const supertest = require('supertest')

it('GET /backend/path/to test', async () => {
    jest.resetModules();
    jest.doMock('../path/to/someServerModule', () => {
        // ../path/to/someServerModule mock used by ./path/to/my/server
        return {
            // someServerModule logic here ...
        };
    });
    // it is necessary to reload ./path/to/my/server module after mocks are defined
    const app = require('./path/to/my/server');
    const requester = supertest(app);
    const response = await requester.get('/backend/path/to'); // by ./path/to/my/server
    // response.body
});

Note: check GitHub repository.

Project details

Project structure:

.
├── node_modules
├── package.json
├── package-lock.json
└── src
    ├── db.js
    ├── index.js
    ├── server.js
    └── test
        └── index.test.js

src/test/index.test.js file:

const supertest = require('supertest');

const createRequester = () => {
    // it is necessary to reload ../server module after mocks are defined
    const app = require('../server');
    return supertest(app);
};

describe('users api', () => {
    // always we want to use default defined modules as initial state
    beforeEach(() => jest.resetModules());

    it('getting users', async () => {
        jest.doMock('../db', () => {
            // ../db mock
            return {
                getUsers: () => ['mocked john', 'mocked ann', 'mocked matt']
            };
        });
        // we need to create requester after mock is defined
        const requester = createRequester();
        const users = await requester.get('/backend/users');
        expect(users.body[0]).toBe('mocked john');
        expect(users.body[1]).toBe('mocked ann');
        expect(users.body[2]).toBe('mocked matt');
    });
})

src/db.js file: <--- this module was reloaded in the test with mock

exports.getUsers = () => ['john', 'ann', 'matt'];

src/server.js file:

const express = require('express');
const db = require('./db');

const app = express();

app.get('/backend/users', async (req, res) => {
    const users = db.getUsers()
    res.json(users);
});

module.exports = app;

src/index.js file:

const app = require('./server');

app.listen(3000);

package.json file:

{
  "name": "jest-test",
  "version": "1.0.0",
  "description": "",
  "main": "src/index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.17.1"
  },
  "devDependencies": {
    "jest": "^26.6.3",
    "supertest": "^6.1.3"
  }
}

Running tests

Run the following command indicating test name:

node 'node_modules/.bin/jest' 'src/test/index.test.js' -t 'some feature'

Note: to install Jest, run in project directory: npm install --save-dev jest.

Example output:

 PASS  src/test/index.test.js
  users api
    ✓ getting users (2546 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        4.374 s, estimated 30 s
Ran all test suites matching /\/home\/john\/Desktop\/jest-test\/src\/test\/index.test.js/i with tests matching "users api".
Waiting for the debugger to disconnect...

Running server

In this section, we would like to show when server is run in production mode.

Execute in the command line:

node src/index.js

JavaScript - express server Rest API mock with supertest and Jest

Project details

Running tests

Running server

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