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 40 个练习（共 148 个） · 第 4 / 4 页。Showing 40 of 148 · page 4 / 4.来自From 异步与结构：Promise.all、EventEmitter、LRUAsync and structure: Promise.all, EventEmitter, LRU · 面试八股Interview questions
L3写整块Write a block手写 LRUCache（用 Map，不写链表）Write an LRUCache by hand (use a Map, no linked list)
把「什么都没存」的骨架写成真正的 LRU：超容量淘汰最久未使用， get 和 put 都要刷新「最近用过」。This skeleton stores nothing. Turn it into a real LRU: over capacity, drop the entry that has gone unused the longest, and both get and put must mark an entry as recently used.
要求Requirements
- get / put 基本读写；get 不到返回 undefinedBasic reads and writes through get and put; a miss on get returns undefined
- 超容量时淘汰最久未使用的那条Over capacity, drop the entry that has gone unused the longest
- get 命中要刷新「最近用过」A hit on get marks that entry as recently used
- put 已存在的 key：更新值并刷新put on a key that already exists updates the value and marks it as recently used
- capacity 为 1 也要正确A capacity of 1 still behaves correctly
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 Context 放在哪一层 —— 这道题最容易死的地方Which level the Context goes on — the most common way to fail this task · Cab BookingCab Booking
L3写整块Write a block从签名写出整个 CabContextWrite the whole CabContext from the signature
只给你 import 和导出。三件套自己写出来， 包括那个守卫。检查器会查守卫、查不可变更新、 以及不许用 push。You only get the import and the export. Write the three parts yourself, including the guard. The checker looks for the guard, for an immutable update, and for no use of push.
要求Requirements
- createContext() 不传默认值 —— 这样没套 Provider 时是 undefined，守卫才抓得住createContext() takes no default value — that way it is undefined with no Provider around, which is what the guard catches
- 两个 state：bookedCabDetails 初始 null、rideHistory 初始 []Two states: bookedCabDetails starts null, rideHistory starts []
- updateBookedCabDetails 同时改两个 state，历史用不可变更新（展开运算符），不许 pushupdateBookedCabDetails changes both states, and the history update is immutable (spread), never push
- useCabContext 里有 if 守卫 + throw，错误信息要出现 CabProvideruseCabContext has an if guard plus a throw, and the message mentions CabProvider
- 命名导出 CabProvider 和 useCabContextNamed exports for CabProvider and useCabContext
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 按类型分组渲染六张卡Rendering the six cards grouped by type · Cab BookingCab Booking
L3写整块Write a block从零写出 CabCardWrite CabCard from an empty file
只给你 props 签名。五个 testid 自己写出来， 别忘了
alt 和 type="button"。You only get the props signature. Write the five testids yourself, and do not forget alt and type="button".要求Requirements
- 五个 data-testid 一个不少：cab-card-img / -name / -type / -price / -select-buttonAll five data-testid values: cab-card-img / -name / -type / -price / -select-button
- img 要有 alt（测试不查，但没有 alt 是真实的可访问性缺陷）The img needs an alt (the tests do not check it, but a missing alt is a real accessibility defect)
- 按钮写 type="button" —— 不写的话在 <form> 里会变成提交按钮The button needs type="button" — without it, inside a <form> it becomes a submit button
- onClick 里包一层箭头函数把 cab 传进去，不是直接传 onSelectCabWrap the onClick in an arrow function that passes cab in; do not pass onSelectCab directly
- 价格前面要有 $ —— 历史列表的断言查的是 "$20"The price needs a $ in front — the history assertion looks for "$20"
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 历史与确认页：两个小而致命的细节The history and confirmation pages: two small details that decide pass or fail · Cab BookingCab Booking
L3写整块Write a block从零写出 RideHistoryWrite RideHistory from an empty file
从 Context 读历史，取最新三条、最新在最上， 空的时候显示空状态。检查器会挡住原地修改。Read the history out of the Context, take the three newest with the newest at the top, and show the empty state when there is nothing. The checker blocks changes made in place.
要求Requirements
- 从 useCabContext() 里读 rideHistoryRead rideHistory out of useCabContext()
- 取最新三条并让最新的排在最上面（slice(-3).reverse() 或等价写法）Take the three newest and put the newest at the top (slice(-3).reverse() or an equivalent)
- 不许在 rideHistory 上直接调 reverse / sort —— 那会原地改 stateNever call reverse or sort on rideHistory directly — that changes the state in place
- 每条记录一个 <li data-testid="history-cabs">，里面有车名和 $价格One <li data-testid="history-cabs"> per record, holding the cab name and the $price
- 空历史显示 <p data-testid="no-ride-title">No ride history yet.</p>，且此时不渲染列表An empty history shows <p data-testid="no-ride-title">No ride history yet.</p> and renders no list
- key 不能只用 ride.id —— 同一辆车可以被订两次The key cannot be ride.id alone — the same cab can be booked twice
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.