How to define implementation of function that combine call and construct signatures in typescript

The typescript documentation talks about function combine call and construct signatures。and describes how you can declare the type。https://www.typescriptlang.org/docs/handbook/2/functions.html

interface CallOrConstruct {
  new (s: string): Date;
  (n?: number): number;
}

But it never shows how to define the actual implementation of such functions。 my english's not good, so I make a sample to descript my question, I will appreciate your answer。

interface CallOrConstruct {
    new (s: string): { s: string }
    (n: number): number
}
function invokeF (fn: CallOrConstruct) {
    fn(5)
    new fn('hello world')
}
var fn:CallOrConstruct  // how to define the actual implementation of fn

I want to know how to define the actual implementation of the type that combine call and construct signature。

You'd use new.target to distinguish between the calls. But you'll basically have to implement an overloaded function which requires liberal use of any to get it compile as TypeScript:

const fn = function(this: any, x: any): any {
  if (new.target) {
    // we're in a constructor now
    this.s = x;
  } else {
    // a plain function call
    return x;
  }
} as CallOrConstruct;