Unit testing services in NestJS - Step-by-Step Execution

Concept Flow - Unit testing services

Write test case

↓

Create testing module

↓

Inject service

↓

Call service method

↓

Check result with expect()

↓

Pass or fail test

↓

Repeat for other methods

This flow shows how to write and run a unit test for a NestJS service step-by-step.

Execution Sample

NestJS

describe('CatsService', () => {
  let service: CatsService;

  beforeEach(async () => {
    const module = await Test.createTestingModule({ providers: [CatsService] }).compile();
    service = module.get<CatsService>(CatsService);
  });

  it('should return all cats', () => {
    expect(service.findAll()).toEqual(['cat1', 'cat2']);
  });
});

This code sets up a test for CatsService and checks if findAll() returns the expected list.

Execution Table

Step	Action	Evaluation	Result
1	Create testing module with CatsService	Module created	Module instance ready
2	Get CatsService instance from module	Service injected	service variable assigned
3	Call service.findAll()	Returns ['cat1', 'cat2']	Output: ['cat1', 'cat2']
4	Compare output with expect()	Output matches expected	Test passes
5	End of test case	No more tests	Test suite completes

💡 All test cases executed, no failures

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
service	undefined	CatsService instance	CatsService instance	CatsService instance
output	undefined	undefined	['cat1', 'cat2']	['cat1', 'cat2']

Key Moments - 3 Insights

Why do we create a testing module before getting the service?

The testing module simulates NestJS dependency injection, so the service can be properly created and injected as in the real app. See Step 1 and 2 in execution_table.

What does expect(service.findAll()).toEqual(...) check?

It checks if the service method returns exactly the expected value. If it matches, the test passes (Step 4).

Why do we use beforeEach() to create the module and service?

beforeEach() runs before every test to ensure a fresh service instance, avoiding shared state between tests. This keeps tests independent.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'service' after Step 2?

A null

B CatsService instance

C undefined

D Array of cats

💡 Hint

Check the 'service' variable in variable_tracker after Step 2

At which step does the test check if the output matches the expected value?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for the expect() call

If the service.findAll() returned ['cat1'] instead of ['cat1', 'cat2'], what would happen at Step 4?

A Test fails

B Test passes

C Service instance changes

D Module creation fails

💡 Hint

Step 4 compares output with expected value; mismatch causes failure

Concept Snapshot

Unit testing services in NestJS:
- Use Test.createTestingModule to build a test module
- Inject service with module.get()
- Call service methods inside it() blocks
- Use expect() to check outputs
- Use beforeEach() for fresh setup
- Tests pass if outputs match expected values

Full Transcript

Unit testing services in NestJS involves creating a testing module that mimics the real app environment. We inject the service we want to test from this module. Then, we call the service methods and check their outputs using expect(). The beforeEach() function ensures each test runs with a fresh service instance. If the output matches what we expect, the test passes. This process helps us verify that our service behaves correctly in isolation.