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
练习Exercises
筛出 26 个练习（共 148 个） · 第 1 / 3 页。Showing 26 of 148 · page 1 / 3.来自From 数组与对象：不可变更新三件套Arrays and objects: three ways to update without changing the original · 地基 · 项目与语言Foundations · project and language
L2Debug LabDebug LabDebug Lab · 数据加进去了，界面没反应Debug Lab · the data went in, the screen did not move
这一类 bug 最难查，因为它不报错。 先看现象，判断类型，再找病灶 —— 别跳步。
This kind of bug is the hardest to find, because it reports no error. Read the symptom, classify it, then locate it. Do not skip a step.
# 没有任何报错。控制台干净。
# 现象：填好表单点 Add，表格里什么都不出现。
# 在 handleSubmitNote 里 console.log(notes) —— 长度确实在增加。
notes.length before: 0
notes.length after : 1 ← 数据真的进去了
（但 <NoteTable /> 渲染出来的行数始终是 0）# No error at all. The console is clean.
# Symptom: fill in the form, click Add, and nothing shows up in the table.
# Add console.log(notes) inside handleSubmitNote — the length really does grow.
notes.length before: 0
notes.length after : 1 ← the data really did go in
(But <NoteTable /> always renders 0 rows.)
1const handleSubmitNote = (submittedNote: Note) => {
2 notes.push(submittedNote);
3 setNotes(notes);
4};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From ESM:import / export 与那些莫名其妙的报错ESM: import / export, and the errors that look strange at first · 地基 · 项目与语言Foundations · project and language
L2Debug LabDebug LabDebug Lab · ERR_MODULE_NOT_FOUNDDebug Lab · ERR_MODULE_NOT_FOUND
你在 node-subgraph/ 里跑 npm start, 服务器起不来。报错很长，但关键信息只有两行。
You run npm start inside node-subgraph/ and the server does not come up. The error is long, but only two lines of it matter.
$ npm start
node:internal/modules/esm/resolve:274
Error [ERR_MODULE_NOT_FOUND]: Cannot find module
'/Users/me/node-subgraph/src/dataSources/orderDataSource'
imported from /Users/me/node-subgraph/src/index.js
at finalizeResolution (node:internal/modules/esm/resolve:274:11)
Did you mean to import "./dataSources/orderDataSource.js"?
1import { resolvers } from './resolvers/orderResolvers.js';
2import { OrderDataSource } from './dataSources/orderDataSource';
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From props：数据往下流，事件往上报props: data flows down, events go back up · React 考试React exam
L2Debug LabDebug LabDebug Lab · 页面一打开，所有笔记就消失了Debug Lab · every note disappears the moment the page opens
添加两条笔记后刷新页面（假设有持久化），表格瞬间变空。 有时候浏览器还会卡住。先判断类型，再找病灶。
Add two notes, then reload the page (assume the notes are saved somewhere). The table goes empty at once, and sometimes the browser stops responding. First name the kind of error, then find the line that causes it.
Warning: Maximum update depth exceeded. This can happen when a component
repeatedly calls setState inside componentWillUpdate or componentDidUpdate.
React limits the number of nested updates to prevent infinite loops.
（另一种表现：没有任何报错，但表格永远是空的）Warning: Maximum update depth exceeded. This can happen when a component
repeatedly calls setState inside componentWillUpdate or componentDidUpdate.
React limits the number of nested updates to prevent infinite loops.
(Another symptom: no warning at all, but the table stays empty forever.)
1const NoteItem: React.FC<NoteItemProps> = ({ note, onDelete, onEdit }) => {
2 return (
3 <tr>
4 <td>{note.title}</td>
5 <td>{note.content}</td>
6 <td>
7 <button onClick={onEdit(note)} className="outlined">Edit</button>
8 </td>
9 <td>
10 <button onClick={onDelete(note.id)} className="danger">Delete</button>
11 </td>
12 </tr>
13 );
14};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 受控输入：value + onChange 的闭环Controlled inputs: the loop between value and onChange · React 考试React exam
L2Debug LabDebug LabDebug Lab · 点 Add 之后页面闪一下，笔记没了Debug Lab · the page blinks after Add and the note is gone
填好标题和内容，点 Add。页面明显闪了一下， 地址栏出现了 ?，表格还是空的。
Fill in a title and some content, then press Add. The page clearly blinks, a ? appears in the address bar, and the table is still empty.
