Check if an interface has a required field

Question

Is it possible to check whether an interface has a required field using Typescript's Conditional Types?

type AllRequired = { a: string; b: string }
type PartiallyRequired = { a: string; b?: string }
type Optional = { a?: string; b?: string }

// Is it possible to change this, so the below works
type HasRequiredField<T> = T extends {} ? true : false

type A = HasRequiredField<AllRequired> // true
type B = HasRequiredField<PartiallyRequired> // true
type C = HasRequiredField<Optional> // false

Answer 1

Yes, you can detect if properties are optional. It gets a bit iffy with index signatures, but your types don't have them so I'm not going to worry about them.

Here's how you can extract just the keys of the non-optional properties of an object type

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

And then you can just check if it has any of them or not (if RequiredKeys<T> is never then it does not):

type HasRequiredField<T> = RequiredKeys<T> extends never ? false : true

And that gives your desired results:

type A = HasRequiredField<AllRequired> // true
type B = HasRequiredField<PartiallyRequired> // true
type C = HasRequiredField<Optional> // false

Hope that helps; good luck!