Mocking Inherited Classes with Jest - node.js

Question

I am extending net.Socket. In doing so, I am overriding the connect method as follows.
class ENIP extends Socket {
constructor() {
super();
this.state = {
session: { id: null, establishing: false, established: false },
error: { code: null, msg: null }
};
}
connect(IP_ADDR) {
const { registerSession } = encapsulation; //returns a buffer to send
this.state.session.establishing = true;
return new Promise(resolve => {
super.connect(EIP_PORT, IP_ADDR, () => { // This is what i want to mock -> super.connect
this.state.session.establishing = false;
this.write(registerSession());
resolve();
});
});
}
}
I want to mock the underlying Socket class so that I can simulate super.connect. Having viewed Facebook's docs on the matter, I am unsure on how to proceed with developing tests for this class as all methods will extend super.someMethodToMock. Does anyone know an approach I should take? Please let me know if there are any clarifying details I can provide.

Answer 1

You can use jest.spyOn(object, methodName) to create mock methods on Socket.prototype.
E.g.
index.js:
import { Socket } from 'net';
const EIP_PORT = 3000;
const encapsulation = {
registerSession() {
return 'session';
},
};
export class ENIP extends Socket {
constructor() {
super();
this.state = {
session: { id: null, establishing: false, established: false },
error: { code: null, msg: null },
};
}
connect(IP_ADDR) {
const { registerSession } = encapsulation;
this.state.session.establishing = true;
return new Promise((resolve) => {
super.connect(EIP_PORT, IP_ADDR, () => {
this.state.session.establishing = false;
this.write(registerSession());
resolve();
});
});
}
}
index.test.js:
import { ENIP } from './';
import { Socket } from 'net';
describe('ENIP', () => {
afterAll(() => {
jest.restoreAllMocks();
});
describe('#connect', () => {
it('should pass', async () => {
const writeSpy = jest.spyOn(Socket.prototype, 'write').mockImplementation();
const connectSpy = jest.spyOn(Socket.prototype, 'connect').mockImplementationOnce((port, addr, callback) => {
callback();
});
const enip = new ENIP();
await enip.connect('localhost');
expect(writeSpy).toBeCalledWith('session');
expect(connectSpy).toBeCalledWith(3000, 'localhost', expect.any(Function));
});
});
});
unit test result with coverage report:
PASS src/stackoverflow/48888509/index.test.jsx (10.43s)
ENIP
#connect
✓ should pass (7ms)
-----------|----------|----------|----------|----------|-------------------|
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s |
-----------|----------|----------|----------|----------|-------------------|
All files | 100 | 100 | 100 | 100 | |
index.jsx | 100 | 100 | 100 | 100 | |
-----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 12.357s
source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/48888509