Sinon Calling a function passed as an argument to a constructor - node.js

Question

I have the following classes A and B,
class A {
listenResponse(port, callback) {
const server = new CustomServer(port, (error, response) => {
if (error) return callback(error);
callback(null, response);
});
}
}
class CustomServer {
constructor(port, callback) {
this.port = port;
this.server = http.createServer((request, response) => {
if(// condition) { return callback(error); }
callback(null, response);
});
}
}
How do I test the function passed to CustomServer constructor while unit testing class A? Any help is highly appreciated.

Answer 1

Here is the unit test solution using an additional module proxyquire:
a.js:
const CustomServer = require('./customServer');
class A {
listenResponse(port, callback) {
const server = new CustomServer(port, (error, response) => {
if (error) return callback(error);
callback(null, response);
});
}
}
module.exports = A;
customServer.js:
class CustomServer {
constructor(port, callback) {
this.port = port;
this.server = http.createServer((request, response) => {
callback(null, response);
});
}
}
module.exports = CustomServer;
a.test.js:
const sinon = require('sinon');
const proxyquire = require('proxyquire');
describe('60084056', () => {
it('should get response', () => {
const error = null;
const response = {};
const customServerStub = sinon.stub().yields(error, response);
const A = proxyquire('./a.js', {
'./customServer': customServerStub,
});
const a = new A();
const port = 3000;
const callback = sinon.stub();
a.listenResponse(port, callback);
sinon.assert.calledWithExactly(customServerStub, port, sinon.match.func);
sinon.assert.calledWithExactly(callback, null, response);
});
it('should handle error', () => {
const error = new Error('network');
const response = {};
const customServerStub = sinon.stub().yields(error, response);
const A = proxyquire('./a.js', {
'./customServer': customServerStub,
});
const a = new A();
const port = 3000;
const callback = sinon.stub();
a.listenResponse(port, callback);
sinon.assert.calledWithExactly(customServerStub, port, sinon.match.func);
sinon.assert.calledWithExactly(callback, error);
});
});
Unit test results with coverage report:
60084056
✓ should get response (1684ms)
✓ should handle error
2 passing (2s)
-----------------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
-----------------|---------|----------|---------|---------|-------------------
All files | 70 | 100 | 50 | 66.67 |
a.js | 100 | 100 | 100 | 100 |
customServer.js | 25 | 100 | 0 | 25 | 3-5
-----------------|---------|----------|---------|---------|-------------------
Source code: https://github.com/mrdulin/expressjs-research/tree/master/src/stackoverflow/60084056