feat(types): Type check Field defaultValue

[martinlehoux]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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: Typing improvement

What is the current behavior?

Currently, when using @Field(() => MyType, { defaultValue: ... }), the defaultValue type is not checked as MyType at compile time. It can lead to runtime errors if you refactor MyType but forget to update the default value.

What is the new behavior?

When using the @Field(() => MyType, { defaultValue: ... }) or the array one @Field(() => [MyType], { defaultValue: ... }), the defaultValue type is checked and there is a Typescript error if it's not MyType or MyType[] respectively.

The feature is not available when using @Field({ defaultValue: ... }), it defaults to the current behavior of allowing any value.

Does this PR introduce a breaking change?

• Yes
• No

I think it doesn't, given there is an any fallback, but I may need to test more thoroughly (I don't know how to add type tests in the test suite).

Other information

The same behavior could be implemented for others decorators to some extend.

[kamilmysliwiec]

Thanks for your contribution @martinlehoux! Would you like to update tests as well (possibly leveraging the // @ts-expect-error helper https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-9.html#-ts-expect-error-comments)?

[martinlehoux]

@kamilmysliwiec Thanks for the link, I didn't know it existed! As for the tests, could you point me to where I should add it. I have a hard time finding the tests related to the @field decorator

[martinlehoux]

@kamilmysliwiec I added a test file for the field.decorator.ts file, please tell me if the format of the test is OK

[martinlehoux]

@kamilmysliwiec Hi, is there anything I can do to push this forward?

[martinlehoux]

@kamilmysliwiec Is it still something you'd like to see added to the codebase?

[jmcdo29]

Overall, I think the type looks good. This shouldn't cause any issues with older code bases, yeah? As any is the default used here

[jmcdo29]

Should this be Inner[]?

[martinlehoux]

I guess you're right ^^

[kamilmysliwiec]

PR looks great! However, we can't merge it in yet as it introduces a (minor but still) breaking change (even though in theory it shouldn't break when defaultValue was used as expected)