数组可以直接由.reduce()生成，但回调变得复杂，因为它需要标识要更新的数组的正确项。
让它保持原样，然后处理groupByCategory的条目以获得预期结果：

const list = Object.entries(groupByCategory).map(
  ([key, value]) => ({ category: key, user_guides: value })
);

查看实际应用：

const newUserGuides = [
  {
    category: "Data",
    description: "user guide 1",
    name: "User Guide 1",
  },
  {
    category: "Data",
    description: "user guide 2",
    name: "User Guide 2",
  },
  {
    category: "Pupils",
    description: "user guide 3",
    name: "User Guide 3",
  },
];

const groupByCategory = newUserGuides.reduce((group, userGuide) => {
  const { category } = userGuide;
  group[category] = group[category] ?? [];
  group[category].push(userGuide);
  return group;
}, {});

const list = Object.entries(groupByCategory).map(
  ([key, value]) => ({ category: key, user_guides: value })
);
console.log(list);

了解Object.entries()。

const source = [{
    category: "Data",
    description: "user guide 1",
    name: "User Guide 1",
  },
  {
    category: "Data",
    description: "user guide 2",
    name: "User Guide 2",
  },
  {
    category: "Pupils",
    description: "user guide 3",
    name: "User Guide 3",
  }
]

// DESIRED OUTPUT
// [{category: 'Data', user_guides: [{...}]},
// {category: 'Pupils', user_guides: [{...}]}]

const newData = source.reduce((arr, item) => {
  const data = arr.find(i => i.category === item.category) || { category: item.category, user_guides: []}
  
  data.user_guides.push({ description: item.description, name: item.name })
  arr.push(data);

  return arr;
}, [])

console.log(newData)