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how to create instance of class with jest

I am a beginner in writing tests and in jest also. I want to test that this function to be called and return the success promise. I write a unit test for function using jest. The function gets like parameter instance of class that I can't create because its constructor requires parameters that I haven't access. I have a lot of functions that have Session like parametrs. How can test function when you cant provide parametrs for it? Can I mock instance of class or function and handle it without parameter?
async initFlow(session: Session) {
const nextAtomId = session.userInput.getParam('NEXT_ATOM');
if (nextAtomId) {
const nextAtom = await AtomManager.findActiveAtom(nextAtomId);
if (!session.features.useTerms || ['beforeTerms', 'TermsAndConditions'].includes(nextAtom.type)) {
return AtomProcessor.processAtom(session, nextAtom);
}
}
const start = await AtomManager.getStartAtom(`${session.botId}`);
if (!start) {
throw new Error('Could not find start atom');
}
session.user = await UserManager.getGlobalUser(session); // getGlobalUser makes initUser under the hood.
return AtomProcessor.processAtom(session, start);
}

You can mock both AtomManager & UserManager and provide a mock session object when calling initFlow.
jest.mock("./path/to/AtomManager");
jest.mock("./path/to/UserManager");
it("works", async () => {
const mockSession = {
userInput: {
getParam: jest.fn(),
},
botId: "123",
};
const mockUser = "user123";
const mockStartAtom = "atom123";
AtomManager.getStartAtom.mockResolveValue(mockStartAtom);
UserManager.getGlobalUser.mockResolveValue(mockUser);
await initFlow(mockSession);
expect(mockSession.user).toBe(mockUser);
expect(AtomManager.getStartAtom).toHaveBeenCalledTimes(1);
expect(AtomManager.getStartAtom).toHaveBeenCalledWith(mockSession.botId);
expect(UserManager.getGlobalUser).toHaveBeenCalledTimes(1);
expect(UserManager.getGlobalUser).toHaveBeenCalledWith(mockSession);
expect(AtomProcessor.processAtom).toHaveBeenCalledTimes(1);
expect(AtomProcessor.processAtom).toHaveBeenCalledWith(mockSession, mockStartAtom);
});
The snippet above makes the following assertions:
AtomManager.getStartAtom is called once and it's called with the mock botId.
UserManager.getGlobalUser is called once and it's called with the mock session object.
UserManager.getGlobalUser has successfully added the user property on the passed session object.
AtomProcessor.processAtom is called once and it's called with the mock session and the mock start atom.
You can similarly the test other branches of code.

How to stub a libary function in JavaScript

For example, if I have main.js calling a defined in src/lib/a.js, and function a calls node-uuid.v1, how can I stub node-uuid.v1 when testing main.js?
main.js
const a = require("./src/lib/a").a
const main = () => {
return a()
}
module.exports = main
src/lib/a.js
const generateUUID = require("node-uuid").v1
const a = () => {
let temp = generateUUID()
return temp
}
module.exports = {
a
}
tests/main-test.js
const assert = require("assert")
const main = require("../main")
const sinon = require("sinon")
const uuid = require("node-uuid")
describe('main', () => {
it('should return a newly generated uuid', () => {
sinon.stub(uuid, "v1").returns("121321")
assert.equal(main(), "121321")
})
})
The sinon.stub(...) statement doesn't stub uuid.v1 for src/lib/a.js as the above test fails.
Is there a way to globally a library function so that it does the specified behavior whenever it gets called?

