Q121- Why do we mock in Jest? How do you mock a module or API call.

Mocking in Jest means replacing a real function, module, or API call with a fake implementation to control behavior in tests. It allows you to test components in isolation without relying on actual network calls or external dependencies.

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1️⃣ Why We Mock

1. Isolate Tests: Focus on the component logic, not external APIs
2. Control Responses: Simulate success, failure, or edge cases
3. Improve Test Performance: Avoid slow network or database calls
4. Avoid Side Effects: Prevent real HTTP requests or file system changes

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2️⃣ Mocking a Function

const fetchData = jest.fn(); // mock function

fetchData.mockReturnValue("Hello World");

console.log(fetchData()); // "Hello World"
expect(fetchData).toHaveBeenCalledTimes(0); // check call count

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3️⃣ Mocking an API Call (Module)

Example: Using axios

// api.js
import axios from "axios";
export const getUsers = () => axios.get("/users");

// api.test.js
import { getUsers } from "./api";
import axios from "axios";

jest.mock("axios"); // mock the axios module

test("fetch users", async () => {
  const users = [{ id: 1, name: "Alice" }];
  axios.get.mockResolvedValue({ data: users });

  const result = await getUsers();
  expect(result.data).toEqual(users);
  expect(axios.get).toHaveBeenCalledWith("/users");
});

Explanation:

• jest.mock("axios") → replaces axios with a mock
• mockResolvedValue → simulates a successful API response
• Can also use mockRejectedValue for errors

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4️⃣ Mocking Modules with Custom Implementation

jest.mock("./utils", () => ({
  generateId: jest.fn(() => "12345"),
}));

• Replaces the generateId function with a custom implementation

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⚡ Key Points:

• Mocking allows predictable and isolated tests
• Useful for functions, modules, timers, and network requests
• Use jest.fn(), jest.mock(), and mock return values

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In short:
We mock in Jest to control external dependencies, simulate different scenarios, and ensure tests run fast and reliably.