NestJS NestJS Testing & Debugging 3

Question 1

How do you test a NestJS `CanActivate` guard in isolation?

Accepted Answer

Instantiate the guard directly and call its `canActivate()` method with a mock `ExecutionContext`

Answer

Mount it in an e2e test and check the HTTP response status

Answer

Use `app.useGlobalGuards()` inside the test module

Answer

Decorate the test controller with `@UseGuards()` and verify route access

Question 2

What is the purpose of the `mockReturnValue()` method on a Jest mock function in NestJS tests?

Accepted Answer

It sets the value the mock function will return on every subsequent call

Answer

It runs the real implementation once and then returns undefined

Answer

It asserts the mock was called with the specified value

Answer

It resets the mock's call history and sets a new implementation

Question 3

Which Jest method should you use when a NestJS service method returns a Promise and you want to mock its resolved value?

Accepted Answer

mockResolvedValue(data)

Answer

mockReturnValue(Promise.resolve(data))

Answer

mockImplementation(async () => data)

Answer

mockReturnValueOnce(data)

Question 4

In NestJS testing, what is the recommended way to test an `ExceptionFilter` that transforms `HttpException` into a custom response?

Accepted Answer

Both B and C are valid approaches depending on whether you want unit or e2e coverage

Answer

Import the filter in a test module and call the route that throws

Answer

Instantiate the filter and call `catch(exception, host)` with mock arguments

Answer

Use `app.useGlobalFilters()` in an e2e test and assert on the response body

Question 5

What does the `--coverage` flag do when running `jest` in a NestJS project?

Accepted Answer

Generates a code coverage report showing which lines/branches are tested

Answer

Runs only tests marked with @CoverageTest() decorator

Answer

Limits test execution to files in the coverage whitelist

Answer

Enables verbose output for each test case

Question 6

Which NestJS feature allows you to log detailed request/response information during development to help with debugging?

Accepted Answer

The built-in `Logger` class with `app.useLogger()`

Answer

The `@Debug()` decorator applied to controllers

Answer

The `NestDevTools` package integrated via `NestFactory`

Answer

Setting `NODE_ENV=debug` automatically enables verbose output

Question 7

What is the role of `TestingModule.resolve()` as opposed to `TestingModule.get()` in NestJS testing?

Accepted Answer

`resolve()` creates a new scoped/transient instance; `get()` returns the singleton

Answer

`resolve()` is synchronous; `get()` is asynchronous

Answer

`resolve()` retrieves global providers; `get()` retrieves module-local providers

Answer

`resolve()` compiles the module lazily; `get()` compiles it eagerly