how to write test for socket emit with acknowledge function - socket.io

Question

I have a socket emit function which has acknowledge function as
socket.emit('ferret', 'tobi', (data) => {
const a = 5;
this.sum(a,data);
});
I want to have a test that covers the line const a = 5; and this.sum(a,data);

Answer 1

In order to testing easily, you should make a little refactor about your code like this:
index.ts:
import { socket } from './socket';
export const obj = {
someMethod() {
this.handleFerret = this.handleFerret.bind(this);
socket.emit('ferret', 'tobi', this.handleFerret);
},
handleFerret(data) {
const a = 5;
this.sum(a, data);
},
sum(a, b) {
//
}
};
socket.ts: simulate socket.io module, you can replace it with socket.io
export const socket = {
emit(event, data, callback) {
// real implemetation
}
};
index.spec.ts:
import { obj } from './';
import { socket } from './socket';
describe('obj', () => {
it('should emit event and execute callback correctly', () => {
const emitSpy = jest.spyOn(socket, 'emit');
obj.someMethod();
expect(emitSpy).toBeCalledWith('ferret', 'tobi', obj.handleFerret);
});
it('should handle ferret correctly', () => {
const sumSpy = jest.spyOn(obj, 'sum');
obj.handleFerret(2);
expect(sumSpy).toBeCalledWith(5, 2);
});
});
Unit test result with 100% coverage report:
PASS src/stackoverflow/57461614/index.spec.ts (12.977s)
obj
✓ should emit event and execute callback correctly (13ms)
✓ should handle ferret correctly (1ms)
-----------|----------|----------|----------|----------|-------------------|
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s |
-----------|----------|----------|----------|----------|-------------------|
All files | 100 | 100 | 100 | 100 | |
index.ts | 100 | 100 | 100 | 100 | |
socket.ts | 100 | 100 | 100 | 100 | |
-----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests: 2 passed, 2 total
Snapshots: 0 total
Time: 14.159s
Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/57461614