Create a fresh user interface that mandates the presence of all optionals

Is there a way to modify an interface by making optional properties required?

Consider the following interface:

interface Foo {
  test1: string;
  test2?: string;
}

I want to create a new interface based on this, with the optional property now being required:

interface FooDefaults {
  test2: string;
}

This new interface can be used for setting default values like so:

const defaults: FooDefaults = {
  test2: 'bar'
}

Answer №1

This particular issue is quite complex, as there are ongoing debates about the syntax. You can find more details in this thread. The solution provided below is influenced by @jcalz's response on this post.

interface Bar {
  example1: string;
  example2?: string;
}

type Defaults<T, OptionalKeys extends keyof T = { [K in keyof T]:
  ({} extends { [P in K]: T[K] } ? K : never)
}[keyof T]> = {
  [K in OptionalKeys]-?: T[K]
}

type BarDefaults = Defaults<Bar>

In my opinion, this solution appears to be more of a workaround than a definitive fix. It relies on the behavior where an empty object {} is considered to extend an object with an optional property ({a?: string}), but not one with a required property ({a: string}).

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