Confusion with the "extends" keyword in Typescript

Issue

Think of the following line of code in Typescript:

let x: 'a' | 'b' extends 'a' ? true : false;

I was wondering that the type of x would be true since intuitively 'a' | 'b' is an extended version of 'a' (at least my intuition says so). I thought extends would work like subsets in math. A extends B iff B ⊆ A.

However, it seems that the actual type x is false here. I guess I don't understand how exactly the extends keyword works.

Solution

Per the Liskov Subsitution Principle, if "Y extends X" or equivalently "Y is a subtype of X", then a value conforming to type Y can be used wherever a value conforming to type X is requested. This leads to a counterintuitive conclusion: when referring to the universe of possible values, if Y extends X, Y is more constrained than X. All Ys are Xs, but not all Xs are Ys.

In your example, because 'a' | 'b' could be either 'a' or 'b', that union type does not extend type 'a', because 'b' wouldn't substitute for 'a'. Instead, 'a' extends ('a' | 'b'), because all values that match 'a' would work where 'a' | 'b' is requested.

As such, A extends B iff A ⊆ B.

One reason this is less intuitive in TypeScript is that your example deals in unions of literal values. It might make more sense for us to think of this in terms of objects, where {foo: number, bar: number} extends {foo: number}. The latter, {foo: number}, could have a bar property of any type or no bar at all. The former {foo: number, bar: number} is more specific and more constrained: not only is foo a number, but bar is also a number. This also matches the use of extends in class or interface definitions: The subclass or subinterface adds properties and methods, which further constrains instances compared to the superclass or superinterface.

This is also why never is assignable to everything: never is the most constrained type because no actual values match it, so never extends everything. The empty set is a subset of every set; the empty type never is the subtype of every type and can be assigned to every other type.

type Foo = { foo: number };
type Bar = never extends Foo ? true : false;  // true

Answered By - Jeff Bowman