How to reuse function signature definition in TypeScript

Question

I've got the following code:

class A {
  constructor(public n: number) {}  

  defaultFn(a: number): number {
    return a + 1;
  }

  doStuff(callback?): number {
    return callback ? callback(this.n) : this.defaultFn(this.n);
  }
}

How can I tell TypeScript that the optional callback function passed to doStuff method should have the same signature as defaultFn method?

Answer 1

One more option is to "query" type of defaultFn. In this case you won't need to maintain the type of callback manually:

class A {
    constructor(public n: number) {}  

    defaultFn(a: number): number {
        return a + 1;
    }

    doStuff(callback?: A['defaultFn']): number {
        return callback ? callback(this.n) : this.defaultFn(this.n);
    }
}

The result of A['deafultFn'] is (a: number) => number