Adding Conditionally Specified Properties to a Parameterized TypeScript Interface Based on Type

Encountering a challenge with TypeScript where I need to selectively add properties to a generic interface depending on the type parameter. Let me explain further with an example:

Consider two interfaces, A and G<T>:

interface A {
    IA: string;
}

interface G<T> {
    IG: string;
    Parent: T;
}

My goal is to include additional properties in G<T> only when it's used with type A. For instance, when using G<A>, I want to introduce properties like prop1, but not when using G<B> (with B being another type). It should look something like this:

inteface G<T> extends A {
    prop1: string;
}

In need of a solution or different approach to achieve this conditional property addition based on the type parameter. Open to ideas and recommendations. Thank you!

Answer №1

If you want to accomplish this, make use of the type method.

Take a look at this illustration:

interface A {
  IA: string
}

type G<T> = {
  IG: string
} & T

// correct
const g: G<A> = {
  IA: 'IA',
  IG: 'IG',
}

// incorrect 
const g: G<A> = {
  IG: 'IG',
}


In case the property does not belong to A, here is how you can handle it

interface A {
  IA: string
}

interface B {
  IB: string
}

type G<T> = {
  IG: string
} & (T extends A ? { prop1: string } : { prop2: string })

// correct
const ga: G<A> = {
  IG: 'IG',
  prop1: 'IA',
}

// error
const gb: G<B> = {
  IG: 'IG',
  prop1: 'IA',
}

Answer №2

I have discovered a method to configure conditional properties:

interface G<T> {
    IG: string;
    Parent: T;

    prop1: T extends A ? string : never;
    prop2: T extends B ? string : never;
}

With this technique, I am now able to assign specific properties based on T. However, the downside is that all properties will be present. For example, G<A> will still include prop2 but it will be of type never. To address this, we need to filter it out to avoid displaying it in intellisense.

type NonNeverType<T> = {
  [K in keyof T as T[K] extends never ? never : K]: T[K] & string;
};

This filters out properties with type never.

While not the most elegant solution, it serves its purpose for my particular situation.

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