編寫涉及模擬或存根 API 呼叫的單元測試可能會讓人感到不知所措——我自己也經歷過這種情況。在本文中，我將指導您透過更簡單、更有效的方法在測試期間模擬 API 請求。
在我們深入之前，這裡是我們將使用的工具清單：
如果您對所有內容都不熟悉，請不要擔心 - 只需按照說明進行操作，我將引導您完成每個步驟。
這是該專案的 Github 連結
概述
我們將建立一個簡單的 React 應用程序，該應用程式使用 Json Placeholder API 取得並顯示 Todos 列表。在此專案中，我們將編寫測試場景來驗證以下內容：
本指南將涵蓋兩種主要方法；
讓我們開始吧！
要開始建立應用程序，請按照以下步驟操作：
1。建立一個新的 React 應用程式： 執行以下命令來建立您的 React 應用程式：
npx create-react-app msw-example
2。啟動應用程式： 設定後，導航至專案資料夾並運行：
npm start
3。安裝必要的套件： 接下來，安裝 @tanstack/react-query 來管理客戶端資料：
npm install @tanstack/react-query
React Query (Tanstack Query) 有助於處理伺服器端狀態管理，包括快取、同步和資料取得。您可以在他們的官方文件中了解更多
您現在可以開始編寫應用程式邏輯並設定 React Query 來有效管理資料擷取。這是設定後的樣子。
import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
import { Todos } from "components/Todos";
const queryClient = new QueryClient();
function App() {
return (
<QueryClientProvider client={queryClient}>
<div data-testid="app" className="App">
<Todos />
</div>
</QueryClientProvider>
);
}
export default App;
接下來我們建立 Todos 元件，呈現我們的待辦事項清單。
Todos.js
import { useQuery } from "@tanstack/react-query";
import { getTodos } from "api/todo";
export function Todos() {
const { data, isError, isLoading } = useQuery({
queryFn: getTodos,
queryKey: ["TODOS"],
});
if (isLoading) {
return <p>loading todo list</p>;
}
if (isError) {
return <p>an error occurred fetching todo list</p>;
}
return (
<div>
{Boolean(data.length) ? (
<ol>
{data.map((item) => (
<li key={item.id}>{item.title}</li>
))}
</ol>
) : (
<p>You do not have todos created yet</p>
)}
</div>
);
}
現在一切都已設定完畢，讓我們開始為我們之前概述的場景編寫測試。首先，我們將使用我們已經熟悉的傳統方法來實現這一點 - 使用 Jest
模擬 API 調用查看 github 儲存庫，以便您可以繼續操作
模擬 API 呼叫的傳統方式
React 和 Jest 開箱即用，無縫協作，因此不需要額外的配置或設定——至少目前是這樣。我們將在 Todo.js 元件旁邊建立一個名為 Todos.test.js 的文件，我們將在其中匯入 Todos 元件並編寫我們的測試..
我們有一個名為 getTodos 的函數，它負責進行 API 呼叫以檢索待辦事項清單並傳回回應。
export async function getTodos() {
const response = await fetch("https://jsonplaceholder.typicode.com/todos", {
headers: {
"Content-Type": "application/json",
},
});
if (!response.ok) {
const res = await response.json();
throw new Error(res.message || response.status);
}
return response.json();
}
在 Todos.test.js 檔案中，您需要匯入 Todos 元件並建立一個實用程式函數，該函數為 React 查詢提供者提供包裝器。這確保了 Todos 元件及其子元件可以使用 React-query 來管理測試中的伺服器狀態。
import { render, screen, waitFor, within } from "@testing-library/react";
import { Todos } from "./Todos";
import { reactQueryWrapper } from "utils/reactQueryWrapper";
import { getTodos } from "api/todo";
const { wrapper, queryCache } = reactQueryWrapper();
接下來，我們需要模擬 getTodos 函數。這將使我們能夠指定每個測試場景的傳回值，使我們能夠控制函數在測試期間傳回的資料。此外，我們將確保清除先前測試案例中的任何剩餘數據，因此每個測試案例都以乾淨的狀態開始。
程式碼範例
jest.mock("api/todo", () => ({
getTodos: jest.fn(),
}));
afterEach(() => {
queryCache.clear();
jest.clearAllMocks();
});
Todos.test.js
import { render, screen, waitFor, within } from "@testing-library/react";
import { Todos } from "./Todos";
import { reactQueryWrapper } from "utils/reactQueryWrapper";
import { getTodos } from "api/todo";
const { wrapper, queryCache } = reactQueryWrapper();
jest.mock("api/todo", () => ({
getTodos: jest.fn(),
}));
afterEach(() => {
queryCache.clear();
});
afterEach(() => {
jest.clearAllMocks();
});
第一個測試場景：渲染載入狀態
我們想要驗證我們的元件在請求進行時是否正確顯示載入狀態。
test("Renders loading state", () => {
getTodos.mockImplementation(() => {
return new Promise((resolve) => {
setTimeout(() => {
resolve();
}, 1000);
});
});
render(<Todos />, { wrapper });
const loadingText = screen.getByText("loading todo list");
expect(loadingText).toBeInTheDocument();
});
第二個測試場景：請求失敗或網路錯誤時呈現錯誤狀態
我們希望驗證當 API 請求失敗或遇到網路錯誤時元件是否正確呈現錯誤狀態。
test("Renders error state when request fails or there is network error", async () => {
getTodos.mockImplementationOnce(() => {
return new Promise((resolve, reject) => {
reject();
});
});
render(<Todos />, { wrapper });
await screen.findByText("an error occurred fetching todo list");
expect(
screen.getByText("an error occurred fetching todo list")
).toBeInTheDocument();
});
第三個測試場景：渲染待辦事項清單
我們希望驗證我們的元件在請求成功時是否正確呈現待辦事項清單。
test("Renders list of todos", async () => {
getTodos.mockImplementation(() => {
return Promise.resolve([
{ id: 1, title: "Exercise" },
{ id: 2, title: "Cook" },
]);
});
render(<Todos />, { wrapper });
const loadingText = screen.queryByText("loading todo list");
await waitFor(() => expect(loadingText).not.toBeInTheDocument());
const list = screen.getByRole("list");
expect(list).toBeInTheDocument();
