DrillLab
开始StartNode.js, npm, node_modules and the lockfile
练习Practice

动手做Get your hands on it

练习跟着课文走 —— 每节课尾都有本课的练习。这一页是全部练习的总库，想集中刷题的时候来。 每个练习都写清了它来自哪一节，卡住了就回去看那一节。Practice follows the lessons — every lesson ends with the exercises for that lesson. This page is the whole library, for when you want to drill in one sitting. Each exercise names the lesson it came from, so you can go back when you stall.

0 / 148个做对过you got right
筛一下Filter theseReact 考试React examL3L3
课程Course全部All地基 · 项目与语言Foundations · project and languageReact 考试React examFederation 考试Federation exam面试八股Interview questionsCab BookingCab Booking
难度LevelL1 → L4 和上面四档同一个意思：给你的东西越来越少L1 → L4: the same idea as the four tiers above — less is handed to you全部难度All levelsL1L1L2L2L3L3L4L4
类型Kind全部类型All kinds认出来Spot it排顺序Order it填空Fill the blanks写整块Write a blockDebug LabDebug Lab从零重写Rebuild from scratch

已筛到你正在学的《React 考试》。想看全部就点上面的「全部」。Filtered to React exam — the course you are on. Use “All” above to see everything.

练习Exercises

筛出 17 个练习（共 148 个） · 第 2 / 2 页。Showing 17 of 148 · page 2 / 2.
来自From 变式三 · fetch 取数：loading、error 与竞态Variation 3 · fetching data: loading, error, and the race between two requests · React 考试React exam
L3Debug LabDebug LabDebug Lab · URL 上是用户 2，界面显示用户 1Debug Lab · the URL says user 2 and the screen shows user 1DrillLab 自出Written by DrillLab

快速点两个用户，界面最后显示的是先点的那个。 慢一点点就没问题。控制台干净。

Click two users quickly and the screen ends up showing the one you clicked first. Click a little slower and it is fine. The console is clean.

第 1 步 · 先看现象和代码，别急着改Step 1 · read the symptom and the code before you touch anything
# 没有任何报错。 $ npx vitest run src/UserCard.test.tsx ✕ a slow stale response must not overwrite the newer one（竞态） Expected element to have text content: 用户2 Received: 用户1 Tests 1 failed | 5 passed (6) # 手动复现： # 1. 点用户 1（这个接口慢，200ms） # 2. 立刻点用户 2（这个快，10ms） # 3. 先看到用户 2 —— 对的 # 4. 200ms 后界面自己变成了用户 1 ← 错的，URL 上还是 2# No error at all. $ npx vitest run src/UserCard.test.tsx ✕ a slow stale response must not overwrite the newer one（竞态） Expected element to have text content: 用户2 Received: 用户1 Tests 1 failed | 5 passed (6) # Manual repro: # 1. Click user 1 (that request is slow, 200ms) # 2. Click user 2 right away (that one is fast, 10ms) # 3. User 2 shows up first — correct # 4. 200ms later the view switches itself to user 1 ← wrong, the URL still says 2
TSXsrc/components/UserCard/index.tsx示意Illustrative
1useEffect(() => {
2 setLoading(true);
3 setError(null);
4
5 (async () => {
6 try {
7 const res = await fetch(`/api/users/${userId}`);
8 if (!res.ok) throw new Error(`HTTP ${res.status}`);
9 setUser(await res.json());
10 } catch (e) {
11 setError((e as Error).message);
12 } finally {
13 setLoading(false);
14 }
15 })();
16}, [userId]);
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 变式四 · 递归读取评论的评论Variation 4 · reading replies to replies with recursion · React 考试React exam
L3写整块Write a block写出树形数据的不可变更新Write an immutable update for tree dataDrillLab 自出Written by DrillLab

这是这道题真正的难点。目标节点可能在任意深度， 要返回一棵新树，而且原树一个字节都不能改。

This is the hard part of the question. The target node can be at any depth, you have to return a new tree, and not one byte of the original may change.

