feat: typed response command

[Farenheith]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

The execute methods of CommandBus and QueryBus return any by default or the return type informed as generics, there's no way to intrinsically associate a return type to a command.

Issue Number: N/A

What is the new behavior?

If the command/query class attends two requisites the return type will be inferred:

• It extends of ICommand<ReturnType>/IQuery<ReturnType>;
• It is merged with an interface declaration of the same name that inherits ICommand<ReturnType>/IQuery<ReturnType>

Example:

class MyCommand implements ICommand<string> {}
interface MyCommand extends ICommand<string> {}

If the command doesn't attend both requisites, commandBus, and queryBus will still return any, making of this change a safe one, as no previous behavior got broken.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I'll update whatever documentation I need if the idea is accepted.

Although it's not ideal to merge a class and interface declaration, in some specific scenarios, like this one, this may bring some benefits and more type-safe code. I hope someday Typescript evolves so that this can work without the merge trick.

[kamilmysliwiec]

Thanks for your contribution! This feature is already implemented in this PR #1861

[Farenheith]

Thank you for your reply. You made the base class in a better way than I proposed, I think it could avoid the need for the type merge my approach needed.

However, it only affected the Handlers, not the command and query buses. I think with a new overload for the execute method in those classes the result of execute can also be strongly typed, which will result in more safe-type operations with the buses!

Can I create a PR for the PR's branch proposing this change?

edit: never mind. It is there too, sorry.