Using a mapped type, can't Exclude "undefined" from an optional property

[acshef]

The example below shows how one ought to be able to implement the opposite of the mapped type Partial<T>, which should remove undefined from the type of every single property in an interface.

TypeScript Version: 2.9.0-dev.201xxxxx

Search Terms:
exclude undefined mapped optional

Code

type NeverUndefined<T> = {
    [K in keyof T]: Exclude<T[K], undefined>;
}

interface MyInterface {
    undef: undefined
    optionalString?: string;
    stringOrUndefined: string | undefined;
    definedString: string;
}

type NeverUndefinedMyInterface = NeverUndefined<MyInterface>;

Expected behavior:

// typeof NeverUndefinedMyInterface
interface NeverUndefinedMyInterface {
    undef: never;
    optionalString: string;
    stringOrUndefined: string;
    definedString: string;
}

Actual behavior:

// typeof NeverUndefinedMyInterface
interface NeverUndefinedMyInterface {
    undef: never;
    optionalString: string | undefined;
    stringOrUndefined: string;
    definedString: string;
}

(notice that optionalString still contains undefined in its type definition)

Playground Link:
https://www.typescriptlang.org/play/#src=type%20NeverUndefined%3CT%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BK%20in%20keyof%20T%5D%3A%20Exclude%3CT%5BK%5D%2C%20undefined%3E%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20MyInterface%20%7B%0D%0A%20%20%20%20undef%3A%20undefined%3B%0D%0A%20%20%20%20optionalString%3F%3A%20string%3B%0D%0A%20%20%20%20stringOrUndefined%3A%20string%20%7C%20undefined%3B%0D%0A%20%20%20%20definedString%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Atype%20NeverUndefinedMyInterface%20%3D%20NeverUndefined%3CMyInterface%3E%3B

Related Issues:
None

[DanielRosenwasser]

You can never truly remove undefined from an optional property, but you remove optionality with mapped type modifiers (#21919), or use the Required type which is part of lib.d.ts now.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.