How to Test Middleware in Express: Simple Guide

To test middleware in Express, you can call the middleware function directly with mock req, res, and next objects or use a testing library like Supertest to simulate HTTP requests. This lets you verify middleware behavior such as modifying requests, sending responses, or calling next() properly.

Syntax

An Express middleware function has the signature (req, res, next). It receives the request and response objects, and a next function to pass control to the next middleware.

• req: The HTTP request object.
• res: The HTTP response object.
• next: A function to call the next middleware.

You test middleware by calling it with mock versions of these objects.

javascript

function middleware(req, res, next) {
  // Middleware logic here
  next();
}

Example

This example shows how to test a simple middleware that adds a property to req. We use Jest for assertions and mock req, res, and next.

javascript

const middleware = (req, res, next) => {
  req.customProperty = 'testValue';
  next();
};

// Mock objects
const req = {};
const res = {};
const next = jest.fn();

// Run middleware
middleware(req, res, next);

// Test results
console.log(req.customProperty); // 'testValue'
console.log(next.mock.calls.length); // 1

Output

testValue
1

Common Pitfalls

Common mistakes when testing middleware include:

• Not mocking next properly, so you can't verify if it was called.
• Forgetting to mock req or res properties your middleware uses.
• Not handling asynchronous middleware correctly by missing async/await or callbacks.

Always ensure mocks match what your middleware expects.

javascript

/* Wrong: next is not mocked, so you can't check if it was called */
const middleware = (req, res, next) => {
  next();
};

// middleware({}, {}, undefined); // This will throw an error if next is undefined

/* Right: next is a jest.fn() mock to track calls */
const next = jest.fn();
middleware({}, {}, next);
console.log(next.mock.calls.length); // 1

Output

1

Quick Reference

Tips for testing Express middleware:

• Mock req, res, and next objects based on middleware needs.
• Use jest.fn() or similar to spy on next calls.
• For middleware that sends responses, mock res.send or res.status to verify output.
• Use Supertest to test middleware in the context of full HTTP requests.
• Handle async middleware with async/await or done callbacks.

Key Takeaways

Test middleware by calling it with mock req, res, and next objects.

Always mock next as a function to verify it is called.

Use Supertest for integration-style tests with HTTP requests.

Mock only the parts of req and res your middleware uses.

Handle async middleware with async/await or callbacks.