node/lib/internal/modules/run_main.js

'use strict';

const {
  ObjectCreate,
} = primordials;
const { getOptionValue } = require('internal/options');
const path = require('path');
const { shouldUseESMLoader } = require('internal/modules/utils');

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 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, ObjectCreate(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,
};