fix(): do not remove undefined from ConfigService.get return type

[MatthiasKunnen]

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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 return type of a validated ConfigService.get excludes undefined. This hides null errors in the following case.

class Env {
    OPT?: string
}
const configService = app.get<ConfigService<Env, true>>(ConfigService);

const opt = configService.get('OPT', {infer: true}) // type is: string, should be string | undefined
const hasCat = opt.includes('🐱') // Will result in runtime error because opt is undefined

Issue Number: #1302

What is the new behavior?

The return type now no longer excludes undefined if WasValidated is true.

Does this PR introduce a breaking change?

• Yes
• No

Users that have strictNullChecks enabled, have optional properties on their Env class, and depend on the behavior of ConfigService.get stripping undefined from the return type will receive type errors. Example of code that would now result in errors:

class Env {
    mandatoryString?: string
}

const configService = app.get<ConfigService<Env, true>>(ConfigService);
const hasCat = configService.get('mandatoryString', {infer: true}).includes('🐱')

The TypeScript error raised:

src/file.ts:??:20 - error TS2532: Object is possibly 'undefined'.

??     const hasCat = configService.get('mandatoryString', {infer: true}).includes('🐱')
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I would argue that their types are wrong and this is exposed by this PR.

[micalevisk]

just to clarify here, can you share what's the result of this:

interface Env {
  optionalString?: string
  mandatoryString: string
  nilField: null | undefined
}

const configService = app.get<ConfigService<Env, true>>(ConfigService)

const opt = configService.get('optionalString', {infer: true}) // type ??
const man = configService.get('mandatoryString', {infer: true}) // type ??
const nil = configService.get('nilField', {infer: true}) // type ??

[MatthiasKunnen]

The types from your example are:

const opt = configService.get('optionalString', {infer: true}) // type string | undefined
const man = configService.get('mandatoryString', {infer: true}) // type string
const nil = configService.get('nilField', {infer: true}) // type null | undefined

[kamilmysliwiec]

LGTM