Is it possible to view Intersection types in Typescript as a subtype of the types included in the intersection?

I've been exploring the concepts of generic types and subtypes in TypeScript, but despite my efforts to delve into the official documentation, I'm still left with a lingering question. Specifically, I'm curious: Is an intersection type potentially a subtype of the types it intersects?

Let's consider the following types:

type Dog = { color: string }
type Cat = { gender: boolean }
type Fish = { weight: number }

type Dog_Cat_Fish = Dog & Cat & Fish;
type Dog_Cat = Dog & Cat;

Would it be accurate to say:

  • Dog_Cat_Fish is a subtype of Cat (1)
  • Dog_Cat_Fish is a subtype of Dog_Cat (2)
  • T & U is a subtype of T (3)
  • T & U & V is a subtype of T & V (4)

In this context, T, U, and V represent *type variables or generic types.

Thank you for entertaining my inquiry.

typescriptgenericstypescript-genericssubtypeintersection-types

Answer №1

Your four bullet points can all be answered with a resounding yes.

Even if the members of your types are incompatible, this statement holds true.

type Dog = { color:string }
type Cat = { color:boolean }
type Dog_Cat = Dog & Cat

While no values of this type can actually exist, it still functions as a subtype of both Dog and Cat. It essentially equates to never, although some tools may not always display it as such

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