code: function mustContain(object = {}, ...keys) {
    let missingKeys = [];
    let promise;
    keys.forEach((key) => {
      if (object[key] === undefined) {
        missingKeys.push(key);
      }
    });
    if (missingKeys.length === 0) {
      missingKeys = undefined;
      promise = Promise.resolve();
    } else {
      promise = Promise.reject(
        new Error(`Missing keys ${missingKeys.join(', ')}`)
      );
    }
  
    return {
      missingKeys,
      promise,
    };
  }
docs: ----------------------------------------------------------------------------