要求Requirements
  • 找到 id === parentId 的节点，把 reply 追加到它的 replies 末尾Find the node whose id === parentId and append reply to the end of its replies
  • 返回新数组、新节点对象，不修改原数据Return a new array and new node objects, without changing the original data
  • 目标可能在任意深度，需要递归往下找The target can be at any depth, so recurse downwards to find it
  • 不许用 JSON.parse(JSON.stringify(...)) 深拷贝Do not deep-copy with JSON.parse(JSON.stringify(...))
  • 不许用 push / splice / 直接赋值Do not use push / splice / direct assignment
TypeScriptsrc/components/CommentTree/index.tsx
This check is textual: it looks for the right constructs, it does not run your code
提示Hints共 4 级，已看 0 级4 levels, 0 opened
先自己想两分钟。想不出来再点右上角 —— 提示是一级一级放的，不会一次给完。Think for two minutes first. Then use the button above — hints come one level at a time, never all at once.

看答案之前，先确认你已经自己动手写过一遍。看懂别人的答案和自己写出来，是两种能力。Before you open this, make sure you have written it yourself once. Following someone else's answer and producing your own are two different skills.

来自From 变式四 · 递归读取评论的评论Variation 4 · reading replies to replies with recursion · React 考试React exam
L3Debug LabDebug LabDebug Lab · 回复加进去了，界面不动Debug Lab · the reply went in and the screen never movedDrillLab 自出Written by DrillLab

给深层评论加回复，console.log 打出来的树里 新回复确实在，但界面没变化。控制台干净。

You add a reply to a deep comment. The tree printed by console.log really does contain the new reply, but the screen does not change. The console is clean.

第 1 步 · 先看现象和代码，别急着改Step 1 · read the symptom and the code before you touch anything
# 没有任何报错。 $ npx vitest run src/CommentTree.test.tsx ✕ addReply 挂到深层节点，且不改原树 TypeError: Cannot add property 0, object is not extensible （测试把原树深冻结了，实现试图直接修改它） ✕ 给三层的评论再回复，落在正确的位置 Unable to find an element with the text: 第四层 # 手动复现：点某条评论的 Reply、输入、发送 # console.log(comments) -> 新回复确实在树里 # 屏幕 -> 一点变化都没有# No error at all. $ npx vitest run src/CommentTree.test.tsx ✕ addReply 挂到深层节点，且不改原树 TypeError: Cannot add property 0, object is not extensible (The test deep-froze the original tree; the implementation edits it in place.) ✕ 给三层的评论再回复，落在正确的位置 Unable to find an element with the text: 第四层 # Manual repro: click Reply on a comment, type something, send it # console.log(comments) -> the new reply really is in the tree # the screen -> nothing changes at all
TSXsrc/components/CommentTree/index.tsx示意Illustrative
1function addReply(nodes: Comment[], parentId: number, reply: Comment) {
2 for (const node of nodes) {
3 if (node.id === parentId) {
4 node.replies.push(reply); // 找到就塞进去
5 return nodes;
6 }
7 addReply(node.replies, parentId, reply);
8 }
9 return nodes;
10}
11
12const handleReply = (parentId: number, text: string) => {
13 const reply = { id: Date.now(), author: "我", body: text, replies: [] };
14 setComments(addReply(comments, parentId, reply));
15};
1function addReply(nodes: Comment[], parentId: number, reply: Comment) {
2 for (const node of nodes) {
3 if (node.id === parentId) {
4 node.replies.push(reply); // found it, so push it in
5 return nodes;
6 }
7 addReply(node.replies, parentId, reply);
8 }
9 return nodes;
10}
11
12const handleReply = (parentId: number, text: string) => {
13 const reply = { id: Date.now(), author: "我", body: text, replies: [] };
14 setComments(addReply(comments, parentId, reply));
15};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 变式五 · 主题切换：Context 怎么用Variation 5 · theme switching: how to use Context · React 考试React exam
L3写整块Write a block自己写出 ThemeProvider 和 useThemeWrite ThemeProvider and useTheme yourselfDrillLab 自出Written by DrillLab

类型已给好。写出 context、Provider、自定义 hook 三部分。 检查器会查记忆化、函数式更新和守卫。

The types are given. Write all three parts: the context, the Provider, and the custom hook. The checker looks for the memoization, the updater form and the guard.

