Regression: Partial properties became undefined instead of optional

TypeScript Version: 3.9.0-dev.20200310

Search Terms:
Partial

Code

export interface Messages {
  [key: string]: string
  $: string
}
export type PartialMessages<T extends Messages> = Partial<T> & {
  $: string
}
export function createMessages<T extends Messages>(messages: T): T {
  return messages
}
export const en = createMessages({
  $: 'English',
  welcome: 'Welcome!',
}
export const fr_ca = createMessages<PartialMessages<typeof en>>({
  $: 'Français (Canada)',
})

Expected behavior:

Should compile. Messages in PartialMessages are either complying or absent.

last tested working version: Typescript 3.7.5
first tested failing version: Typescript 3.8.2

Actual behavior:

Fail in:

export const fr_ca = createMessages<PartialMessages<typeof en>>(

Consider that all messages are present, but may be undefined.

Type 'PartialMessages<{ $: string; welcome: string; }>' does not satisfy the constraint 'Messages'.
  Property 'welcome' is incompatible with index signature.
    Type 'string | undefined' is not assignable to type 'Message'.
      Type 'undefined' is not assignable to type 'Message'.

Anders Hejlsberg (@ahejlsberg)

Anders Hejlsberg (@ahejlsberg) thoughts on this?

Partial<T> seems to make property values optional rather than the keys. Even though it seems like a semantic difference, a non-existent property cannot be accessed by messages.<prop_name>, while a property with a potentially non-existent value may be accessed to give type | undefined. Quick fix for now would be something like:

export type Messages = Partial<{
    [key: string]: string,
}> & {
    $: string
}

This is working as intended and is an effect of #35143 (which was in the 3.8 release). Consider:

// Compile with --strictNullChecks
declare let t: { [key: string]: string, $: string };
declare let s1: { welcome?: string } & { $: string };
declare let s2: { welcome?: string, $: string };

t = s1;  // Error, but wasn't before 3.8
t = s2;  // Error

Clearly the two assignments should be checked the same and they are now. Note that neither assignment is an error when --strictNullChecks is off.

Anders Hejlsberg (@ahejlsberg)

Thank you Anders Hejlsberg (@ahejlsberg). So, if I understand, the best way to write what I expect is how suggested by Muthu Kumar (@MKRhere)?
I suggest to add this behavior (and the solution) in the « breaking change » list of 3.8…

Stéphane Veyret (@sveyret)

Stéphane Veyret (@sveyret) I would probably change the declaration of Messages to

export interface Messages {
  [key: string]: string | undefined;
  $: string;
}

Yes, that's what I finally did. Thank you.
Shall I close the issue?

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Regression: Partial properties became undefined instead of optional #37324

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Regression: Partial properties became undefined instead of optional #37324

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RyanCavanaugh commented on Mar 11, 2020

MKRhere commented on Mar 11, 2020

ahejlsberg commented on Mar 12, 2020

sveyret commented on Mar 12, 2020

ahejlsberg commented on Mar 12, 2020

sveyret commented on Mar 12, 2020

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