Testing module setup in NestJS - Step-by-Step Execution

Concept Flow - Testing module setup

Create TestingModuleBuilder

↓

Configure providers, controllers

↓

Compile TestingModule

↓

Retrieve instances with get()

↓

Run tests using instances

↓

END

This flow shows how NestJS testing modules are created, configured, compiled, and used to get instances for testing.

Execution Sample

NestJS

const module = await Test.createTestingModule({
  providers: [MyService],
}).compile();
const service = module.get(MyService);

This code creates a testing module with MyService, compiles it, and retrieves the service instance for testing.

Execution Table

Step	Action	Input	Output	Notes
1	Call Test.createTestingModule	{ providers: [MyService] }	TestingModuleBuilder instance	Builder created with providers config
2	Call compile()	TestingModuleBuilder instance	TestingModule instance	Module compiled, dependencies resolved
3	Call get(MyService)	TestingModule instance	MyService instance	Service instance retrieved for tests
4	Use service in tests	MyService instance	Test results	Service methods can be called
5	End	-	-	Testing module setup complete

💡 Testing module setup ends after service instance is retrieved and ready for tests.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
module	undefined	TestingModuleBuilder	TestingModule	TestingModule	TestingModule
service	undefined	undefined	undefined	MyService instance	MyService instance

Key Moments - 2 Insights

Why do we need to call compile() after createTestingModule()?

compile() finalizes the module by resolving dependencies and preparing instances, as shown in step 2 of the execution_table.

What does module.get(MyService) return?

It returns the instance of MyService created inside the testing module, as shown in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after calling compile()?

A TestingModule instance

B TestingModuleBuilder instance

C MyService instance

D Undefined

💡 Hint

Check step 2 in the execution_table where compile() is called.

At which step is the MyService instance first available?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at when module.get(MyService) is called in the execution_table.

If you forget to call compile(), what will module.get(MyService) return?

A MyService instance

B TestingModuleBuilder instance

C Undefined or error

D Empty object

💡 Hint

Refer to the variable_tracker and the role of compile() in the key_moments.

Concept Snapshot

NestJS Testing Module Setup:
- Use Test.createTestingModule({ providers, controllers }) to start.
- Call compile() to finalize and resolve dependencies.
- Use module.get(Service) to retrieve instances.
- Instances are ready for testing after compile.
- Always compile before getting instances.

Full Transcript

In NestJS, testing modules are created using Test.createTestingModule with configuration for providers and controllers. This returns a TestingModuleBuilder. Calling compile() on this builder finalizes the module by resolving dependencies and preparing instances. After compilation, you can retrieve service instances using module.get(Service). These instances are then used in your tests. Skipping compile() will cause errors or undefined instances. This process ensures your tests run with properly configured and isolated modules.