Jest: mocking dependencies that are passed in to constructor - node.js

Question

I'm stuck trying to mock a class that is passed in through constructor and all the examples I've encountered so far have initialized dependencies inside constructors.
From my understanding jest replaces such dependencies with its overridden constructor but because I'm passing in the dependencies myself I'm in need of a passable instance when initialising UnderTest.
For what it's worth I'm ideally looking for a Mockito-ish behaviour, something along the lines of.
const mockedDependency = ???
const underTest = new UnderTest(mockedDependency)
...
Proceed to write tests for underTest
This is the code I'm trying to test. Please assume that MockedDependency has its own dependencies, passed in to constructor as well.
export default class UnderTest {
private mockedDependency : MockedDependency
constructor(mockedDependency: MockedDependency) {
this.mockedDependency = mockedDependency
}
public methodUnderTest(parameter: string) {
const mockedResult = this.mockedDependency.returnSomething(parameter)
return this.doSomethingElse(mockedResult)
}
public methodUnderTest2(parameter1: string, parameter2: string) {
const mockedResult = this.mockedDependency.returnSomething2(parameter1, parameter2)
return this.doSomethingElse(mockedResult)
}
private doSomethingElse(mockedResult: string) {
return mockedResult
}
}
How would you go about unit testing UnderTest class?
Bonus points for a way to set the result of mockedDependency methods either in each test or by input.
Edit:
A possible solution to those who stumble upon the same issue:
It's possible to cast variable as the desired object and override the methods like so:
const mockedDependency = {
returnSomething(parameter: string) {
return parameter
}
} as MockedDependency
const underTest = new UnderTest(mockedDependency)
It's far from perfect but will do for simpler cases.
Thanks in advance!

Answer 1

Here is the solution:
UnderTest.ts:
export interface MockedDependency {
returnSomething(...args: any[]): any;
returnSomething2(...args: any[]): any;
}
export default class UnderTest {
private mockedDependency: MockedDependency;
constructor(mockedDependency: MockedDependency) {
this.mockedDependency = mockedDependency;
}
public methodUnderTest(parameter: string) {
const mockedResult = this.mockedDependency.returnSomething(parameter);
return this.doSomethingElse(mockedResult);
}
public methodUnderTest2(parameter1: string, parameter2: string) {
const mockedResult = this.mockedDependency.returnSomething2(parameter1, parameter2);
return this.doSomethingElse(mockedResult);
}
private doSomethingElse(mockedResult: string) {
return mockedResult;
}
}
UnderTest.spec.ts:
import UnderTest, { MockedDependency } from './UnderTest';
const mockedDeps: jest.Mocked<MockedDependency> = {
returnSomething: jest.fn(),
returnSomething2: jest.fn()
};
const underTest = new UnderTest(mockedDeps);
describe('UnderTest', () => {
afterEach(() => {
jest.resetAllMocks();
});
describe('#methodUnderTest', () => {
it('should correctly', () => {
mockedDeps.returnSomething.mockReturnValueOnce('mocked result');
const actualValue = underTest.methodUnderTest('1');
expect(actualValue).toBe('mocked result');
expect(mockedDeps.returnSomething).toBeCalledWith('1');
});
});
describe('#methodUnderTest2', () => {
it('should correctly', () => {
mockedDeps.returnSomething2.mockReturnValueOnce('mocked result');
const actualValue = underTest.methodUnderTest2('2', '3');
expect(actualValue).toBe('mocked result');
expect(mockedDeps.returnSomething2).toBeCalledWith('2', '3');
});
});
});
Unit test result with 100% coverage report:
PASS src/stackoverflow/55966013/UnderTest.spec.ts
UnderTest
#methodUnderTest
✓ should correctly (5ms)
#methodUnderTest2
✓ should correctly (1ms)
--------------|----------|----------|----------|----------|-------------------|
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s |
--------------|----------|----------|----------|----------|-------------------|
All files | 100 | 100 | 100 | 100 | |
UnderTest.ts | 100 | 100 | 100 | 100 | |
--------------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests: 2 passed, 2 total
Snapshots: 0 total
Time: 3.597s, estimated 8s
Here is the completed demo: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/55966013