Creating a type or interface within a class in TypeScript allows for encapsulation of

I have a situation where I am trying to optimize my code by defining a derivative type inside a generic class in TypeScript. The goal is to avoid writing the derivative type every time, but I keep running into an error.

Here is the current version that is working:

class MyClass<T> {
    t: T

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

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

However, the version I want to implement, which is not working as intended, looks like this:

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) {} 
}

If you have any insights on the right approach and solution for this problem, I would greatly appreciate it. Thank you.

Answer №1

You have the option to utilize a declared prop, or possibly a namespace

class NewClass<T> {
    // this is an instance property that is not actually instantiated.
    declare readonly FT: (t: T) => void;
    
    doSomething(f: typeof this.FT) {}
    doAnotherThing(g: NewClass.FT<T>) {} 
}
declare namespace NewClass {
    export type FT<T> = (t: T) => void
}

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

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

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