# 没有 JavaScript 报错。
# 现象：点击 Add 后
# - 页面整体刷新了一次
# - 地址栏从 http://localhost:5173/ 变成 http://localhost:5173/?
# - 输入框被清空，表格依然是空的
# - React DevTools 里所有 state 都回到了初始值# No JavaScript error.
# Symptom: after clicking Add
# - the whole page reloaded once
# - the address bar changed from http://localhost:5173/ to http://localhost:5173/?
# - the inputs were cleared and the table is still empty
# - in React DevTools every piece of state is back to its initial value
1const handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
2 if (isFormInvalid) return;
3 const newNote = { id: Date.now(), title: title.trim(), content: content.trim() };
4 onSubmit(newNote);
5 setTitle("");
6 setContent("");
7};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From useEffect：把 props 的变化同步进 stateuseEffect: copying a change in props into state · React 考试React exam
L3Debug LabDebug LabDebug Lab · 点 Edit 之后页面卡死Debug Lab · the page freezes after you press Edit
点某一行的 Edit 按钮，浏览器标签页转圈，控制台刷出大量警告。 请判断类型并定位。
Press the Edit button on any row. The browser tab spins and the console fills up with warnings. Classify the error, then locate it.
Warning: Maximum update depth exceeded. This can happen when a component
calls setState inside useEffect, but useEffect either doesn't have a dependency
array, or one of the dependencies changes on every render.
at NoteForm (src/components/NoteForm/index.tsx:13:3)
at NoteManager (src/components/NoteManager/index.tsx:6:3)
Uncaught Error: Too many re-renders. React limits the number of renders to
prevent an infinite loop.
1useEffect(() => {
2 if (noteToEdit) {
3 setTitle(noteToEdit.title);
4 setContent(noteToEdit.content);
5 } else {
6 setTitle("");
7 setContent("");
8 }
9}, [noteToEdit, title, content]);
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Task 2 · Delete：点 Delete，该行按 id 被移除Task 2 · Delete: click Delete and that one row is removed by id · React 考试React exam
L2Debug LabDebug LabDebug Lab · 删一条，同名的全没了Debug Lab · delete one note and every note with the same title goes too
测试全过，但手动测试时发现：三条标题相同的笔记， 点其中一条的 Delete，三条一起消失。
Every test passes, but a manual check shows this: with three notes that share a title, clicking Delete on one of them makes all three disappear.
# 测试：4 passed (4) ← 测试全过！
# 手动复现步骤：
# 1. 添加 "会议记录 / 内容1"
# 2. 添加 "会议记录 / 内容2"
# 3. 添加 "会议记录 / 内容3"
# 4. 点第 2 行的 Delete
# 期望：只剩「内容1」「内容3」
# 实际：表格全空# Tests: 4 passed (4) ← every test passes!
# Manual repro steps:
# 1. Add "会议记录 / 内容1"
# 2. Add "会议记录 / 内容2"
# 3. Add "会议记录 / 内容3"
# 4. Click Delete on the second row
# Expected: only 内容1 and 内容3 are left
# Actual: the table is empty
1const handleDelete = (id: number) => {
2 const target = notes.find((n) => n.id === id);
3 setNotes((prev) => prev.filter((note) => note.title !== target?.title));
4};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Task 3 · Edit：回填、改文字、就地更新、退出编辑Task 3 · Edit: refill the form, change the button text, update the row where it is, leave edit mode · React 考试React exam
L3Debug LabDebug LabDebug Lab · 点 Update 之后毫无反应Debug Lab · nothing happens after you click Update
点 Edit，输入框正常回填，按钮变成 Update。 改完内容点 Update —— 列表一点变化都没有， 表单也没清空。控制台干净。
Click Edit and the inputs prefill correctly, and the button becomes Update. Change the content and click Update — the list does not change at all, and the form does not clear either. The console is clean.