You should configure the stub before importing the main module. In this way the module will call the stub instead of the original function.
const assert = require("assert")
const sinon = require("sinon")
const uuid = require("node-uuid")
describe('main', () => {
it('should return a newly generated uuid', () => {
sinon.stub(uuid, "v1").returns("121321")
const main = require("../main")
assert.equal(main(), "121321")
})
})

Bear in mind that node-uuid is deprecated as you can see by this warning
[Deprecation warning: The use of require('uuid') is deprecated and
will not be supported after version 3.x of this module. Instead, use
require('uuid/[v1|v3|v4|v5]') as shown in the examples below.]
About how to stub that for testing would be a bit more harder than before as actually there is no an easy way to mock a standalone function using sinon
Creating a custom module
//custom uuid
module.exports.v1 = require('uuid/v1');
Requiring uuid from the custom module in your project
const uuid = require('<path_to_custom_module>');
Sinon.stub(uuid, 'v1').returns('12345');

How to mock pg Pool with Sinon

In a previous project I mocked the mysql library with Sinon. I did this like so:
X.js:
const con = mysql.createPool(config.mysql);
...
Some other place in the project:
const rows = await con.query(query, inserts);
...
X.test.js:
const sinon = require('sinon');
const mockMysql = sinon.mock(require('mysql'));
...
mockMysql.expects('createPool').returns({
query: () => {
// Handles the query...
},
...
It worked perfectly.
In another project I am trying to mock pg, again with Sinon.
pool.js:
const { Pool } = require('pg');
const config = require('#blabla/config');
const pool = new Pool(config.get('database'));
module.exports = pool;
Some other place in the project:
const con = await pool.connect();
const result = await con.query(...
Y.test.js:
???
I can't understand how to mock connect().query(). None of the following approaches work:
1:
const { Pool } = require('pg');
const config = require('#blabla/config');
const mockPool = sinon.mock(new Pool(config.get('database')));
...
mockPool.expects('connect').returns({
query: () => {
console.log('query here');
},
});
1 results in no error but the real db connection is used.
2:
const { Pool } = sinon.mock(require('pg'));
const config = require('#blabla/config');
const pool = new Pool(config.get('database'));
pool.expects('connect').returns({
query: () => {
console.log('query here');
},
});
2 => TypeError: Pool is not a constructor
3:
const { Pool } = sinon.mock(require('pg'));
const config = require('#blabla/config');
const pool = sinon.createStubInstance(Pool);
pool.connect.returns({
query: () => {
console.log('query here');
},
});
3 => TypeError: The constructor should be a function.
Can anybody point me in the right direction with how to mock my PostgreSQL connection?

Example: I have postgres.js like this.
const { Pool } = require('pg');
const handler = {
count: async (pgQuery) => {
try {
const pool = new Pool();
const res = await pool.query(pgQuery);
return { count: parseInt(res.rows[0].counter, 10) };
} catch (error) {
// Log/Throw error here.
}
return false;
}
}
module.exports = handler;
The spec test I created on postgres.spec.js is like this.
const { expect } = require('chai');
const sinon = require('sinon');
const pgPool = require('pg-pool');
const handler = require('postgres.js');
describe('Postgres', function () {
it('should have method count that bla bla', async function () {
// Create stub pgPool query.
const postgreeStubQuery = sinon.stub(pgPool.prototype, 'query');
postgreeStubQuery.onFirstCall().throws('XXX');
postgreeStubQuery.onSecondCall().resolves({
rows: [{ counter: 11 }],
});
// Catch case.
const catcher = await handler.count('SELECT COUNT()..');
expect(catcher).to.equal(false);
expect(postgreeStubQuery.calledOnce).to.equal(true);
// Correct case.
const correct = await handler.count('SELECT COUNT()..');
expect(correct).to.deep.equal({ count: 11 });
expect(postgreeStubQuery.calledTwice).to.equal(true);
// Restore stub.
postgreeStubQuery.restore();
});
});
To stub pool.query(), you need to stub pg-pool prototype and method query.
Hope this helps.

