What is the reason for the truth value of `type a = {} extends {a?:number}? true:false;`?

Why is 

type a = {} extends {a?:number}? true:false;
evaluated as true, while 
type b = {a?:number} extends {}? true:false;
is also true!
It seems like the empty object {} acts as a supertype that can extend other types. This raises some questions about how compatibility between types is judged. Typically, I've used Set theory to explain type relationships, but it doesn't seem to fully address this situation with optional properties and the empty object extension.
Could TypeScript be doing something special with the concept of {} in these cases?

typescript

Answer №1

When using the extends keyword, you can think of it as meaning 'can be assigned to'. This means that an object like {} can extend any type as long as all properties are optional.

const example: { property1?: string, property2?: number } = {}; // this is valid and will not throw an error

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