NestJS Commander Testing

So you;'ve built a CLI application, but you want to test it, and you want to be able to do your
usual NestJS DI mocking. Well, here's your solution :fireworks:

Installation

Before you get started, you'll need to install a few packages. First and foremost, this one:
nest-commander-testing (name pending). You'll also need to install @nestjs/testing as this
package makes use of them under the hood, but doesn't want to tie you down to a specific version,
yay peerDependencies!

``sh
npm i nest-commander-testing @nestjs/testing


OR

yarn add nest-commander-testing @nestjs/testing

OR

pnpm i nest-commander-testing @nestjs/testing
`

Testing With Mocks

So what's the use of writing a super awesome command line script if you can't test it super easily,
right? Fortunately, nest-commander has some utilities you can make use of that fits in perfectly
with the NestJS ecosystem, it'll feel right at home to any Nestlings out there. Instead of using the
CommandFactory for building the command in test mode, you can use CommandTestFactory and pass in
your metadata, very similarly to how Test.createTestingModule from @nestjs/testing works. In
fact, it uses this package under the hood. You're also still able to chain on the overrideProvider
methods before calling compile() so you can swap out DI pieces right in the test.
A nice example of this can be seen in the basic.command.factory.spec.ts file.

Testing Inquirer Questions

If you are making use of the InquirerService, you can use CommandTestFactory.setAnswers method
and pass either a single answer or multiple answers depending on how many answers you need to mock.
In doing so, a mock inquirer service will act similarly to inquirer without needing to modify
process.stdin` or make use of any user input, allowing smooth testing in your CI pipelines. For an
example of this, please check the
pizza command integration test.