TypeScript conditional types - filter out readonly properties / pick only required properties

Question

Using the new conditional types in TypeScript (or maybe another technique), is there a way to pick only certain properties from an interface based on their modifiers? For example, having...

interface I1 {
    readonly n: number
    s: string
}

I would like to create a new type based on the previous one which looks like this:

interface I2 {
    s: string
}

Answer 1

Update 2018-10: @MattMcCutchen has figured out that it is possible to detect readonly fields (invalidating the struck-out passage below), as shown in this answer. Here is a way to build it:

type IfEquals<X, Y, A=X, B=never> =
  (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? A : B;

type WritableKeys<T> = {
  [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P>
}[keyof T];

type ReadonlyKeys<T> = {
  [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, never, P>
}[keyof T];

If you want to extract the writable fields from an interface, you can use the above WritableKeys definition and Pick together:

interface I1 {
    readonly n: number
    s: string
}

type I2 = Pick<I1, WritableKeys<I1>>; 
// equivalent to { s: string; }

Hooray!

For `readonly`, I don't think you can extract those. I've [looked at this issue before](https://github.com/Microsoft/TypeScript/issues/13257#issuecomment-308528175) and it wasn't possible then; and I don't think anything has changed.

Since the compiler doesn't soundly check readonly properties, you can always assign a {readonly n: number} to a {n: number} and vice-versa. And therefore the obvious TSv2.8 conditional type check doesn't work. If, for example, {n: number} were not considered assignable to {readonly n: number} then you could do something like:

// does not work, do not try this
type ExcludeReadonlyProps<T> = Pick<T,
  { [K in keyof T]-?:
    ({ readonly [P in K]: T[K] } extends { [P in K]: T[K] } ? never : K)
  }[keyof T]>

type I2 = ExcludeReadonlyProps<I1> // should be {s: string} but is {} 🙁

But you can't. There's some interesting discussion about this in a GitHub issue originally named "readonly modifiers are a joke".

Sorry! Good luck.

---

For optional properties, you can indeed detect them and therefore extract or exclude them. The insight here is that {} extends {a?: string}, but {} does not extend {a: string} or even {a: string | undefined}. Here's how you could build a way to remove optional properties from a type:

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

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

type ExcludeOptionalProps<T> = Pick<T, RequiredKeys<T>>

type I3 = { 
  a: string, 
  b?: number, 
  c: boolean | undefined
}

type I4 = ExcludeOptionalProps<I3>;
// {a: string; c: boolean | undefined} 🙂

So that's good.

---

Finally, I don't know if you want to be able to do stuff with the class-only property modifiers like public, private, protected, and abstract, but I would doubt it. It happens that the private and protected class properties can be excluded pretty easily, since they are not present in keyof:

class Foo {
  public a = ""
  protected b = 2
  private c = false
}
type PublicOnly<T> = Pick<T, keyof T>; // seems like a no-op but it works
type PublicFoo = PublicOnly<Foo>; // {a: string} 🙂

But extracting the private or protected properties might be impossible, for the same reason that excluding them is so easy: keyof Foo doesn't have them. And for all of these including abstract, you can't add them to properties in type aliases (they are class-only modifiers), so there's not much I can think of to do to touch them.