Query Parameter Edge Case Testing

In this lesson, you will learn how to harden your API endpoints against malformed, malicious, or unexpected query string inputs that could crash your database queries or application logic.

The Problem: Beyond Happy Paths

When building search or filter functionality, we often test the "happy path" (e.g., ?category=books). However, query parameters are strings provided by the client, and they are inherently untrusted. An edge case occurs when parameters arrive in unexpected formats, such as arrays where a single value is expected, missing keys, or reserved characters that break your query builder.

Preventing Injection and Logic Crashes

Consider an endpoint that filters users by age: GET /users?age=25. A naive implementation might pass req.query.age directly into a database driver. What happens if a user sends ?age[$gt]=10 or ?age[0]=25? Depending on your parser (like qs vs querystring), your code might inject an object into a place expecting a string, potentially causing an Unhandled Promise Rejection or leaking database schema details.

You should implement test cases that specifically inject these "structural anomalies" to ensure your validation layer catches them before they reach the data layer:

// Example: Testing for prototype pollution or unexpected object injection
it('should reject non-string filter values', async () => {
  const response = await request(app)
    .get('/users')
    .query({ age: { $gt: 10 } }); // Malicious object injection

  expect(response.status).toBe(400);
});

it('should handle missing or empty query strings gracefully', async () => {
  const response = await request(app).get('/users?');
  expect(response.status).toBe(200);
  expect(response.body).toHaveProperty('data');
});

Strategy for Robustness

To effectively test these edge cases:

1. Type Coercion Tests: Ensure that numeric filters (like price or limit) fail gracefully when strings like "abc" or null are passed.
2. Boundary Testing: Verify how your application reacts to extreme lengths, such as ?q= followed by a 10,000-character string, which might trigger a Denial of Service (DoS) if your database doesn't have length limits on that column.
3. Array/Object Collisions: Test what happens when you send duplicate keys: ?sort=asc&sort=desc. Does your controller crash, or does it correctly pick the first/last value?

Key Takeaway: Treat all query parameters as untrusted input by explicitly testing how your application handles structural anomalies like unexpected objects, arrays, and extreme payloads.