Javascript add to string with each function call

Question

I have the following situation where I have a function f which takes an argument input.

I want to be able to have f such that

Answer 1

In JS you can even achieve this dualitiy of a function also "being" a String. Yet this is nothing you should ever use in production code.

const f = function _f(prefix){
  const toString = () => prefix;
  return Object.assign(function(suffix="o"){ 
    return _f(prefix + suffix);
  }, {
    valueOf: toString,
    toString
  });
}("f");

console.log(""+f('l'))
console.log(""+f())
console.log(""+f()('l'))
console.log(""+f()()('l'))
console.log(""+f()()()('l'))

let foo = f()();
let fool = foo("l");
console.log(""+foo("l"));
console.log(""+fool);
console.log(""+foo("bar"));
console.log(""+fool("bar"));