nodejs Cannot stub a singleton in component test - node.js

Question

I am trying to stub the getDB function within the Connection class , which is a singleton.
When i am making a unit test on Class B it works fine.
Now i am trying to create a component test, which means using Class A and stub the getDB function again (Class A uses Class B which uses the Connection class - getDb).
But the stubbing doesn't work.
Not sure why.
Here are the classes:
Class Connection
"use strict";
const config = {};
const pgp = require("pg-promise")();
const db = pgp(config);
class Connection {
constructor() {
console.log("Connection ctor")
}
getDb() {
console.log("getDb")
return db;
}
}
module.exports = new Connection();
Class B
"use strict";
class B {
constructor() {
console.log("B ctor")
this.db = require("./Connection").getDb();
}
async getDataFromDb() {
console.log("B getDataFromDb")
let res = null;
res = await this.db.one("SELECT guid FROM table", ["NONE"]);
return res.guid;
}
}
module.exports = B;
Class A
"use strict";
const bDao = new (require("../db/B"))();
class A {
async getGuid() {
console.log("A getGuid")
let guid = await bDao.getDataFromDb();
console.log("got guid " + guid);
return guid;
}
}
module.exports = A;
The Test - the B_unit_test works ,
but when i try to stub the getDb from the connection class, it doesn't stub it and the component_test fails.
const chai = require("chai");
const expect = chai.expect;
const A = require("../src/service/A");
const Connection = require("../src/db/Connection");
const B = require("../src/db/B");
const sinon = require("sinon");
describe("component_test", function () {
before(() => {
var db = {
one: async () => {
return {
guid: '24f475b6-f9fa-495b-83f4-16f6b45b0a6a',
created_on: "123"
}
},
none: async () => {
}
};
this.dbStub = sinon.stub(Connection, "getDb").returns(db);
});
after(() => {
this.dbStub.restore();
});
it("B_unit_test", async function () {
var dao = new B();
var guid = await dao.getDataFromDb();
expect(guid).to.equal('24f475b6-f9fa-495b-83f4-16f6b45b0a6a');
});
it("A_component_test", async function () {
var dao = new A();
var guid = await dao.getGuid();
expect(guid).to.equal('24f475b6-f9fa-495b-83f4-16f6b45b0a6a');
});
});

Answer 1

You'd better stub getDataFromDb of B which is the class that A used directly. The unit test solution:
connection.js:
const config = {};
const pgp = require('pg-promise')();
const db = pgp(config);
class Connection {
constructor() {
console.log('Connection ctor');
}
getDb() {
console.log('getDb');
return db;
}
}
module.exports = new Connection();
b.js:
class B {
constructor() {
console.log('B ctor');
this.db = require('./connection').getDb();
}
async getDataFromDb() {
console.log('B getDataFromDb');
let res = null;
res = await this.db.one('SELECT guid FROM table', ['NONE']);
return res.guid;
}
}
module.exports = B;
b.test.js:
class B {
constructor() {
console.log('B ctor');
this.db = require('./connection').getDb();
}
async getDataFromDb() {
console.log('B getDataFromDb');
let res = null;
res = await this.db.one('SELECT guid FROM table', ['NONE']);
return res.guid;
}
}
module.exports = B;
a.js:
const bDao = new (require('./b'))();
class A {
async getGuid() {
console.log('A getGuid');
let guid = await bDao.getDataFromDb();
console.log('got guid ' + guid);
return guid;
}
}
module.exports = A;
a.test.js:
const B = require('./b');
const sinon = require('sinon');
const { expect } = require('chai');
describe('a', () => {
it('should pass', async () => {
const getDataFromDbStub = sinon.stub(B.prototype, 'getDataFromDb').resolves('24f475b6-f9fa-495b-83f4-16f6b45b0a6a');
const A = require('./a');
const aDao = new A();
const guid = await aDao.getGuid();
expect(guid).to.be.equal('24f475b6-f9fa-495b-83f4-16f6b45b0a6a');
sinon.assert.calledOnce(getDataFromDbStub);
});
});
unit test result with coverage report:
a
B ctor
getDb
A getGuid
got guid 24f475b6-f9fa-495b-83f4-16f6b45b0a6a
✓ should pass (47ms)
b
B ctor
B getDataFromDb
✓ should pass
2 passing (65ms)
---------------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
---------------|---------|----------|---------|---------|-------------------
All files | 100 | 100 | 100 | 100 |
a.js | 100 | 100 | 100 | 100 |
b.js | 100 | 100 | 100 | 100 |
connection.js | 100 | 100 | 100 | 100 |
---------------|---------|----------|---------|---------|-------------------