custom-import.ts

const cache: {
  [key: string]: any;
} = {};

function getModuleCode(path: string): Promise<string> {
  return fetch(path).then((res) => res.text());
}

async function import<T = unknown>(path: string): Promise<T> {
  if (cache.hasOwnProperty(path)) {
    return cache[path];
  }

  const code = await getModuleCode(path);
  const module = cache[path] = {
    exports: {} as T
  };
  Function('module, exports', code)(module, module.exports);
  return module.exports;
}

(async () => {
  const value = await import<number>('./value.js'); // 42
})();