expect(within(list).getAllByRole("listitem")).toHaveLength(2);
});
第四個測試場景：渲染待辦事項清單
我們希望驗證當 API 要求傳回空的待辦事項清單時，您的元件是否正確呈現回饋訊息。
test("Renders feedback message when user has an empty list of todos", async () => {
getTodos.mockImplementationOnce(() => {
return Promise.resolve([]);
});
render(<Todos />, { wrapper });
await waitFor(() =>
expect(screen.queryByText("loading todo list")).not.toBeInTheDocument()
);
expect(
screen.getByText("You do not have todos created yet")
).toBeInTheDocument();
});
Great! Now that we've covered mocking API calls with Jest, let’s explore a better approach using Mock Service Worker (MSW). MSW provides a more elegant and maintainable way to mock API calls by intercepting network requests at the network level rather than within your tests.
Introducing MSW (Mock Service Worker)
Mock Service Worker (MSW) is an API mocking library designed for both browser and Node.js environments. It allows you to intercept outgoing requests, observe them, and provide mocked responses. MSW helps you simulate real-world scenarios in your tests, making them more robust and reliable.
Read more about MSW
Setting Up MSW
Step 1: Install MSW using the following command:
npm install msw@latest --save-dev
Step 2: Set up the environment you wish to intercept requests in—either Browser or Node. Before doing so, create a mock directory within your src directory. Inside this directory, you'll create the following files and directories:
browser.js: Handles request interception in the browser environment.
server.js: Handles request interception in the Node.js environment.
handlers: A directory containing files that define the API endpoints to intercept.
Here’s how your folder structure should look:
src/
└── mock/
├── browser.js
├── server.js
└── handlers/
This setup ensures that you have a clear organization for intercepting and handling requests in both browser and Node.js environments using MSW.
Browser Environment Setup
To set up MSW for intercepting requests in the browser, follow these steps:
1. Create the browser.js File
In your src/mock directory, create a file named browser.js. This
file will set up the MSW worker to intercept requests in the
browser environment.
// src/mock/browser.js
import { setupWorker } from 'msw/browser';
// Create a worker instance to intercept requests
export const worker = setupWorker();
2. Generate the mockServiceWorker.js File
This file is required for MSW to function properly in the browser.
Generate it using the following command from the root directory of
your application:
npx msw init <PUBLIC_DIR> --save
This command initializes the MSW service worker and places the mockServiceWorker.js file into the public directory of your React app.
3. Start the Service Worker
Import and start the worker in your application entry point
(typically index.js or App.js).
// src/index.js
import React from "react";
import ReactDOM from "react-dom/client";
import "./index.css";
import App from "./App";
import reportWebVitals from "./reportWebVitals";
if (process.env.NODE_ENV === "development") {
const mswState = localStorage.getItem("mswState") === "enabled";
if (mswState) {
const { worker } = require("./mocks/browser");
worker.start();
window.__mswStop = worker.stop;
}
}
const root = ReactDOM.createRoot(document.getElementById("root"));
root.render(
<React.StrictMode>
<App />
</React.StrictMode>
);
4. Verify the Setup
To ensure that the service worker is correctly set up, navigate to
the URL of your application in the browser:
http://localhost:3000/mockServiceWorker.js
You should see the service worker script displayed in your browser. This confirms that the service worker is correctly installed and ready to intercept requests.
If your MSW setup is correct and enabled, you should see a console message indicating that MSW is active. Your browser console should display logs similar to this:
These logs confirm that the MSW service worker is properly intercepting network requests and is ready to mock API responses according to the handlers you’ve defined.
Node Environment Setup
To set up MSW for intercepting requests in a Node.js environment (for example, in server-side tests), follow these steps:
Step 1: Create the server.js File
In the src/mock directory, create a file named server.js. This file sets up the MSW server to intercept requests in a Node.js environment.