要求Requirements
  • theme 初始为 'light'theme starts as 'light'
  • toggleTheme 在 light / dark 之间翻转，必须用函数式更新toggleTheme flips between light and dark, using the updater form
  • context value 要记忆化，theme 不变时不产生新对象The context value has to be memoized, so no new object appears while theme is unchanged
  • toggleTheme 引用要稳定（theme 变了它也不变）The reference of toggleTheme has to be stable, unchanged even when theme changes
  • 没套 Provider 就用 useTheme() 时抛出一句能看懂的错误Calling useTheme() with no Provider above it throws a message a person can read
  • 不许把 theme 存到组件外的全局变量里Do not keep theme in a global variable outside the component
TSXsrc/context/ThemeContext.tsx
This check is textual: it looks for the right constructs, it does not run your code
提示Hints共 4 级，已看 0 级4 levels, 0 opened
先自己想两分钟。想不出来再点右上角 —— 提示是一级一级放的，不会一次给完。Think for two minutes first. Then use the button above — hints come one level at a time, never all at once.

看答案之前，先确认你已经自己动手写过一遍。看懂别人的答案和自己写出来，是两种能力。Before you open this, make sure you have written it yourself once. Following someone else's answer and producing your own are two different skills.

来自From Debug Lab · React 十种典型故障Debug Lab · ten typical React failures · React 考试React exam
L3Debug LabDebug Lab故障 4 · 编辑后列表毫无变化（综合题）Fault 4 · the list does not change after an edit (mixed question)

这一题不告诉你是哪一类。控制台干净，console.log 显示数据是对的。 自己分诊。

This one does not tell you which category it is. The console is clean, and console.log shows the data is correct. Sort it yourself.

第 1 步 · 先看现象和代码，别急着改Step 1 · read the symptom and the code before you touch anything
# 没有任何报错。 # 复现：添加 "A"、"B" 两条 → 点 B 的 Edit → 改成 "B2" → 点 Update # 期望：列表变成 A、B2 # 实际：列表还是 A、B # 在 handleSubmitNote 里插了日志： console.log("submitted:", submittedNote); // → submitted: { id: 1785737900978, title: 'B2', content: '...' } ← 数据是对的 console.log("after:", notes); // → after: [ {title:'A'...}, {title:'B2'...} ] ← 数组里也是对的！ # 但屏幕上还是 B。 # 测试结果： # ✕ edits a note in place# No error at all. # Repro: add "A" and "B" → click Edit on B → change it to "B2" → click Update # Expected: the list becomes A, B2 # Actual: the list is still A, B # Logs added inside handleSubmitNote: console.log("submitted:", submittedNote); // → submitted: { id: 1785737900978, title: 'B2', content: '...' } ← the data is right console.log("after:", notes); // → after: [ {title:'A'...}, {title:'B2'...} ] ← the array is right too! # But the screen still shows B. # Test result: # ✕ edits a note in place
TSX有问题的 handleSubmitNoteThe handleSubmitNote with the problem示意Illustrative
1const handleSubmitNote = (submittedNote: Note) => {
2 if (noteToEdit) {
3 const i = notes.findIndex((n) => n.id === submittedNote.id);
4 notes[i] = submittedNote;
5 setNotes(notes);
6 setNoteToEdit(null);
7 } else {
8 setNotes((prev) => [...prev, submittedNote]);
9 }
10};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this