🧪 Mocking APIs and Network Requests

🔹 What is API Mocking?

Mocking an API means simulating the behavior of a real API so that you can test your application without making actual network requests. This helps in development and testing by:

Avoiding reliance on unstable or unavailable external services

Controlling response data (success, error, delay)

Speeding up tests and reducing costs

🔹 Why Mock APIs?

Reason Benefit

API not ready Begin frontend/backend development early

Faster tests No delays from real network calls

More control Simulate edge cases, errors, timeouts

Isolate system components Focus on logic, not API behavior

🔹 Common Use Cases

Frontend Testing (e.g., React, Vue)

Unit Tests for Services

Integration Testing

CI/CD pipelines

Offline development

🔹 How to Mock APIs: Tools and Approaches

✅ 1. Frontend (JavaScript/TypeScript)

a. Using fetch-mock or msw (Mock Service Worker)

js

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import { setupServer } from 'msw/node';

import { rest } from 'msw';

const server = setupServer(

  rest.get('/api/user', (req, res, ctx) => {

    return res(ctx.status(200), ctx.json({ name: 'John Doe' }));

  })

);

beforeAll(() => server.listen());

afterEach(() => server.resetHandlers());

afterAll(() => server.close());

b. Axios Interceptors

js

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axios.interceptors.request.use(config => {

  if (config.url === '/api/user') {

    return Promise.resolve({ data: { name: 'Mock User' } });

  }

  return config;

});

✅ 2. Python

a. Using unittest.mock or responses library

python

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import requests

import responses

@responses.activate

def test_mock_api():

    responses.add(

        responses.GET, 'https://api.example.com/user',

        json={'name': 'Alice'}, status=200

    )

    response = requests.get('https://api.example.com/user')

    assert response.json()['name'] == 'Alice'

✅ 3. Node.js (Backend Testing)

a. Using nock

js

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const nock = require('nock');

nock('https://api.example.com')

  .get('/user')

  .reply(200, { name: 'Jane Doe' });

✅ 4. Postman Mock Server

Create a Mock Server in Postman.

Define endpoints and sample responses.

Use the mock URL in your application instead of the real API.

🔹 Best Practices

Practice Description

Use realistic data Mimic actual API responses

Test both success and error Include 404, 500, timeouts, etc.

Isolate test environment Don’t rely on real API calls in tests

Keep mocks versioned Sync with real API contracts/schema

🔹 Summary

Mocking APIs and network requests helps you:

Develop independently of real APIs

Write fast and reliable tests

Simulate all kinds of scenarios (good and bad)

🧰 Tools vary based on your stack:

Frontend: MSW, fetch-mock, MirageJS

Backend: nock, responses, unittest.mock

General: Postman, WireMock

Would you like help setting up a mock server for your current project, or generating mock API documentation with OpenAPI?

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