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.
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练习Exercises
筛出 16 个练习（共 148 个） · 第 2 / 2 页。Showing 16 of 148 · page 2 / 2.来自From Debug Lab · Federation 十种典型故障Debug Lab · ten common Federation failures · Federation 考试Federation exam
L2Debug LabDebug Lab故障 2 · Cannot return null for non-nullable fieldFault 2 · Cannot return null for non-nullable field
查一个没有订单的用户，整个 data 变成了null，而且 errors 里有一条很长的消息。
You query a user who has no orders, the whole data turns into null, and errors carries one very long message.
$ node verify-schema.mjs
Query.orders: {"orders":null}
errors: [{
"message": "Cannot return null for non-nullable field Query.orders.",
"path": ["orders"],
"extensions": { "code": "INTERNAL_SERVER_ERROR" }
}]
# 更严重的情况：如果查询是嵌套的，整个 data 会变成 null$ node verify-schema.mjs
Query.orders: {"orders":null}
errors: [{
"message": "Cannot return null for non-nullable field Query.orders.",
"path": ["orders"],
"extensions": { "code": "INTERNAL_SERVER_ERROR" }
}]
# Worse case: if the query is nested, the whole data object turns into null
1async orders(_, { userId }, { dataSources, correlationId }) {
2 const orders = await dataSources.orderDataSource.getOrdersByUserId(userId);
3 return orders; // 数据源可能返回 undefined
4}
5
6// 参考 schema.graphql：
7// type Query {
8// orders(userId: ID!): [Order!]! ← 双重非空
9// }
1async orders(_, { userId }, { dataSources, correlationId }) {
2 const orders = await dataSources.orderDataSource.getOrdersByUserId(userId);
3 return orders; // the data source may return undefined
4}
5
6// For reference, schema.graphql says:
7// type Query {
8// orders(userId: ID!): [Order!]! ← non-null twice
9// }
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Debug Lab · Federation 十种典型故障Debug Lab · ten common Federation failures · Federation 考试Federation exam
L2Debug LabDebug Lab故障 4 · PATCH 传了小写状态，返回 500Fault 4 · PATCH sends a lowercase status and gets a 500
Java 那边。mvn test 全过， 但客户端传小写的 shipped 时服务返回 500。
This one is on the Java side. mvn test passes everything, but the service returns 500 when the client sends the lowercase shipped.
$ curl -i -X PATCH localhost:8080/api/orders/1/status \
-H 'Content-Type: application/json' -d '{"status":"shipped"}'
HTTP/1.1 500
{"timestamp":"...","status":500,"error":"Internal Server Error"}
# 服务端日志：
java.lang.IllegalArgumentException: No enum constant
com.techflow.orders.model.OrderStatus.shipped
at java.base/java.lang.Enum.valueOf(Enum.java:293)
at com.techflow.orders.model.OrderStatus.valueOf(OrderStatus.java:3)
at c.t.orders.controller.OrderController.updateOrderStatus(OrderController.java:71)
# mvn test：Tests run: 5, Failures: 0 ← 测试全过$ curl -i -X PATCH localhost:8080/api/orders/1/status \
-H 'Content-Type: application/json' -d '{"status":"shipped"}'
HTTP/1.1 500
{"timestamp":"...","status":500,"error":"Internal Server Error"}
# Server log:
java.lang.IllegalArgumentException: No enum constant
com.techflow.orders.model.OrderStatus.shipped
at java.base/java.lang.Enum.valueOf(Enum.java:293)
at com.techflow.orders.model.OrderStatus.valueOf(OrderStatus.java:3)
at c.t.orders.controller.OrderController.updateOrderStatus(OrderController.java:71)
# mvn test: Tests run: 5, Failures: 0 ← every test passes
1@PatchMapping("/api/orders/{id}/status")
2public ResponseEntity<Order> updateOrderStatus(
3 @PathVariable Long id,
4 @RequestBody Map<String, String> statusUpdate) {
5 OrderStatus status = OrderStatus.valueOf(statusUpdate.get("status"));
6 return ResponseEntity.ok(orderService.updateOrderStatus(id, status));
7}
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Loading：一秒之后自己跳走Loading: it moves to the next page by itself after one second · Cab BookingCab Booking
