We can unit test Express.js route handler functions using a mocking library called node-mocks-http

Let’s say we’ve a simple express app

// index.js
const express = require("express");
const exampleRouter = require("./example-router");
const app = express();

app.use("/example", exampleRouter);

app.listen(3000);

With route handler defined separately as

// example-router.js
function exampleRouteHandler(req, res) {
  res.send("hello world!");
}

module.exports = exampleRouteHandler;

For unit testing, we should be able to pass various inputs and see if we get correct outputs. Here, we should be able to pass valid request (req) and response (res) objects as inputs and since this function doesn’t return anything, we should be able to make assertions on the response object (res).

We can do this by using node-mocks-http’s createRequest and createResponse apis.

Let’s write a simple test for this using mocha

// example-router.test.js
const assert = require("assert");
const httpMocks = require("node-mocks-http");
const exampleRouteHandler = require("./example-router");

describe("Example Router", () => {

  it("should return 'hello world' for GET /example", () => {
    const mockRequest = httpMocks.createRequest({
      method: "GET",
      url: "/example"
    });
    const mockResponse = httpMocks.createResponse();

    exampleRouteHandler(mockRequest, mockResponse);

    const actualResponseBody = mockResponse._getData();
    const expectedResponseBody = "hello world!";
    assert(actualResponseBody, expectedResponseBody);
  });

});

Checkout the node-mocks-http repo for more info.