doc: fix transpiler loader hooks documentation

The loader hooks examples have been broken for a while:

1. The nextLoad() hook cannot be used on a .coffee file that ends
   up going to the default load step without an explict format,
   which would cause a ERR_UNKNOWN_FILE_EXTENSION. Mention
   adding a package.json with a type field to work around it
   in the example.
2. Pass the context parameter to the nextLoad() invocation and
   document that context.format is mandatory when module type
   is not explicitly inferrable from the module.
3. Correct the getPackageType() implementation which returns
   false instead of undefined in the absence of an explict format,
   which is not a valid type for format.

PR-URL: nodejs#57037
Refs: nodejs#57030
Reviewed-By: Jacob Smith <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>

Author: joyeecheung

doc/api/module.md

@@ -1164,7 +1164,11 @@ changes:
   Node.js default `load` hook after the last user-supplied `load` hook
   * `url` {string}
   * `context` {Object|undefined} When omitted, defaults are provided. When provided, defaults are
-    merged in with preference to the provided properties.
+    merged in with preference to the provided properties. In the default `nextLoad`, if
+    the module pointed to by `url` does not have explicit module type information,
+    `context.format` is mandatory.
+    <!-- TODO(joyeecheung): make it at least optionally non-mandatory by allowing
+         JS-style/TS-style module detection when the format is simply unknown -->
 * Returns: {Object|Promise} The asynchronous version takes either an object containing the
   following properties, or a `Promise` that will resolve to such an object. The
   synchronous version only accepts an object returned synchronously.
@@ -1362,36 +1366,32 @@ transpiler hooks should only be used for development and testing purposes.
 ```mjs
 // coffeescript-hooks.mjs
 import { readFile } from 'node:fs/promises';
-import { dirname, extname, resolve as resolvePath } from 'node:path';
-import { cwd } from 'node:process';
-import { fileURLToPath, pathToFileURL } from 'node:url';
+import { findPackageJSON } from 'node:module';
 import coffeescript from 'coffeescript';
 
 const extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;
 
 export async function load(url, context, nextLoad) {
   if (extensionsRegex.test(url)) {
-    // CoffeeScript files can be either CommonJS or ES modules, so we want any
-    // CoffeeScript file to be treated by Node.js the same as a .js file at the
-    // same location. To determine how Node.js would interpret an arbitrary .js
-    // file, search up the file system for the nearest parent package.json file
-    // and read its "type" field.
-    const format = await getPackageType(url);
-
-    const { source: rawSource } = await nextLoad(url, { ...context, format });
+    // CoffeeScript files can be either CommonJS or ES modules. Use a custom format
+    // to tell Node.js not to detect its module type.
+    const { source: rawSource } = await nextLoad(url, { ...context, format: 'coffee' });
     // This hook converts CoffeeScript source code into JavaScript source code
     // for all imported CoffeeScript files.
     const transformedSource = coffeescript.compile(rawSource.toString(), url);
 
+    // To determine how Node.js would interpret the transpilation result,
+    // search up the file system for the nearest parent package.json file
+    // and read its "type" field.
     return {
-      format,
+      format: await getPackageType(url),
       shortCircuit: true,
       source: transformedSource,
     };
   }
 
   // Let Node.js handle all other URLs.
-  return nextLoad(url);
+  return nextLoad(url, context);
 }
 
 async function getPackageType(url) {
@@ -1402,72 +1402,51 @@ async function getPackageType(url) {
   // this simple truthy check for whether `url` contains a file extension will
   // work for most projects but does not cover some edge-cases (such as
   // extensionless files or a url ending in a trailing space)
-  const isFilePath = !!extname(url);
-  // If it is a file path, get the directory it's in
-  const dir = isFilePath ?
-    dirname(fileURLToPath(url)) :
-    url;
-  // Compose a file path to a package.json in the same directory,
-  // which may or may not exist
-  const packagePath = resolvePath(dir, 'package.json');
-  // Try to read the possibly nonexistent package.json
-  const type = await readFile(packagePath, { encoding: 'utf8' })
-    .then((filestring) => JSON.parse(filestring).type)
-    .catch((err) => {
-      if (err?.code !== 'ENOENT') console.error(err);
-    });
-  // If package.json existed and contained a `type` field with a value, voilà
-  if (type) return type;
-  // Otherwise, (if not at the root) continue checking the next directory up
-  // If at the root, stop and return false
-  return dir.length > 1 && getPackageType(resolvePath(dir, '..'));
+  const pJson = findPackageJSON(url);
+
+  return readFile(pJson, 'utf8')
+    .then(JSON.parse)
+    .then((json) => json?.type)
+    .catch(() => undefined);
 }
 ```
 
 ##### Synchronous version
 
 ```mjs
 // coffeescript-sync-hooks.mjs
-import { readFileSync } from 'node:fs/promises';
-import { registerHooks } from 'node:module';
-import { dirname, extname, resolve as resolvePath } from 'node:path';
-import { cwd } from 'node:process';
-import { fileURLToPath, pathToFileURL } from 'node:url';
+import { readFileSync } from 'node:fs';
+import { registerHooks, findPackageJSON } from 'node:module';
 import coffeescript from 'coffeescript';
 
 const extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;
 
 function load(url, context, nextLoad) {
   if (extensionsRegex.test(url)) {
-    const format = getPackageType(url);
-
-    const { source: rawSource } = nextLoad(url, { ...context, format });
+    const { source: rawSource } = nextLoad(url, { ...context, format: 'coffee' });
     const transformedSource = coffeescript.compile(rawSource.toString(), url);
 
     return {
-      format,
+      format: getPackageType(url),
       shortCircuit: true,
       source: transformedSource,
     };
   }
 
-  return nextLoad(url);
+  return nextLoad(url, context);
 }
 
 function getPackageType(url) {
-  const isFilePath = !!extname(url);
-  const dir = isFilePath ? dirname(fileURLToPath(url)) : url;
-  const packagePath = resolvePath(dir, 'package.json');
-
-  let type;
+  const pJson = findPackageJSON(url);
+  if (!pJson) {
+    return undefined;
+  }
   try {
-    const filestring = readFileSync(packagePath, { encoding: 'utf8' });
-    type = JSON.parse(filestring).type;
-  } catch (err) {
-    if (err?.code !== 'ENOENT') console.error(err);
+    const file = readFileSync(pJson, 'utf-8');
+    return JSON.parse(file)?.type;
+  } catch {
+    return undefined;
   }
-  if (type) return type;
-  return dir.length > 1 && getPackageType(resolvePath(dir, '..'));
 }
 
 registerHooks({ load });
@@ -1489,6 +1468,21 @@ console.log "Brought to you by Node.js version #{version}"
 export scream = (str) -> str.toUpperCase()
 ```
 
+For the sake of running the example, add a `package.json` file containing the
+module type of the CoffeeScript files.
+
+```json
+{
+  "type": "module"
+}
+```
+
+This is only for running the example. In real world loaders, `getPackageType()` must be
+able to return an `format` known to Node.js even in the absence of an explicit type in a
+`package.json`, or otherwise the `nextLoad` call would throw `ERR_UNKNOWN_FILE_EXTENSION`
+(if undefined) or `ERR_UNKNOWN_MODULE_FORMAT` (if it's not a known format listed in
+the [load hook][] documentation).
+
 With the preceding hooks modules, running
 `node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./coffeescript-hooks.mjs"));' ./main.coffee`
 or `node --import ./coffeescript-sync-hooks.mjs ./main.coffee`