gender, select, f {Sie} m {Er}} ist toll.
   */
  export function saySomething(data: { gender: 'f' | 'm' }): string;
}

We currently need to pre-compile our translations and emit a .d.ts and a .js, so we import the .js instead of the .proprties. Only then we get this complex interface.

Use case 2

We add new exported functions to existing modules so we can mock them easily. What will be added depends on the query parameters of the loader. It could look like this: 'import-inject-loader?fetch!../src/get-users'.

E.g. using import-inject-loader?fetch will add two new exported functions to ../src/get-users: ilOverwriteFetch and ilResetAll.

Currently we need declare every module which uses import-inject-loader as any (declare module 'import-inject-loader?*';). Everything else would be too hard to maintain.

What I'd like to see

• I would like to get proper typings for imported files where the content changes the interface (= use case 1) without precompiling, but with importing the file directly.

• I would like to get proper typings for files with existing interfaces, but which are transformed (in the easiest case just extended) into a different interface without loosing the original interface (= use case 2).

• I would like to get proper typings for imported modules, when the interface changes, because of loader options (= use case 2).

• I would like to get assistance when configuring loaders through query parameters (= use case 2, e.g. show an error, when I write 'import-inject-loader?fetch!../src/get-users', if there is no fetch used inside ../src/get-users).

---

If something like that isn't in the scope of a TypeScript core team, I could think of giving a new "TypeScript tooling team" this scope which develops closely together with the TypeScript core team in a similar fashion like the VS code team.

I think it is important to address these meta language issues somehow in an official way to get a nice TypeScript ecosystem.

[mohsen1]

I thought TypeScript language plugins allow extending the type checker as well as module resolution so I decided to write a TypeScript plugin for typed-css-modules but it's not possible with current plugin architecture.

Support for loaders will open up tons of opportunities. Here are a few:

• Loading JSON

  import data from './data.json'

• Loading CSS (typed css modules)

  import * as style from './style.css'
  <div className={style.myClass} />

• Loading images

  import * as image from './image.png'
  <img src={image.name} width={image.width} />

• Loading API definition files

  import APIClient from './api.swagger.yaml'
  APIClient.getPets();

[donaldpipowitch]

I thought TypeScript language plugins allow extending the type checker as well as module resolution so I decided to write a TypeScript plugin for typed-css-modules but it's not possible with current plugin architecture.

Yeah, currently it is not possible. Taken from RyanCavanaugh/sample-ts-plugin:

Examples of things language plugins cannot do:

• Customize the type system to change what is or isn't an error when running tsc

[DanielRosenwasser]

This might be related to #10988.

[raix]

It might also be related to https://github.com/Microsoft/TypeScript/issues/3136

We currently preprocess language files creating a typescript file with comments containing english translation - we parse the ICU format providing types for the translation functions/react components giving us full typings. (code documentation is also important for us)

Working on the translations we have to rerun the translation generator when editing and we cannot look at the json files to see parsing errors or code references.

The typescript loader could provide:

• ast
• typings
• code documentation
• source map
• compile errors

[RyanCavanaugh]

@rbuckton this is related to your current work. Thoughts?

[mohsen1]

Also related: https://github.com/microsoft/TypeScript/issues/16607

[nadavwix]

I would love to see typescript support for loaders.

the way is see it, a loader should transform a file type ( identified by regex on the file path) into a "d.ts" file. allowing typescript to use the typing information.

Ideally, we could configure this in tsConfig:

typeLoaders:{ "*.css": "css-loader/dts-transformer" }