0 | "" | {} extends 0 | "" // false
0 | "" | {} extends 0 | {} // true
Comparing the union 0 | "" | {} to 0 | "", it seems like the former technically extends from the latter. However, I am puzzled by the fact that it returns false.
This query arose from a Typescript type-challenge solution for AnyOf. It would be really helpful if someone could break down how Typescript determines whether UnionA extends UnionB is true or false.
The keyword ES6 classes is used to illustrate the concept of specialization:
class SubClass extends BaseClass...However, a problem arises when considering 0 | "" | {} versus 0 | "". Instead of narrowing down the possibilities, adding {} expands the options, making false the correct return value.
To test the idea of "MoreSpecific extends LessSpecific", consider:
0 | "" extends 0 | "" | {} ? true : falseAnother perspective involves assignments.
let variable: BaseClass = some_instance_of_SubClasslet myVar1: 0 | "" = value;In the case of
0 | "" | {} extends 0 | {}let test1: {} = ""
let test2: {} = 34
let test3: {} = {"anything you like": "other than null or undefined"}
(historically for compatibility, see https://github.com/microsoft/TypeScript/issues/44520 “It's a concession to back-compat because { } was the top type when generics were introduced”)
Since anything other than null or undefined can be assigned to {}, it follows that 0 | "" | {} is assignable to {} and consequently to 0 | {}. Therefore, the statement ""... extends ..."" holds true.
The inclusion of "" as an 'extra possibility' is encompassed within the {} part of 0 | {}.
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