test: add more build assertions. (#4)

README.md

@@ -12,16 +12,17 @@ Early stages of development. Inspired by https://github.com/microsoft/TypeScript
 
 * Node >= 16.19.0.
 * TypeScript, `npm i typescript`.
+* A `tsconfig.json` with `outDir` defined.
 
 ## Example
 
-First, install the package to create the `duel` executable inside your `node_modules/.bin` directory.
+First, install this package to create the `duel` executable inside your `node_modules/.bin` directory.
 
 ```console
 user@comp ~ $ npm i @knighted/duel
 ```
 
-Then, given a `package.json` that defines `"type": "module"` and  a `tsconfig.json` file that looks like the following:
+Then, given a `package.json` that defines `"type": "module"` and  a `tsconfig.json` file that looks something like the following:
 
 ```json
 {
@@ -51,9 +52,9 @@ And then running it:
 user@comp ~ $ npm run build
 ```
 
-If everything worked, you should have an ESM build inside of `dist` and a CJS build inside of `dist/cjs`. Now you can update your `exports` in package.json to match the build output.
+If everything worked, you should have an ESM build inside of `dist` and a CJS build inside of `dist/cjs`. Now you can update your [`exports`](https://nodejs.org/api/packages.html#exports) to match the build output.
 
-It should work similarly for a CJS first project. Except, your `tsconfig.json` would define `--module` and `--moduleResolution` differently, and you'd want to pass `-x .mjs`.
+It should work similarly for a CJS first project. Except, your `tsconfig.json` would define `--module` and `--moduleResolution` differently, and you'd want to pass `--target-extension .mjs`.
 
 See the available [options](#options).
 
@@ -71,13 +72,15 @@ You can run `duel --help` to get more info. Below is the output of that:
 Usage: duel [options]
 
 Options:
---project, -p 			 Compile the project given the path to its configuration file, or to a folder with a 'tsconfig.json'.
---target-extension, -x 		 Sets the file extension for the dual build. [.cjs,.mjs]
---help, -h 			 Print this message.
+--project, -p 		 Compile the project given the path to its configuration file, or to a folder with a 'tsconfig.json'.
+--target-extension, -x 	 Sets the file extension for the dual build. [.cjs,.mjs]
+--help, -h 		 Print this message.
 ```
 
 ## Gotchas
 
-* Unfortunately, TypeScript doesn't really understand dual packages very well. For instance, it will **always** create CJS exports when `--module commonjs` is used, even on files with an `.mts` extension. One reference issue is https://github.com/microsoft/TypeScript/issues/54573. If you use `.mts` extensions to enforce an ESM module system, this might break in the corresponding dual CJS build.
-* If targeting a dual CJS build, and you are using [top level `await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#top_level_await), you will most likely encounter the compilation error `error TS1378: Top-level 'await' expressions are only allowed when the 'module' option is set to 'es2022', 'esnext', 'system', 'node16', or 'nodenext', and the 'target' option is set to 'es2017' or higher.` during the CJS build. This is because `duel` creates a temporary `tsconfig.json` from your original and overwrites the `--module` and `--moduleResolution` based on the provided `--target-ext`.
+* Unfortunately, TypeScript doesn't really build [dual packages](https://nodejs.org/api/packages.html#dual-commonjses-module-packages) very well in regards to preserving module system by file extension. For instance, it will **always** create CJS exports when `--module commonjs` is used, _even on files with an `.mts` extension_, which is contrary to [how Node determines module systems](https://nodejs.org/api/packages.html#determining-module-system). The `tsc` compiler is fundamentally broken in this regard. One reference issue is https://github.com/microsoft/TypeScript/issues/54573. If you use `.mts` extensions to enforce an ESM module system, this will break in the corresponding dual CJS build. There is no way to fix this until TypeScript fixes their compiler.
+
+* If targeting a dual CJS build, and you are using [top level `await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#top_level_await), you will most likely encounter the compilation error `error TS1378: Top-level 'await' expressions are only allowed when the 'module' option is set to 'es2022', 'esnext', 'system', 'node16', or 'nodenext', and the 'target' option is set to 'es2017' or higher.` during the CJS build. This is because `duel` creates a temporary `tsconfig.json` from your original and necessarily overwrites the `--module` and `--moduleResolution` based on the provided `--target-ext`. There is no workaround other than to **not** use top level await if you want a dual build.
+
 * If doing an `import type` across module systems, i.e. from `.mts` into `.cts`, or vice versa, you might encounter the compilation error ``error TS1452: 'resolution-mode' assertions are only supported when `moduleResolution` is `node16` or `nodenext`.``. This is a [known issue](https://github.com/microsoft/TypeScript/issues/49055) and TypeScript currently suggests installing the nightly build, i.e. `npm i typescript@next`.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@knighted/duel",
-  "version": "1.0.0-alpha.2",
+  "version": "1.0.0-alpha.3",
   "description": "TypeScript dual packages.",
   "type": "module",
   "main": "dist",

src/init.js

@@ -41,12 +41,12 @@ const init = async args => {
     log('Usage: duel [options]\n')
     log('Options:')
     log(
-      "--project, -p \t\t\t Compile the project given the path to its configuration file, or to a folder with a 'tsconfig.json'.",
+      "--project, -p \t\t Compile the project given the path to its configuration file, or to a folder with a 'tsconfig.json'.",
     )
     log(
-      '--target-extension, -x \t\t Sets the file extension for the dual build. [.cjs,.mjs]',
+      '--target-extension, -x \t Sets the file extension for the dual build. [.cjs,.mjs]',
     )
-    log('--help, -h \t\t\t Print this message.')
+    log('--help, -h \t\t Print this message.')
   } else {
     const { project, 'target-extension': targetExt } = parsed
     let configPath = resolve(project)

test/integration.js

@@ -3,6 +3,7 @@ import assert from 'node:assert/strict'
 import { fileURLToPath } from 'node:url'
 import { dirname, resolve } from 'node:path'
 import { rm } from 'node:fs/promises'
+import { existsSync } from 'node:fs'
 
 import { duel } from '../src/duel.js'
 
@@ -87,6 +88,13 @@ describe('duel', () => {
     assert.ok(
       spy.mock.calls[2].arguments[0].startsWith('Successfully created a dual CJS build'),
     )
+    // Check that the expected files and extensions are there
+    assert.ok(existsSync(resolve(dist, 'index.js')))
+    assert.ok(existsSync(resolve(dist, 'index.d.ts')))
+    assert.ok(existsSync(resolve(dist, 'cjs.cjs')))
+    assert.ok(existsSync(resolve(dist, 'cjs/index.cjs')))
+    assert.ok(existsSync(resolve(dist, 'cjs/index.d.cts')))
+    assert.ok(existsSync(resolve(dist, 'cjs/esm.mjs')))
   })
 
   it('reports compilation errors during a build', async t => {