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

Guards expose a `canActivate(context)` method; you can unit test it by instantiating the guard class and passing a crafted mock `ExecutionContext` to verify its logic.

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

`mockReturnValue(val)` configures a `jest.fn()` mock to return `val` synchronously on every call, allowing you to control dependent service behavior in tests.

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

`mockResolvedValue(data)` is shorthand for `mockReturnValue(Promise.resolve(data))` and is the idiomatic way to mock async service methods in NestJS tests.

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

Exception filters can be unit tested by calling `catch()` directly with mock `ArgumentsHost`, or e2e tested by checking the full HTTP response — both approaches provide complementary coverage.

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

The `--coverage` flag instruments the code and produces a report (HTML, text, lcov) showing the percentage of statements, branches, functions, and lines covered by tests.

NestJS Practice Test

NestJS Practice Test

NestJS NestJS Testing & Debugging 3