Support esm config in node commonjs projects

load-config.js

const fs = require('fs');
const path = require('path');

const extensions = ['.mjs', '.cjs', '.js'];

async function loadConfig(name) {
  // check if node version supports dynamic imports
  const supportsEsm = await (async () => {
    try {
      await import(__filename);
    } catch (error) {
      return false;
    }
    return true;
  })();

  const configName = path.join(process.cwd(), `${name}.config`);

  if (!supportsEsm) {
    const resolvedPath = require.resolve(configName);
    // node version doesn't support ESM, load it as cjs
    return fs.existsSync(resolvedPath) ? require(resolvedPath) : undefined;
  }

  // load config using dynamic import, resolving .mjs over .cjs over .js.
  // this supports configs written in CJS and ESM, using node's project configuration
  for (const extension of extensions) {
    const resolvedPath = `${configName}${extension}`;
    if (fs.existsSync(resolvedPath)) {
      return import(resolvedPath);
    }
  }
}

// example usage
(async () => {
  const config = await loadConfig('my-project');
  console.log('config', config);
})();

This code snippet allows node projects shipped as commonjs to support user configs written in es modules as well as common js.

It uses node's logic for deciding if a file is ESM or CJS, based on package.json or file extension. On node versions which don't support es modules at all, commonjs is always used.

Examples:

node v10

<my-project>.config.js is always loaded

node v12 (latest), v13 & v14

<my-project>.config.mjs -> config is loaded as esm
<my-project>.config.cjs -> config is loaded as cjs
<my-project>.config.js -> config is loaded based on project package.json type