Since you're needing to mock the returned results of a query, I think the easiest solution would be to abstract your database from the the code needing the query results. Example being, your query results are returning information about a person. Create a person.js module with specific methods for interacting with the database.
Your other code needing the person information from the database won't know or care what type of database you use or how you connect to it, all they care to know is what methods are exposed from person.js when they require it.
//person.js
const { Pool } = require('pg')
// do other database connection things here
const getPersonById = function (id) {
// use your query here and return the results
}
module.exports = { getPersonById }
Now in your tests, you mock the person module, not the pg module. Imagine if you had 20 some odd tests that all had the mock MySQL pool set up then you changed to pg, you'd have to change all of those, nightmare. But by abstracting your database connection type/setup, it makes testing much easier, because now you just need to stub/mock your person.js module.
const person = require('../person.js') //or whatever relative file path it's in
const sinon = require('sinon')
describe('person.js', function () {
it('is stubbed right now', function () {
const personStub = sinon.stub(person)
personStub.getPersonById.returns('yup')
expect(personStub.getPersonById()).to.eq('yup')
})
})

Below is a simpler approach that means the system-under-test doesn't need any special tricks.
It is comprised of two parts, though the first is "nice to have":
Use a DI framework to inject the pg.Pool. This is a better approach IMO anyway, and fits really well with testing.
In the beforeEach() of the tests, configure the DI framework to use a mock class with sinon.stub instances.
If you aren't using a DI framework, pass the mock as a Pool parameter... but DI is better ;)
The code below is TypeScript using tsyringe, but similar approaches will work fine with plain JavaScript etc.
Somewhere you'll have code that uses pg.Pool. A contrived example:
import { Pool } from 'pg'
...
function getPets(pool: Pool): Promise<Pet[]> {
return pool.connect()
.then(db => db.query(SQL_HERE)
.then(result => {
db.release()
return result.rows // or result.rows.map(something) etc
})
.catch(error => {
db.release()
throw error
})
)
}
That works, and it's fine if you want to pass the Pool instance in. I'd prefer not to, so I use tsyringe like this:
import { container } from 'tsyringe'
...
function getPets(): Promise<Pet[]> {
return container.resolve<Pool>().connect()
.then(...)
}
Exactly the same outcome, but getPets() is cleaner to call - it can be a pain to lug around a Pool instance.
The main of the program would set up an instance in one of a few ways. Here's mine:
...
container.register(Pool, {
useFactory: instanceCachingFactory(() => {
return new Pool(/* any config here */)
})
})
The beauty of this comes out in tests.
The code above (the "system under test") needs a Pool instance, and that instance needs a connect() method that resolves to a class with query() and release() methods.
This is what I used:
class MockPool {
client = {
query: sinon.stub(),
release: sinon.stub()
}
connect () {
return Promise.resolve(this.client)
}
}
Here's the setup of a test using MockPool:
describe('proof', () => {
let mockPool: MockPool
beforeEach(() => {
// Important! See:
// https://github.com/microsoft/tsyringe#clearing-instances
container.clearInstances()
mockPool = new MockPool()
container.registerInstance(Pool, mockPool as unknown as Pool)
})
})
The cast through unknown to Pool is needed because I'm not implementing the whole Pool API, just what I need.
Here's what a test looks like:
it('mocks postgres', async () => {
mockPool.client.query.resolves({
rows: [
{name: 'Woof', kind: 'Dog'},
{name: 'Meow', kind: 'Cat'}
]
})
const r = await getPets()
expect(r).to.deep.equal([
{name: 'Woof', kind: 'Dog'},
{name: 'Meow', kind: Cat'}
])
})
You can easily control what data the mock Postgres Pool returns, or throw errors, etc.

How to connect to mongoDB and test for drop collection?

