How to mock Fastify plugin

Question

I am trying to write unit testing for fastify application which also has custom fastify plugin. Is there a way we can mock fastify plugin? I tried mocking using Jest and Sinon without much success.

Answer 1

Giorgios link to the file is broken, the mocks folder is now absent from the master branch. I dug the commit history to something around the time of his answer and I found a commit with the folder still there. I leave it here for those who will come in the future!

Answer 2

This is what works for me

Setup your plugin according to Fastify docs https://www.fastify.io/docs/latest/Reference/Plugins/

// establishDbConnection.ts
import fp from 'fastify-plugin';
import {FastifyInstance, FastifyPluginAsync} from 'fastify';
import { initDbConnection } from './myDbImpl';

const establishDbConnection: FastifyPluginAsync = async (fastify: FastifyInstance, opts) => {
  fastify.addHook('onReady', async () => {
    await initDbConnection()
  });
};
export default fp(establishDbConnection);

mock the plugin with jest, make sure you wrap the mock function in fp() so that Fastify recognizes it as a plugin.

// myTest.ts
import fp from 'fastify-plugin';
const mockPlugin = fp(async () => jest.fn());
jest.mock('../../../fastifyPlugin/establishDbConnection', (() => {
  return mockPlugin;
}));

Answer 3

Your question is a bit generic but if you are using Jest it must be enough for mocking a fastify plugin. You can take a look in this repo and more specifically this file . This is a mock file of fastify and you add the registered plugins and in the specific example addCustomHealthCheck and then in your test files you can just call jest.mock('fastify').

Answer 4

You do not give a specific use case and there are lot of reasons you might want to mock a plugin. The nature of the plugin to be mocked is important to giving a good answer. Because I don't know that specific information I will show how to mock a plugin that creates a decorator that stores data that can be retrieved with fastify.decorator-name. This is a common use case for plugins that connect to databases or store other widely needed variables.

In the below case, the goal is to test a query function that queries a db; a plugin stores the connection information via a fastify decorator. So, in order to unit test the query we specifically need to mock the client data for the connection.

First create an instance of fastify. Next, set up a mock to return the desired fake response. Then, instead of registering the component with fastify (which you could also do), simply decorate the required variables directly with mock information.

Here is the function to be tested. We need to mock a plugin for a database which creates a fastify decorator called db. Specifically, in the below case the function to be tested uses db.client:

const fastify = require("fastify")({ //this is here to gather logs
    logger: {
        level: "debug",
        file: "./logs/combined.log"
    }
});


const HOURS_FROM_LOADDATE = "12";     

const allDataQuery = `
SELECT * 
FROM todo_items 
WHERE a."LOAD_DATE" > current_date - interval $1 hour
`;
const queryAll = async (db) => {
    return await sendQuery(db, allDataQuery, [HOURS_FROM_LOADDATE]);
};


//send query to db and receive data
const sendQuery = async (db, query, queryParams) => {
    var res = {};
    try {
        const todo_items = await db.client.any(query, queryParams);
        res = todo_items;
    } catch (e) {
        fastify.log.error(e);
    }
    return res;
};

module.exports = {
    queryByAsv
};

Following is the test case. We will mock db.client from the db plugin:

 const { queryAll } = require("../src/query");
    
    const any = {
        any: jest.fn(() => {
            return "mock response";
        })
    };
    
    describe("should return db query", () => {
        beforeAll(async () => {
            // set up fastify for test instance      
            fastify_test = require("fastify")({
                logger: {
                    level: "debug",
                    file: "./logs/combined.log",
                    prettyPrint: true
                }
            });
    
    
            
        });

        test("test Query All", async () => {
            // mock client
            const clientPromise = {
                client: any
            };
            //
            fastify_test.decorate("db", clientPromise);
            const qAll = await queryAll(fastify_test.db);
            expect(qAll).toEqual("mock response");
        });
        
    
    
    
    
    
        
    });