Question:

How to get the type of a class property's getter/setter in TypeScript?

Problem

As TypeScript class properties can have getters and setters of different types >since 4.3, I don't know who to get correctly a property's getter and setter's types.

===

As a class property is considered as a variable, I can't distinguish properly if the given property does have a getter nor/and setter and their type (as Parameters<I> and ReturnType<I> can only work on a function/method).

Some ideas of the expected result:

class Foo {

get bar(): number { /** ... */ }

set bar(baz: string) { /** ... */ }

}

type GetterOfFooBar = GetterOf<typeof Foo.prototype.bar>; // number

type SetterOfFooBar = SetterOf<typeof Foo.prototype.bar>; // string

class Foo {

get bar(): number { /** ... */ }

}

type GetterOfFooBar = GetterOf<typeof Foo.prototype.bar>; // number

type SetterOfFooBar = SetterOf<typeof Foo.prototype.bar>; // never

Solution

Any use of the type system to observe the property type of a variant accessor will result in the type that you read, not the type that you write. Which means GetterOf<T> is easy but SetterOf<T> is impossible.

This doesn't seem to be very well documented. It is mentioned in the discussion of >microsoft/TypeScript#53417 at >microsoft/TypeScript#53594:

We always pick the read types.

as well as the implementation notes at >microsoft/TypeScript#42425:

The type system remains entirely covariant in other operations! This has some effects that may not be immediately apparent: ... Types with variant getters/setters are effectively reduced to their get side when put through mapped types ... The type T[K] still means the read type of K on T

There is a request at >microsoft/TypeScript#43729 to allow the setter type to be queried. It's also possible that if the request at >microsoft/Typescript#43826 to support variant accessors in mapped types and index signatures is implemented, it will allow the setter type to be isolated and thus queried. But for now, this is not part of the language.

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