node/lib/internal/modules/esm/formats.js

'use strict';

const {
  RegExpPrototypeTest,
} = primordials;
const { getOptionValue } = require('internal/options');


const experimentalWasmModules = getOptionValue('--experimental-wasm-modules');
const experimentalSpecifierResolution =
  getOptionValue('--experimental-specifier-resolution');

const extensionFormatMap = {
  '__proto__': null,
  '.cjs': 'commonjs',
  '.js': 'module',
  '.json': 'json',
  '.mjs': 'module',
};

const legacyExtensionFormatMap = {
  '__proto__': null,
  '.cjs': 'commonjs',
  '.js': 'commonjs',
  '.json': 'commonjs',
  '.mjs': 'module',
  '.node': 'commonjs',
};

if (experimentalWasmModules) {
  extensionFormatMap['.wasm'] = legacyExtensionFormatMap['.wasm'] = 'wasm';
}

function mimeToFormat(mime) {
  if (
    RegExpPrototypeTest(
      /\s*(text|application)\/javascript\s*(;\s*charset=utf-?8\s*)?/i,
      mime
    )
  ) return 'module';
  if (mime === 'application/json') return 'json';
  if (experimentalWasmModules && mime === 'application/wasm') return 'wasm';
  return null;
}

let experimentalSpecifierResolutionWarned = false;
function getLegacyExtensionFormat(ext) {
  if (
    experimentalSpecifierResolution === 'node' &&
    !experimentalSpecifierResolutionWarned
  ) {
    process.emitWarning(
      'The Node.js specifier resolution in ESM is experimental.',
      'ExperimentalWarning');
    experimentalSpecifierResolutionWarned = true;
  }
  return legacyExtensionFormatMap[ext];
}

module.exports = {
  extensionFormatMap,
  getLegacyExtensionFormat,
  legacyExtensionFormatMap,
  mimeToFormat,
};