L2Debug LabDebug LabDebug Lab：定时器永远不到期Debug Lab: the timer never fires
测试 3 报「找不到
confirm-message」， 而 DOM 快照显示页面还停在 loading。 先读报错，再看下面那个 Loading 组件 ——它和源项目差一个东西。Test 3 reports that it cannot find confirm-message, and the DOM snapshot shows the page still sitting on loading. Read the error first, then look at the Loading component below — one thing in it differs from the source project.FAIL src/test/App.test.jsx > React: Cab Booking > completes a booking and adds it to ride history
TestingLibraryElementError: Unable to find an element by: [data-testid="confirm-message"]
Ignored nodes: comments, script, style
<body>
<div>
<div class="App">
<header …>…</header>
<main class="loading-container" data-testid="loading">
<div class="spinner" aria-hidden="true" />
<h1>Loading...</h1>
</main>
</div>
</div>
</body>
❯ src/test/App.test.jsx:52:17
1const Loading = ({ onComplete }) => {
2 useEffect(() => {
3 const timer = setTimeout(() => {
4 if (onComplete) onComplete();
5 }, 1000);
6
7 return () => clearTimeout(timer);
8 }); // ← 依赖数组呢？
9
10 return (
11 <main data-testid="loading" className="loading-container">
12 <div className="spinner" aria-hidden="true" />
13 <h1>Loading...</h1>
14 </main>
15 );
16};
1const Loading = ({ onComplete }) => {
2 useEffect(() => {
3 const timer = setTimeout(() => {
4 if (onComplete) onComplete();
5 }, 1000);
6
7 return () => clearTimeout(timer);
8 }); // ← where is the dependency array?
9
10 return (
11 <main data-testid="loading" className="loading-container">
12 <div className="spinner" aria-hidden="true" />
13 <h1>Loading...</h1>
14 </main>
15 );
16};
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From 完整答案跑不起来 —— 一个扩展名的事The complete answer does not run — the cause is one file extension · Cab BookingCab Booking
L2Debug LabDebug LabDebug Lab：0 个测试跑起来Debug Lab: zero tests run
README 说「先运行完整答案熟悉流程」。
npm install 成功，npx vitest run 却是下面这个输出。注意最后一行的「no tests」。The README says to run the finished answer first to get used to the flow. npm install succeeds, and npx vitest run gives the output below. Look at the “no tests” on the last line. RUN v2.1.8 /Users/you/cab-booking-context
❯ src/test/App.test.jsx (0 test)
⎯⎯⎯⎯⎯⎯ Failed Suites 1 ⎯⎯⎯⎯⎯⎯⎯
FAIL src/test/App.test.jsx [ src/test/App.test.jsx ]
Error: Failed to parse source for import analysis because the content contains invalid JS syntax. If you are using JSX, make sure to name the file with the .jsx or .tsx extension.
Plugin: vite:import-analysis
File: /Users/you/cab-booking-context/src/context/CabContext.js:19:27
17 | >
18 | {children}
19 | </CabContext.Provider>
| ^
20 | );
21 | };
❯ TransformPluginContext._formatError node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:49255:41
❯ TransformPluginContext.error node_modules/vite/dist/node/chunks/dep-CB_7IfJ-.js:49250:16
Test Files 1 failed (1)
Tests no tests
1import { createContext, useContext, useState } from "react";
2
3const CabContext = createContext();
4
5const CabProvider = ({ children }) => {
6 const [bookedCabDetails, setBookedCabDetails] = useState(null);
7 const [rideHistory, setRideHistory] = useState([]);
8
9 const updateBookedCabDetails = (details) => {
10 setBookedCabDetails(details);
11 setRideHistory([...rideHistory, details]);
12 };
13
14 return (
15 <CabContext.Provider
16 value={{ bookedCabDetails, updateBookedCabDetails, rideHistory }}
17 >
18 {children}
19 </CabContext.Provider>
20 );
21};
22
23const useCabContext = () => {
24 const context = useContext(CabContext);
25
26 if (!context) {
27 throw new Error("useCabContext must be used within a CabProvider");
28 }
29
30 return context;
31};
32
33export { CabProvider, useCabContext };
Source:
cab-booking-context/src/context/CabContext.js第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this