A function to determine if a path is going to be loaded as ESM or CJS

esmOrCjs.js

import assert from 'node:assert';
import path from 'node:path';

async function esmOrCjs(filepath) {
  const absolutePath = import.meta.resolve(filepath).replace('file://', '');
  const ext = path.extname(absolutePath);

  switch (ext) {
    case '.mjs':
      return 'esm';
    case '.js': {
      let currentDir = path.dirname(absolutePath);
      while (currentDir !== path.dirname(currentDir)) {
        const packageJsonPath = path.join(currentDir, 'package.json');
        try {
          const { default: packageJson } = await import(packageJsonPath, { with: { type: 'json' }});
          const { type = 'commonjs' } = packageJson;
          return type === 'module' ? 'esm' : 'cjs';
        } catch {
          // Ignore and move up one directory
        }
        currentDir = path.dirname(currentDir);
      }
      // Falls through
    }
    default:
      return 'cjs';
  }
}

// Test the function
// Known CJS module: bcryptjs
// Known ESM module: is-valid-nip
assert.equal(await esmOrCjs('bcryptjs'), 'cjs');
assert.equal(await esmOrCjs('is-valid-nip'), 'esm');