ESM loader

package.json

@@ -19,6 +19,15 @@
   "main": "./lib/coffeescript/index",
   "module": "./lib/coffeescript-browser-compiler-modern/coffeescript.js",
   "browser": "./lib/coffeescript-browser-compiler-legacy/coffeescript.js",
+  "exports": {
+    ".": "./lib/coffeescript/index.js",
+    "./esm": "./lib/coffeescript/esm.js",
+    "./register": "./lib/coffeescript/register.js",
+    "./*": "./*",
+    "./lib/coffeescript/*": "./lib/coffeescript/*",
+    "./lib/coffeescript-browser-compiler-legacy/*": "./lib/coffeescript-browser-compiler-legacy/*",
+    "./lib/coffeescript-browser-compiler-modern/*": "./lib/coffeescript-browser-compiler-modern/*"
+  },
   "bin": {
     "coffee": "./bin/coffee",
     "cake": "./bin/cake"

src/coffeescript.coffee

@@ -251,6 +251,16 @@ parser.yy.parseError = (message, {token}) ->
   helpers.throwSyntaxError "unexpected #{errorText}", errorLoc
 
 exports.patchStackTrace = ->
+  # Check if Node's built-in source map stack trace transformations are enabled.
+  nodeSourceMapsSupportEnabled = process? and (
+    process.execArgv.includes('--enable-source-maps') or
+    process.env.NODE_OPTIONS?.includes('--enable-source-maps')
+  )
+
+  # Skip if node source maps are already enabled,
+  # or Error.prepareStackTrace is already defined.
+  return if Error.prepareStackTrace or nodeSourceMapsSupportEnabled
+
   # Based on http://v8.googlecode.com/svn/branches/bleeding_edge/src/messages.js
   # Modified to handle sourceMap
   formatSourcePosition = (frame, getSourceMapping) ->

src/esm.coffee

@@ -0,0 +1,55 @@
+# CoffeeScript ESM loader
+#
+# Usage: node --loader coffeescript/esm source.coffee
+#
+# Based on https://nodejs.org/api/esm.html#esm_transpiler_loader and
+# https://github.com/DanielXMoore/Civet/blob/main/source/esm.civet
+
+CoffeeScript = require './'
+fs           = require 'fs'
+module       = require 'module'
+{extname}    = require 'path'
+{fileURLToPath, pathToFileURL} = require 'url'
+
+CoffeeScript.patchStackTrace()
+
+baseURL = pathToFileURL(process.cwd() + '/').href
+
+exports.resolve = (specifier, context, next) ->
+  if CoffeeScript.FILE_EXTENSIONS.includes extname specifier
+    {parentURL} = context
+    parentURL ?= baseURL
+
+    shortCircuit: true
+    format: "coffee"
+    url: new URL(specifier, parentURL).href
+  else
+    # Not CoffeeScript; pass on to next resolver
+    next specifier, context
+
+exports.load = (url, context, next) ->
+  # Support only local .coffee files for now
+  unless context.format is "coffee" and url.startsWith 'file:'
+    return next url, context
+
+  options = module.options or
+            CoffeeScript.helpers.getRootModule(module).options or {}
+
+  # Currently `CoffeeScript.compile` caches all source maps if present. They
+  # are available in `getSourceMap` retrieved by `filename`.
+  options = {...options, inlineMap: true}
+
+  path = fileURLToPath url
+  js = CoffeeScript._compileFile path, options
+
+  # Add .js extension to enable other JavaScript ESM loaders (e.g. Babel)
+  result = await next url + '.js',
+    # Currently assume that, if you're using the loader, you want ESM format.
+    format: "module"
+    source: js
+
+  # Remove .js extension from final URL
+  result.responseURL = (result.responseURL ? transpiledUrl)
+  .replace /\.js$/, ''
+
+  result

src/helpers.coffee

@@ -329,3 +329,9 @@ UNICODE_CODE_POINT_ESCAPE = ///
   |
   \\u\{ ( [\da-fA-F]+ ) \}
 ///g
+
+# Find the root of a module by repeatedly following parent points.
+# Used to find the `options` object attached to the topmost module.
+exports.getRootModule = (module) ->
+  module = module.parent while module.parent
+  module

src/register.coffee

@@ -3,26 +3,17 @@ child_process = require 'child_process'
 helpers       = require './helpers'
 path          = require 'path'
 
-{patchStackTrace} = CoffeeScript
-
-# Check if Node's built-in source map stack trace transformations are enabled.
-nodeSourceMapsSupportEnabled = process? and (
-  process.execArgv.includes('--enable-source-maps') or
-  process.env.NODE_OPTIONS?.includes('--enable-source-maps')
-)
-
-unless Error.prepareStackTrace or nodeSourceMapsSupportEnabled
-  cacheSourceMaps = true
-  patchStackTrace()
+CoffeeScript.patchStackTrace()
 
 # Load and run a CoffeeScript file for Node, stripping any `BOM`s.
 loadFile = (module, filename) ->
-  options = module.options or getRootModule(module).options or {}
+  options = module.options or
+            CoffeeScript.helpers.getRootModule(module).options or {}
 
-  # Currently `CoffeeScript.compile` caches all source maps if present. They
+  # Currently `CoffeeScript.compile` caches all source maps if present.  They
   # are available in `getSourceMap` retrieved by `filename`.
-  if cacheSourceMaps or nodeSourceMapsSupportEnabled
-    options.inlineMap = true
+  options = {...options, inlineMap: true}
+
   js = CoffeeScript._compileFile filename, options
 
   module._compile js, filename