node/lib/internal/modules/run_main.js

'use strict';

const {
  StringPrototypeEndsWith,
} = primordials;

const { getOptionValue } = require('internal/options');
const path = require('path');

function resolveMainPath(main) {
  // Note extension resolution for the main entry point can be deprecated in a
  // future major.
  // Module._findPath is monkey-patchable here.
  const { Module, toRealPath } = require('internal/modules/cjs/loader');
  let mainPath = Module._findPath(path.resolve(main), null, true);
  if (!mainPath) { return; }

  const preserveSymlinksMain = getOptionValue('--preserve-symlinks-main');
  if (!preserveSymlinksMain) {
    mainPath = toRealPath(mainPath);
  }

  return mainPath;
}

function shouldUseESMLoader(mainPath) {
  /**
   * @type {string[]} userLoaders A list of custom loaders registered by the user
   * (or an empty list when none have been registered).
   */
  const userLoaders = getOptionValue('--experimental-loader');
  /**
   * @type {string[]} userImports A list of preloaded modules registered by the user
   * (or an empty list when none have been registered).
   */
  const userImports = getOptionValue('--import');
  if (userLoaders.length > 0 || userImports.length > 0) { return true; }
  const { readPackageScope } = require('internal/modules/cjs/loader');
  // Determine the module format of the main
  if (mainPath && StringPrototypeEndsWith(mainPath, '.mjs')) { return true; }
  if (!mainPath || StringPrototypeEndsWith(mainPath, '.cjs')) { return false; }
  const pkg = readPackageScope(mainPath);
  return pkg && pkg.data.type === 'module';
}

function runMainESM(mainPath) {
  const { loadESM } = require('internal/process/esm_loader');
  const { pathToFileURL } = require('internal/url');

  handleMainPromise(loadESM((esmLoader) => {
    const main = path.isAbsolute(mainPath) ?
      pathToFileURL(mainPath).href : mainPath;
    return esmLoader.import(main, undefined, { __proto__: null });
  }));
}

async function handleMainPromise(promise) {
  const {
    handleProcessExit,
  } = require('internal/modules/esm/handle_process_exit');
  process.on('exit', handleProcessExit);
  try {
    return await promise;
  } finally {
    process.off('exit', handleProcessExit);
  }
}

// For backwards compatibility, we have to run a bunch of
// monkey-patchable code that belongs to the CJS loader (exposed by
// `require('module')`) even when the entry point is ESM.
function executeUserEntryPoint(main = process.argv[1]) {
  const resolvedMain = resolveMainPath(main);
  const useESMLoader = shouldUseESMLoader(resolvedMain);
  if (useESMLoader) {
    runMainESM(resolvedMain || main);
  } else {
    // Module._load is the monkey-patchable CJS module loader.
    const { Module } = require('internal/modules/cjs/loader');
    Module._load(main, null, true);
  }
}

module.exports = {
  executeUserEntryPoint,
  handleMainPromise,
};