Typescript: Mapped type optional -> required property issue

[JakeGinnivan]

TypeScript Version: 2.1.4

Code
https://github.com/JakeGinnivan/TypeScriptMappedTypeIssue

Expected behavior:
The mapped type

export type PropDescriptions<T> = {
    [P in keyof T]: string
}

Is marking all the properties as required, I would expect this to fail:

interface ThingProps {
    prop1: string
    prop2?: string
}

const expected: PropDescriptions<ThingProps> = {
    prop1: 'Ok'
}

with Property 'prop2' is missing in type {...}

Actual behavior:
No compilation error

[JakeGinnivan]

As a side note, it would be awesome if you could iterate at runtime on the keys of an interface. I guess that starts moving into the reflection questions, but it would be super handy if code could be emitted based on type information. So not runtime reflection but static templating.

I probably need to generate the styleguide from the typescript source rather than the approach I am taking.

[mhegazy]

Mapped types that are a homomorphic transformation are treated specially in the type system. the above type PropDescriptions is one of such. The type system knows that PropDescriptions<T> is a transformation of T, and it copies the readonly and ? modifiers on the new type as it is generated from the source. Think of ReadOnly<T>, where T has optional properties, you would expect the result to have optional properties as well.

You however change the mapping to make it a non-homomorphic transformation as such:

type PropDescriptions<T, K extends string> = {
  [P in K]: string
}

and you would instantiate it with:

const expected: PropDescriptions<ThingProps, keyof ThingProps> = ...

Notice the constraint on K is string; this way the type system can not assert that the result of this transformation is a homomorphic mapping on T, and thus no modifiers are copied through.

It is worth noting that if the constraint was K extends keyof T (e.g. the definition of type Pick in lib.d.ts`), the transformation would still be considered a homomorphic one.

[JakeGinnivan]

Thanks, have opened a PR in the docs which adds some of this information. Appreciate the good explanation.

[JakeGinnivan]

It would also be good to extend the docs to explain how to use the Pick and Record types.