# 没有任何报错。
# 复现：
# 1. 添加 "Old / c1"
# 2. 点 Edit → 输入框显示 Old / c1，按钮变 Update ✓ 这两步正常
# 3. 把标题改成 "New"，点 Update
# 期望：列表里那条变成 New，表单清空，按钮回到 Add
# 实际：列表还是 Old，表单还留着 New，按钮还是 Update
# 测试结果：
# ✓ adds a note
# ✓ submit button disabled when inputs empty
# ✓ deletes a note
# ✕ edits a note in place
# Unable to find text content "New" in element [data-testid="notes-list"]# No error at all.
# Repro:
# 1. Add "Old / c1"
# 2. Click Edit → the inputs show Old / c1, the button reads Update ✓ both fine
# 3. Change the title to "New" and click Update
# Expected: that row becomes New, the form clears, the button goes back to Add
# Actual: the row is still Old, the form still holds New, the button still reads Update
# Test results:
# ✓ adds a note
# ✓ submit button disabled when inputs empty
# ✓ deletes a note
# ✕ edits a note in place
# Unable to find text content "New" in element [data-testid="notes-list"]
1const handleSubmitNote = (submittedNote: Note) => {
2 const note = { ...submittedNote, id: Date.now() };
3
4 if (noteToEdit) {
5 setNotes((prev) =>
6 prev.map((n) => (n.id === note.id ? note : n)),
7 );
8 setNoteToEdit(null);
9 } else {
10 setNotes((prev) => [...prev, note]);
11 }
12};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 实现：worker pool（工人池）Building it: a worker pool, meaning a fixed number of workers sharing one queue · React 考试React exam
L2Debug LabDebug LabDebug Lab · 一行 START 都没打印Debug Lab · not one START line prints
跑 npm run q2，没有报错，但一行task N START 都没有，直接就出结果了 —— 而且结果里的 value 长得很奇怪。
You run npm run q2. Nothing reports an error, but not one task N START line appears; the results come out right away. And the value in each result looks strange.
$ npm run q2
=== FINAL RESULTS (must be in original order) ===
#1 { status: 'fulfilled', value: [Function (anonymous)] }
#2 { status: 'fulfilled', value: [Function (anonymous)] }
#3 { status: 'fulfilled', value: [Function (anonymous)] }
#4 { status: 'fulfilled', value: [Function (anonymous)] }
#5 { status: 'fulfilled', value: [Function (anonymous)] }
#6 { status: 'fulfilled', value: [Function (anonymous)] }
# 注意：
# - 一行 "task N START" 都没有
# - 应该 reject 的 task 3 也变成了 fulfilled
# - value 是函数，不是 "result of task N"$ npm run q2
=== FINAL RESULTS (must be in original order) ===
#1 { status: 'fulfilled', value: [Function (anonymous)] }
#2 { status: 'fulfilled', value: [Function (anonymous)] }
#3 { status: 'fulfilled', value: [Function (anonymous)] }
#4 { status: 'fulfilled', value: [Function (anonymous)] }
#5 { status: 'fulfilled', value: [Function (anonymous)] }
#6 { status: 'fulfilled', value: [Function (anonymous)] }
# Note:
# - not one "task N START" line was printed
# - task 3, which should reject, came back fulfilled
# - value is a function, not "result of task N"
1const worker = async () => {
2 while (nextIndex < tasks.length) {
3 const i = nextIndex;
4 nextIndex++;
5
6 try {
7 const value = await tasks[i];
8 results[i] = { status: "fulfilled", value };
9 } catch (reason) {
10 results[i] = { status: "rejected", reason };
11 }
12 }
13};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 变式二 · 计时器：useEffect 的清理函数Variation 2 · a timer: the useEffect cleanup function · React 考试React exam
L2Debug LabDebug LabDebug Lab · 计时器越跑越快Debug Lab · the timer keeps getting fasterDrillLab 自出Written by DrillLab
点了几次 Start / Pause 之后，秒数开始一次跳好几秒。 下面是真实的测试输出。
After a few clicks of Start and Pause, the seconds start jumping several at a time. Below is the real test output.
