API Mocking for QA Testing

How do you test what happens when the payment gateway returns a 503? Or when the API takes 30 seconds to respond? With real APIs, you can't. With mocks, you control everything.

The Testing Challenge

QA engineers face a fundamental problem: real APIs don't fail on demand. You can't ask Stripe to return a "card declined" error. You can't make your database timeout for testing.

This leaves critical code paths untested:

• Error handling for 4xx and 5xx responses
• Timeout and retry logic
• Rate limiting behavior
• Partial failure scenarios
• Edge cases with unusual data

Common Test Scenarios

Example: Testing Error Scenarios

404 Not Found

{
  "error": "Not Found",
  "message": "User with ID 'nonexistent' does not exist",
  "code": "USER_NOT_FOUND"
}

500 Internal Server Error

{
  "error": "Internal Server Error",
  "message": "An unexpected error occurred",
  "requestId": "{{$randomUUID}}"
}

429 Rate Limited

{
  "error": "Too Many Requests",
  "message": "Rate limit exceeded. Please retry after 60 seconds.",
  "retryAfter": 60
}

Testing Timeouts

MockStation lets you add response delays to test timeout handling:

• 2 second delay: Test loading indicators
• 10 second delay: Test patience thresholds
• 30 second delay: Test timeout logic

Does your app retry? Show an error message? Give up gracefully? Now you can verify.

Testing with Cypress

Here's how to use MockStation endpoints in Cypress tests:

describe('Error Handling', () => {
  it('shows error message on 500', () => {
    // Use MockStation endpoint that returns 500
    cy.intercept('GET', '/api/users/*', {
      statusCode: 500,
      body: { error: 'Internal Server Error' }
    });

    cy.visit('/dashboard');
    cy.contains('Something went wrong').should('be.visible');
    cy.contains('Try again').should('be.visible');
  });

  it('handles timeout gracefully', () => {
    cy.intercept('GET', '/api/users/*', {
      delay: 35000, // 35 second delay
      body: { user: {} }
    });

    cy.visit('/dashboard');
    cy.contains('Request timed out', { timeout: 40000 });
  });
});

Testing with Playwright

import { test, expect } from '@playwright/test';

test('displays error on API failure', async ({ page }) => {
  // Route to MockStation error endpoint
  await page.route('**/api/users/**', route => {
    route.fulfill({
      status: 500,
      body: JSON.stringify({ error: 'Server Error' })
    });
  });

  await page.goto('/dashboard');
  await expect(page.locator('.error-message')).toBeVisible();
});

Edge Case Testing

Use Faker to generate edge case data:

• Empty arrays: Test empty state UI
• Very long strings: Test text overflow
• Special characters: Test encoding
• Null values: Test null handling
• Unicode: Test internationalization

{
  "user": {
    "name": "Müller-Schmidt O'Brien III",
    "bio": "{{$faker.lorem.paragraphs 5}}",
    "website": null,
    "followers": 0,
    "posts": []
  }
}

Deterministic Testing

Unlike real APIs, mocks are deterministic. The same request always returns the same response structure. This makes tests:

• Reproducible: Same result every time
• Fast: No network latency to real servers
• Reliable: No external dependencies
• Debuggable: You control every variable

Get Started

Start testing edge cases today:

1. Create a free account
2. Create mock endpoints for success and error scenarios
3. Add response delays for timeout testing
4. Use mock URLs in your test suite