express-dummy-server

Dummy server used for integration testing built on top of express.js.

Spins up an express server on a random available port and provides basic utility functions to easy testing against a real HTTP server.

npm install express-dummy-server

Usage

It's easier to create a standalone file that will have the dummy http service which can be re-used with different values across tests.

dummy-user-server.ts

import {createDummyServer, DummyServer, RequestSnapshot} from "express-dummy-server";
import {RequestHandler} from "express";

// Enum is used for mapping handlers so that we can change handlers later on
export enum DummyUserServiceHook {
    getUser = 'getUser'
}

export const createDummyUserService = async (
    requestHooks?: Map<DummyUserServiceHook, RequestHandler | undefined>
): Promise<DummyServer> =>
    createDummyServer(async (app) => {
        app.get('/users/:id', (req, res, next) => {
            const {id} = req.params;
            // Here we check if there is a new mapped handler that we should use
            const getUserHook = requestHooks?.get(DummyUserServiceHook.getUser);
            if (getUserHook) {
                return getUserHook(req, res, next)
            }
            res.json({id: Number(id), name: 'John'})
        });
    });

import axios from "axios";
import {RequestSnapshot} from "express-dummy-server";
import {RequestHandler} from "express";
import {DummyUserServiceHook} from './dummy-user-server';


describe('DummyServer', () => {
    let server: DummyServer;
    let hooks = new Map<DummyUserServiceHook, RequestHandler | undefined>();

    beforeAll(async () => {
        server = await createDummyUserService(hooks);
    });

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

    beforeEach(() => {
       server.requestStore.clear();
       hooks.clear();
    });

    it('should return a dummy server response', async () => {
        const res = await axios.get(`${server.url}/users/2`);
        expect(res.data).toEqual({id: 2, name: 'John'});

        const requestSnapshots = server.requestStore.get('/users/2');
        expect(requestSnapshots).toHaveLength(1);
        const {body, headers, method, params, query} = (requestSnapshots as RequestSnapshot[])[0];

        expect(method).toBe('GET');
        expect(body).toEqual({});
    });
})

Mocking during tests

When we want to have a different behaviour for a different test, we can use the hook to select the handler and re-map it's functionality. Note that these hooks need to be created in the dummy server first.

it('should return a mocked dummy server response', async () => {
    hooks.set(DummyUserServiceHook.getUser, (req, res) => {
        const {id} = req.params;
        res.json({id: Number(id), name: 'Hello Mike'});
    })
    const res = await axios.get(`${server.url}/users/2`);
    expect(res.data).toEqual({id: 2, name: 'Hello Mike'})

    const requestSnapshots = server.requestStore.get('/users/2');
    expect(requestSnapshots).toHaveLength(1);
    const {body, headers, method, params, query} = (requestSnapshots as RequestSnapshot[])[0];
    expect(method).toBe('GET');
    expect(body).toEqual({});
});

This allows us high level of flexibility especially since express is familiar to everyone.

Utilities

There are some utility functions in the library that speed up writing certain hooks, like:

import {respondJson} from "./request-handler-utils";

// respondJson
hooks.set(DummyUserServiceHook.getUser, respondJson({id: Number(id), name: 'Hello Mike'}));

// Which is identical to
hooks.set(DummyUserServiceHook.getUser, (req, res) => {
    const {id} = req.params;
    res.json({id: Number(id), name: 'Hello Mike'});
})

Debugging

If you want to debug requests and responses made against the server you can turn on the debug mode to console log them and (if needed) pass the logger to it.

import {DummyServerOptions, createDummyServer} from "express-dummy-server";

const options: DummyServerOptions = {debug: true};

const dummyUserServer = await createDummyServer(async (app) => {
    app.get('/users/:id', respondJson({id: 1, name: 'John'}));
}, options);