A toy CommonJS implementation

mini-cjs.js

import fs from "node:fs";
import path from "node:path";
import url from "node:url";

const cache = {};

function createRequire(filename) {
  function require(specifier) {
    const resolved = require.resolve(specifier);
    if (resolved in cache) {
      return cache[resolved].exports;
    }

    const contents = fs.readFileSync(resolved, "utf8");
    const module = {
      loaded: false,
      exports: {},
    };
    const fn = new Function(
      "module",
      "exports",
      "require",
      "__dirname",
      "__filename",
      contents,
    );

    let innerRequire;
    cache[resolved] = module;

    fn(
      module,
      module.exports,
      function require(specifier) {
        if (!innerRequire) {
          innerRequire = createRequire(resolved);
        }
        return innerRequire(specifier);
      },
      path.dirname(resolved),
      resolved,
    );

    module.loaded = true;

    return module.exports;
  }

  require.resolve = (specifier) => {
    let resolved = path.resolve(path.dirname(filename), specifier);
    if (fs.existsSync(resolved)) {
      return resolved;
    }
    if (fs.existsSync(resolved + ".js")) {
      return resolved + ".js";
    }
  };

  require.cache = cache;

  return require;
}

const require = createRequire(url.fileURLToPath(import.meta.url));
require(process.argv[2]);