babel-plugin-module-resolver-loader

Webpack loader for the plugin: babel-plugin-module-resolver by @Tommy

---

1. Installation
2. Who Needs It
3. How To Use
4. Typescript Won't Fix It (Soon)
5. Alternative solutions
6. Implementation Notes

Installation

webpack 4+

There shouldn't be any reason to lock you on specific versions on the following packages. As a result, you must have under devDependencies:

"@babel/core": "^7.4.4",
"@babel/parser": "^7.4.4"

yarn add --dev @babel/core @babel/parser @stavalfi/babel-plugin-module-resolver-loader

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Who Needs It

Library creators who uses Typescript will probably want to generate *.d.ts files. Incase you are using absolute paths in your code like this:

1// src/some-file-under-src-folder.ts
2export { myType } from 'file-under-src'

1// src/file-under-src.ts
2export const myType = number

Will encounter a serious problem when they will use 'ts-loader' to generate the *.d.ts files because typescript won't transform back those absolute paths to relative paths when generating all the d.ts files.

The genrated *.d.ts files will be:

1// dist/some-file-under-src-folder.d.ts
2export { myType } from 'file-under-src' // BAD - absolute (invalid path)

1// dist/file-under-src.d.ts
2export declare type myType = number

Instead of

1// dist/some-file-under-src-folder.d.ts
2export { myType } from './file-under-src' // GOOD - relative (valid path)

1// dist/file-under-src.d.ts
2export declare type myType = number

Q: Why is this a problem?
A: Your library consumers can't resolve the given absolute path: file-under-src because when typescript compiler will load the file dist/some-file-under-src-folder.d.ts, he won't be able to find the module file-under-src. Error!

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How To Use

I advise to use this plugin only in production build because it's not so fast. For development builds, I recommend to use a build system that is similar as https://github.com/stavalfi/jstream.

You have two options:

1. You can use the example-project folder in this reporisotry to see how a real-minimal project use it.
2. Lerna project with lib that produces *.d.ts files and a consumer who uses them: https://github.com/stavalfi/jstream
3. [must be done in any option] Add this loader before ts-loader:

1{
2  module: {
3    rules: [
4      {
5        test: /\.(ts|js)x?$/,
6        exclude: /(node_module|dist)/,
7        use: [
8          {
9            loader: 'babel-loader',
10            options: {
11              cacheDirectory: true,
12            },
13          },
14          {
15            loader: 'ts-loader',
16          },
17          {
18            loader: '@stavalfi/babel-plugin-module-resolver-loader',
19            options: {
20              // all those options will go directly to babel-plugin-module-resolver plugin.
21              // Read babel-plugin-module-resolver DOCS to see all options:
22              // https://github.com/tleunen/babel-plugin-module-resolver/blob/master/DOCS.md
23              root: ['./src'],
24              extensions: ['.js', '.jsx', '.d.ts', '.ts', '.tsx'],
25            },
26          },
27        ],
28      },
29    ],
30  },
31}

Note1: You ONLY want to pass the paths of the modules you built WITHOUT passing the location of node_modules and node_modules/@types. Why?

1. Because if you pass node_modules, the plugin I use will find the external module there and replace the path with a relative path to there. if that external module has types file in node_module/@types, then typescript won't find it.
2. if you pass node_modules/@types, the plugin I use will find the external module there and replace the path with a relative path to there. There are no source files of that library under node_module/@types/your_external_lib so every loader you will ever use will fail to find the source files of the external modules you want to use.

1{
2  baseUrl: '.',
3  paths: {
4    '*': ['src', 'node_modules'],
5  },
6}

Note2: The default extensions doesn't ionclude typescript extentions so we overrire them.

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Typescript Won't Fix It (Soon)

From https://github.com/Microsoft/TypeScript/issues/15479:

@RyanCavanaugh

Our general take on this is that you should write the import path that works at runtime, and set your TS flags to satisfy the compiler's module resolution step, rather than writing the import that works out-of-the-box for TS and then trying to have some other step "fix" the paths to what works at runtime.

We have about a billion flags that affect module resolutions already (baseUrl, path mapping, rootDir, outDir, etc). Trying to rewrite import paths "correctly" under all these schemes would be an endless nightmare.

---

Alternative solutions

1. You can use ttypescript transformers so the paths will be converted when typescript compiles the code (before generating the *.d.ts files): There are some libraries for that. you can look at here: https://github.com/Microsoft/TypeScript/issues/15479

2. generate one big *.d.ts file. There won't be any imports there so all will be good. I couldn't find a working library that does that.

3. Writing multiple/single *.d.ts files by your self and copy them to the dist folder after every webpack build. You can use plugins to run some cli commands after every build. it's easy.

4. Use relative imports when you import types in your *.ts files under src folder.

Implementation Notes

1. I couldn't use babel-loader becuause it uses @babel/core's transform function.

Q: Why is it a problem?
A: transform function accepts a code and return the transpiled code. This function create AST from the code, change the AST and then create code from the new AST. To create the first AST, it needs to recognize typescript code. I couldn't teach him typescript code without also changing the typescript code to js code at the end. so If you run ts-loader after that, ts-loader will get ts files with pure-js code and complain that everything has any type.

So that leaved me with out much options. I couldn't use @babel/core's transform function also. I must create the AST by my self and do th

The solution is to first create an AST from the code webpack gave me:

1const ast = parser.parse(source, {
2  // source ==== code of a file
3  sourceFilename: this.resourcePath, // webpack gives me absolute path of the source file. Maybe I don't need to set this param here.
4  babelrc: false,
5  sourceType: 'module',
6  plugins: ['jsx', 'typescript'], // these plugins are not available in @babel/core.transform function
7})

so there won't be any transpilation involved.

Then I send this AST directly to the transpilation fucntion (without any extra plugins to transpile anything else bu twhat I want):

1const coreResult = core.transformFromAstSync(ast, source, {
2  filename: this.resourcePath,
3  babelrc: false,
4  plugins: [['module-resolver', options]],
5})

The result (the original source code of this loader):

1const { getOptions } = require('loader-utils')
2const core = require('@babel/core')
3const parser = require('@babel/parser')
4
5module.exports = function(source) {
6  const options = getOptions(this)
7
8  const ast = parser.parse(source, {
9    sourceFilename: this.resourcePath,
10    babelrc: false,
11    sourceType: 'module',
12    plugins: ['jsx', 'typescript'],
13  })
14
15  const coreResult = core.transformFromAstSync(ast, source, {
16    filename: this.resourcePath,
17    babelrc: false,
18    plugins: [['module-resolver', options]],
19  })
20
21  return coreResult.code
22}