$ npx vitest run src/Timer.test.tsx
✕ pause stops the clock and keeps the value
Expected element to have text content: 00:02
Received: 00:07
✕ start/pause many times does not speed up
Expected element to have text content: 00:04
Received: 00:10
✕ reset stops and zeroes
Expected element to have text content: 00:00
Received: 00:03
✕ unmount clears the interval
AssertionError: expected 1 to be +0 // Object.is equality
Tests 4 failed | 4 passed (8)
# 现象：start / pause 来回点四次，每次只走 1 秒，
# 显示却是 00:10 —— 正好是 1+2+3+4。
# 而且 Reset 之后秒数还在自己往上涨。$ npx vitest run src/Timer.test.tsx
✕ pause stops the clock and keeps the value
Expected element to have text content: 00:02
Received: 00:07
✕ start/pause many times does not speed up
Expected element to have text content: 00:04
Received: 00:10
✕ reset stops and zeroes
Expected element to have text content: 00:00
Received: 00:03
✕ unmount clears the interval
AssertionError: expected 1 to be +0 // Object.is equality
Tests 4 failed | 4 passed (8)
# Symptom: click start / pause four times, one second of running each time,
# and the display reads 00:10 — exactly 1+2+3+4.
# After Reset the seconds also keep climbing on their own.
1useEffect(() => {
2 if (!running) return;
3
4 const id = setInterval(() => {
5 setSeconds((s) => s + 1);
6 }, 1000);
7}, [running]);
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.
# 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
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
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.
# 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
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
L2Debug LabDebug LabDebug Lab · Cannot destructure property 'theme'DrillLab 自出Written by DrillLab
按钮好好的，卡片一渲染就整页白屏。这是真实报错。
The button is fine, and the moment the card renders the whole page goes blank. This is the real error.
$ npx vitest run src/Theme.test.tsx
TypeError: Cannot destructure property 'theme' of
'(0 , __vite_ssr_import_1__.useTheme)(...)' as it is undefined.
at ThemedCard (src/components/ThemedCard/index.tsx:6:11)
at ThemeApp
✕ 默认是 light，按钮说 Switch to Dark
✕ 点一下变 dark：按钮文字和卡片底色一起变
✕ 再点一下切回 light
✕ 没套 Provider 就用 useTheme()，必须立刻报错
✕ toggleTheme 是稳定引用：theme 变了它也不变
Tests 5 failed | 3 passed (8)
# 浏览器里的报错略有不同，意思一样：
# Cannot destructure property 'theme' of 'useTheme(...)' as it is undefined.$ npx vitest run src/Theme.test.tsx
TypeError: Cannot destructure property 'theme' of
'(0 , __vite_ssr_import_1__.useTheme)(...)' as it is undefined.
at ThemedCard (src/components/ThemedCard/index.tsx:6:11)
at ThemeApp
✕ 默认是 light，按钮说 Switch to Dark
✕ 点一下变 dark：按钮文字和卡片底色一起变
✕ 再点一下切回 light
✕ 没套 Provider 就用 useTheme()，必须立刻报错
✕ toggleTheme 是稳定引用：theme 变了它也不变
Tests 5 failed | 3 passed (8)
# The browser wording is slightly different but means the same thing:
# Cannot destructure property 'theme' of 'useTheme(...)' as it is undefined.
1// ThemeContext.tsx
2const ThemeContext = createContext<ThemeContextValue>(undefined as never);
3
4export function useTheme() {
5 return useContext(ThemeContext); // 没有守卫
6}
7
8// ThemeApp/index.tsx
9const ThemeApp: React.FC = () => (
10 <>
11 <ThemeProvider>
12 <ThemeToggleButton />
13 </ThemeProvider>
14 <ThemedCard />
15 </>
16);
1// ThemeContext.tsx
2const ThemeContext = createContext<ThemeContextValue>(undefined as never);
3
4export function useTheme() {
5 return useContext(ThemeContext); // no guard
6}
7
8// ThemeApp/index.tsx
9const ThemeApp: React.FC = () => (
10 <>
11 <ThemeProvider>
12 <ThemeToggleButton />
13 </ThemeProvider>
14 <ThemedCard />
15 </>
16);
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this