This is how I connect to a mongoDB using monk(). I'll store it in state.
Assume we want to drop some collections, we call dropDB.
db.js
var state = {
db: null
}
export function connection () {
if (state.db) return
state.db = monk('mongdb://localhost:27017/db')
return state.db
}
export async function dropDB () {
var db = state.db
if (!db) throw Error('Missing database connection')
const Users = db.get('users')
const Content = db.get('content')
await Users.remove({})
await Content.remove({})
}
I'm not quite sure if it is a good approach to use state variable. Maybe someone can comment on that or show an improvement.
Now I want to write a unit test for this function using JestJS:
db.test.js
import monk from 'monk'
import { connection, dropDB } from './db'
jest.mock('monk')
describe('dropDB()', () => {
test('should throw error if db connection is missing', async () => {
expect.assertions(1)
await expect(dropDB()).rejects.toEqual(Error('Missing database connection'))
})
})
This part is easy, but the next part gives me two problems:
How do I mock the remove() methods?
test('should call remove() methods', async () => {
connection() // should set `state.db`, but doesn't work
const remove = jest.fn(() => Promise.resolve({ n: 1, nRemoved: 1, ok: 1 }))
// How do I use this mocked remove()?
expect(remove).toHaveBeenCalledTimes(2)
})
And before that? How do I setup state.db?
Update
As explained by poke the global variable makes the problem. So I switched to a class:
db.js
export class Db {
constructor() {
this.connection = monk('mongdb://localhost:27017/db');
}
async dropDB() {
const Users = this.connection.get('users');
const Content = this.connection.get('content');
await Users.remove({});
await Content.remove({});
}
}
which results in this test file:
db.test.js
import { Db } from './db'
jest.mock('./db')
let db
let remove
describe('DB class', () => {
beforeAll(() => {
const remove = jest.fn(() => Promise.resolve({ n: 1, nRemoved: 1, ok: 1 }))
Db.mockImplementation(() => {
return { dropDB: () => {
// Define this.connection.get() and use remove as a result of it
} }
})
})
describe('dropDB()', () => {
test('should call remove method', () => {
db = new Db()
db.dropDB()
expect(remove).toHaveBeenCalledTimes(2)
})
})
})
How do I mock out any this elements? In this case I need to mock this.connection.get()

Having a global state is definitely the source of your problem here. I would suggest to look for a solution that does not involve global variables at all. As per Global Variables Are Bad, global variables cause tight coupling and make things difficult to test (as you have noticed yourself).
A better solution would be to either pass the database connection explicitly to the dropDB function, so it has the connection as an explicit dependency, or to introduce some stateful object that holds onto the connection and offers the dropDB as a method.
The first option would look like this:
export function openConnection() {
return monk('mongdb://localhost:27017/db');
}
export async function dropDB(connection) {
if (!connection) {
throw Error('Missing database connection');
}
const Users = connection.get('users');
const Content = connection.get('content');
await Users.remove({});
await Content.remove({});
}
This would also make it very easy to test dropDB as you can now just pass a mocked object for it directly.
The other option could look like this:
export class Connection() {
constructor() {
this.connection = monk('mongdb://localhost:27017/db');
}
async dropDB() {
const Users = this.connection.get('users');
const Content = this.connection.get('content');
await Users.remove({});
await Content.remove({});
}
}
A test for the first option could look like this:
test('should call remove() methods', async () => {
const usersRemove = jest.fn().mockReturnValue(Promise.resolve(null));
const contentRemove = jest.fn().mockReturnValue(Promise.resolve(null));
const dbMock = {
get(type) {
if (type === 'users') {
return { remove: usersRemove };
}
else if (type === 'content') {
return { remove: contentRemove };
}
}
};
await dropDB(dbMock);
expect(usersRemove).toHaveBeenCalledTimes(1);
expect(contentRemove).toHaveBeenCalledTimes(1);
});
Basically, the dropDB function expects an object that has a get method which when called returns an object that has a remove method. So you just need to pass something that looks like that, so the function can call those remove methods.
For the class, this is a bit more complicated since the constructor has a dependency on the monk module. One way would be to make that dependency explicit again (just like in the first solution), and pass monk or some other factory there. But we can also use Jest’s manual mocks to simply mock the whole monk module.
Note that we do not want to mock the module containing our Connection type. We want to test that, so we need it in its un-mocked state.
To mock monk, we need to create a mock module of it at __mocks__/monk.js. The manual points out that this __mocks__ folder should be adjacent to the node_modules folder.
In that file, we simply export our custom monk function. This is pretty much the same we already used in the first example, since we only care about getting those remove methods in place:
export default function mockedMonk (url) {
return {
get(type) {
if (type === 'users') {
return { remove: mockedMonk.usersRemove };
}
else if (type === 'content') {
return { remove: mockedMonk.contentRemove };
}
}
};
};
Note that this refers to the functions as mockedMonk.usersRemove and mockedMonk.contentRemove. We’ll use this in the test to configure those function explicitly during the test execution.
Now, in the test function, we need to call jest.mock('monk') to enable Jest to mock the monk module with our mocked module. Then, we can just import it too and set our functions within the test. Basically, just like above:
import { Connection } from './db';
import monk from 'monk';
// enable mock
jest.mock('./monk');
test('should call remove() methods', async () => {
monk.usersRemove = jest.fn().mockReturnValue(Promise.resolve(null));
monk.contentRemove = jest.fn().mockReturnValue(Promise.resolve(null));
const connection = new Connection();
await connection.dropDB();
expect(monk.usersRemove).toHaveBeenCalledTimes(1);
expect(monk.contentRemove).toHaveBeenCalledTimes(1);
});

