type or interface inside class in typescript

Question

I have a generic class that uses another derivative type (based on that generic type). I'd like to define the derivative type inside the class so I do not have to write it every time but TypeScript throws me an error.

current version (working):

class MyClass<T> {
    t: T

    doSomeThing(f: (t: T) => void) {}

    doAnotherThing(g: (t: T) => void) {} 
}

The version I want (not working):

class MyClass<T> {
    static type FT = (t: T) => void
    // throws error: I can't define type or interface inside class
    
    t: T

    doSomeThing(f: FT) {}

    doAnotherThing(g: FT) {} 
}

What is the right approach and solution. Thanks.

Answer 1

You may use a declared prop, or maybe a namspace

class MyClass<T> {
    // it's instance property. It does not exists actually.
    declare readonly FT: (t: T) => void;
    
    doSomeThing(f: typeof this.FT) {}
    doAnotherThing(g: MyClass.FT<T>) {} 
}
declare namespace MyClass {
    export type FT<T> = (t: T) => void
}

new MyClass<123>().doSomeThing
//                 ^?
// (method) MyClass<123>.doSomeThing(f: (t: 123) => void): void

new MyClass<123>().doAnotherThing
//                 ^?
// (method) MyClass<123>.doAnotherThing(g: (t: 123) => void): void