Sources that are in formats Node.js doesn’t understand can be converted into JavaScript using the [transformSource hook][]. Before that hook gets called, however, other hooks need to tell Node.js not to throw an error on unknown file types; and to tell Node.js how to load this new file type.

    This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.

    1. // coffeescript-loader.mjs
    2. import { URL, pathToFileURL } from 'url';
    3. import CoffeeScript from 'coffeescript';
    5. const baseURL = pathToFileURL(`${process.cwd()}/`).href;
    7. // CoffeeScript files end in .coffee, .litcoffee or .coffee.md.
    8. const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;
    10. export function resolve(specifier, context, defaultResolve) {
    11.   const { parentURL = baseURL } = context;
    13.   // Node.js normally errors on unknown file extensions, so return a URL for
    14.   // specifiers ending in the CoffeeScript file extensions.
    15.   if (extensionsRegex.test(specifier)) {
    16.     return {
    17.       url: new URL(specifier, parentURL).href
    18.     };
    19.   }
    21.   // Let Node.js handle all other specifiers.
    22.   return defaultResolve(specifier, context, defaultResolve);
    23. }
    25. export function getFormat(url, context, defaultGetFormat) {
    26.   // Now that we patched resolve to let CoffeeScript URLs through, we need to
    27.   // tell Node.js what format such URLs should be interpreted as. For the
    28.   // purposes of this loader, all CoffeeScript URLs are ES modules.
    29.   if (extensionsRegex.test(url)) {
    30.     return {
    31.       format: 'module'
    32.     };
    33.   }
    35.   // Let Node.js handle all other URLs.
    36.   return defaultGetFormat(url, context, defaultGetFormat);
    37. }
    39. export function transformSource(source, context, defaultTransformSource) {
    40.   const { url, format } = context;
    42.   if (extensionsRegex.test(url)) {
    43.     return {
    44.       source: CoffeeScript.compile(source, { bare: true })
    45.     };
    46.   }
    48.   // Let Node.js handle all other sources.
    49.   return defaultTransformSource(source, context, defaultTransformSource);
    50. }
    1. # main.coffee
    2. import { scream } from './scream.coffee'
    3. console.log scream 'hello, world'
    5. import { version } from 'process'
    6. console.log "Brought to you by Node.js version #{version}"
    1. # scream.coffee
    2. export scream = (str) -> str.toUpperCase()

    With the preceding loader, running node --experimental-loader ./coffeescript-loader.mjs main.coffee causes main.coffee to be turned into JavaScript after its source code is loaded from disk but before Node.js executes it; and so on for any .coffee, .litcoffee or .coffee.md files referenced via import statements of any loaded file.