Mock a dependency's constructor Jest

I'm a newbie to Jest. I've managed to mock my own stuff, but seem to be stuck mocking a module. Specifically constructors.
usage.js
const AWS = require("aws-sdk")
cw = new AWS.CloudWatch({apiVersion: "2010-08-01"})
...
function myMetrics(params) {
cw.putMetricData(params, function(err, data){})
}
I'd like to do something like this in the tests.
const AWS = jest.mock("aws-sdk")
class FakeMetrics {
constructor() {}
putMetricData(foo,callback) {
callback(null, "yay!")
}
}
AWS.CloudWatch = jest.fn( (props) => new FakeMetrics())
However when I come to use it in usage.js the cw is a mockConstructor not a FakeMetrics
I realise that my approach might be 'less than idiomatic' so I'd be greatful for any pointers.
This is a minimal example https://github.com/ollyjshaw/jest_constructor_so
npm install -g jest
jest

Above answer works. However, after some time working with jest I would just use the mockImplementation functionality which is useful for mocking constructors.
Below code could be an example:
import * as AWS from 'aws-sdk';
jest.mock('aws-sdk', ()=> {
return {
CloudWatch : jest.fn().mockImplementation(() => { return {} })
}
});
test('AWS.CloudWatch is called', () => {
new AWS.CloudWatch();
expect(AWS.CloudWatch).toHaveBeenCalledTimes(1);
});
Note that in the example the new CloudWatch() just returns an empty object.

The problem is how a module is being mocked. As the reference states,
Mocks a module with an auto-mocked version when it is being required.
<...>
Returns the jest object for chaining.
AWS is not module object but jest object, and assigning AWS.CloudFormation will affect nothing.
Also, it's CloudWatch in one place and CloudFormation in another.
Testing framework doesn't require to reinvent mock functions, they are already there. It should be something like:
const AWS = require("aws-sdk");
const fakePutMetricData = jest.fn()
const FakeCloudWatch = jest.fn(() => ({
putMetricData: fakePutMetricData
}));
AWS.CloudWatch = FakeCloudWatch;
And asserted like:
expect(fakePutMetricData).toHaveBeenCalledTimes(1);

According to the documentation mockImplementation can also be used to mock class constructors:
// SomeClass.js
module.exports = class SomeClass {
method(a, b) {}
};
// OtherModule.test.js
jest.mock('./SomeClass'); // this happens automatically with automocking
const SomeClass = require('./SomeClass');
const mockMethod= jest.fn();
SomeClass.mockImplementation(() => {
return {
method: mockMethod,
};
});
const some = new SomeClass();
some.method('a', 'b');
console.log('Calls to method: ', mockMethod.mock.calls);
If your class constructor has parameters, you could pass jest.fn() as an argument (eg. const some = new SomeClass(jest.fn(), jest.fn());