// src/mock/server.js
import { setupServer } from "msw/browser";
// Create a server instance with the defined request handlers
export const server = setupServer();
Step 2: Define the API Handlers
Create a file named posts.js in the handlers directory.This file will describe the APIs you want to intercept and the mock responses.
// src/mock/handlers/posts.js
import { http, HttpResponse } from "msw";
export const postHandlers = [
// Handler for GET /todos request
http.get("https://jsonplaceholder.typicode.com/todos", () => {
return HttpResponse.json([
{ id: 1, title: "totam quia non" },
{ id: 2, title: "sunt cum tempora" },
]);
}),
];
Here, we're defining that when MSW intercepts a GET request to https://jsonplaceholder.typicode.com/todos, it should respond with a 200 status code and the provided JSON data.
Step 3: Hook Handlers to the Browser Worker
Update the browser.js file to include the defined handlers.
import { setupWorker } from "msw/browser";
import { postHandlers } from "./handlers/posts";
export const worker = setupWorker(...postHandlers);
Step 4: Hook Handlers to the Node Server
Ensure the handlers are also used in the Node.js environment by updating the server.js file.
import { setupServer } from "msw/node";
import { postHandlers } from "./handlers/posts";
export const server = setupServer(...postHandlers);
With these configurations in place, your MSW setup is complete and ready for both browser and Node.js environments. Congratulations on completing the setup! ?
Using MSW in our Tests
To use MSW in your tests, you need to set up your test environment to utilize the mock server for intercepting API calls. Here’s a guide to setting up and writing tests using MSW with your Todos component.
Create the Test File
Create a new file named Todos.MSW.test.js next to your
Todos.jscomponent. This file will contain your tests that
utilize MSW for mocking API responses.
Set Up Test Environment
In your Todos.MSW.test.js file, import the necessary modules and
set up the environment for using MSW with your tests. Below is an
example setup:
import { render, screen, waitFor, within } from "@testing-library/react";
import { http, delay, HttpResponse } from "msw";
import { Todos } from "./Todos";
import { reactQueryWrapper } from "utils/reactQueryWrapper";
import { server } from "mocks/server";
const { wrapper, queryCache } = reactQueryWrapper();
afterEach(() => {
queryCache.clear();
});
afterEach(() => {
jest.clearAllMocks();
});
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());
First Test Scenario: Renders loading state
We want to verify that our component correctly displays the loading state while the request is in progress.
test("Renders loading state", () => {
server.use(
http.get("https://jsonplaceholder.typicode.com/todos", async () => {
await delay(1000);
return HttpResponse.json([]);
})
);
render(<Todos />, { wrapper });
const loadingText = screen.getByText("loading todo list");
expect(loadingText).toBeInTheDocument();
});
Second Test Scenario: Renders error state when request fails or there is network error
We want to verify that the component correctly renders an error state when the API request fails or encounters a network error.
test("Renders error state when request fails or there is network error", async () => {
server.use(
http.get("https://jsonplaceholder.typicode.com/todos", async () => {
return HttpResponse.json([], {
status: 500,
});
})
);
render(<Todos />, { wrapper });
await screen.findByText("an error occurred fetching todo list");
expect(
screen.getByText("an error occurred fetching todo list")
).toBeInTheDocument();
});
Third Test Scenario: Renders list of todos
We want to verify that our component correctly renders the list of todos when the request is successful.
test("Renders list of todos", async () => {
render(<Todos />, { wrapper });
const loadingText = screen.queryByText("loading todo list");
await waitFor(() => expect(loadingText).not.toBeInTheDocument());
const list = screen.getByRole("list");
expect(list).toBeInTheDocument();
expect(within(list).getAllByRole("listitem")).toHaveLength(2);
});
Fourth Test Scenario: Renders list of todos
We want to verify that your component correctly renders a feedback message when the API request returns an empty list of todos.
test("Renders feedback message when user has an empty list of todos", async () => {
server.use(
http.get("https://jsonplaceholder.typicode.com/todos", () => {
return HttpResponse.json([]);
})
);
render(<Todos />, { wrapper });
await waitFor(() =>
expect(screen.queryByText("loading todo list")).not.toBeInTheDocument()
);
expect(
screen.getByText("You do not have todos created yet")
).toBeInTheDocument();
});
Final Thoughts
Mocking API calls is crucial for effective testing, allowing you to simulate different scenarios without relying on real backend services. While traditional Jest mocking can be effective, MSW offers a more sophisticated solution with better support for various environments and more realistic request handling.
Here are a few tips to keep in mind:
Choose the Right Tool: Use MSW for a more comprehensive
solution that integrates seamlessly with your React application,
especially when dealing with complex request handling.
Organize Your Handlers: Keep your API handlers well-organized
and modular to make them easier to maintain and extend.
Clean Up After Tests: Ensure that your tests clean up properly
by resetting handlers and clearing mocks to avoid interference
between tests.
Verify Setup: Always check your setup by inspecting the network
requests and responses to ensure that everything is working as
expected.
By incorporating MSW into your testing strategy, you'll achieve more reliable and maintainable tests, leading to a smoother development experience and higher quality software.
Happy testing! ?
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